Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 201:

  Fireworks have been used for centuries in celebrations around the world. One of the primary components of these devices, black powder, was developed by the Chinese over a thousand years ago and is still used today as a propellant and explosive. Black powder is composed of potassium nitrate (KNO3), charcoal (primarily C) and sulfur (S8) in a 75:15:10 ratio by weight. It is very stable if kept dry but can easily be ignited by a spark or burning fuse to undergo the following reaction:

  

  Reaction 1 The basic firework is shown in Figure 1. Fireworks rely on a particular kind of combustion in which oxygen is supplied by oxidizing agents included in the pyrotechnic mixture. When ignited, the solid propellant begins to liquefy and vaporize allowing the fuel and oxidizing agents to interact more intimately leading to rapid expansion of gases. Delay fuses time the ignition of the other compartments to occur when the shell is high above ground.

  

  Figure 1

  The light generating units of the firework are called stars and are dispersed and ignited by the bursting charge in each compartment. The intense colors of modern fireworks are generated by molecular emitters. For example, barium chloride emits green light (510?30 nm) and strontium chloride emits vibrant red light (605?50 nm). Many of the molecular emitters are unstable at room temperature and so cannot be placed directly into the firework. Instead, they are synthesized in the flame of the pyrotechnic reaction and exist for a short time before decomposing. The flame temperature must be carefully adjusted so that these emitters do not decompose too rapidly.

  

  The molar ratio of N to C to S in black powder is approximately:

  A. 75:125:35

  B. 85:10:210

  C. 75:15:10

  D. 100:20:7

  Correct Answer: A

  The passage gives the composition by weight of black powder as 75:15:10, potassium nitrate:charcoal: S8. Let's assume we have 100 grams of black powder, composed of 75 grams of potassium nitrate, 15 grams of charcoal, and 10 grams of S8. First, we must determine the mass of one mole of each of the materials from the periodic table:

  

  Now we can determine how many moles of each material is present by dividing the mass by the mass per mole:

  

  Therefore, our mole ratio is 0.75:1.25:0.32, or 75:125:32.

• Question 202:

  Fireworks have been used for centuries in celebrations around the world. One of the primary components of these devices, black powder, was developed by the Chinese over a thousand years ago and is still used today as a propellant and explosive. Black powder is composed of potassium nitrate (KNO3), charcoal (primarily C) and sulfur (S8) in a 75:15:10 ratio by weight. It is very stable if kept dry but can easily be ignited by a spark or burning fuse to undergo the following reaction:

  

  Reaction 1 The basic firework is shown in Figure 1. Fireworks rely on a particular kind of combustion in which oxygen is supplied by oxidizing agents included in the pyrotechnic mixture. When ignited, the solid propellant begins to liquefy and vaporize allowing the fuel and oxidizing agents to interact more intimately leading to rapid expansion of gases. Delay fuses time the ignition of the other compartments to occur when the shell is high above ground.

  

  Figure 1

  The light generating units of the firework are called stars and are dispersed and ignited by the bursting charge in each compartment. The intense colors of modern fireworks are generated by molecular emitters. For example, barium chloride emits green light (510?30 nm) and strontium chloride emits vibrant red light (605?50 nm). Many of the molecular emitters are unstable at room temperature and so cannot be placed directly into the firework. Instead, they are synthesized in the flame of the pyrotechnic reaction and exist for a short time before decomposing. The flame temperature must be carefully adjusted so that these emitters do not decompose too rapidly.

  

  Which of the following chemicals is oxidized in the pyrotechnic reaction of gunpowder (Reaction 1)?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Carbon is oxidized from 0 to the +4 state in carbon dioxide. Both N and S are reduced in the reaction, as shown below:

  

• Question 203:

  Fireworks have been used for centuries in celebrations around the world. One of the primary components of these devices, black powder, was developed by the Chinese over a thousand years ago and is still used today as a propellant and explosive. Black powder is composed of potassium nitrate (KNO3), charcoal (primarily C) and sulfur (S8) in a 75:15:10 ratio by weight. It is very stable if kept dry but can easily be ignited by a spark or burning fuse to undergo the following reaction:

  

  Reaction 1 The basic firework is shown in Figure 1. Fireworks rely on a particular kind of combustion in which oxygen is supplied by oxidizing agents included in the pyrotechnic mixture. When ignited, the solid propellant begins to liquefy and vaporize allowing the fuel and oxidizing agents to interact more intimately leading to rapid expansion of gases. Delay fuses time the ignition of the other compartments to occur when the shell is high above ground.

  

  Figure 1

  The light generating units of the firework are called stars and are dispersed and ignited by the bursting charge in each compartment. The intense colors of modern fireworks are generated by molecular emitters. For example, barium chloride emits green light (510?30 nm) and strontium chloride emits vibrant red light (605?50 nm). Many of the molecular emitters are unstable at room temperature and so cannot be placed directly into the firework. Instead, they are synthesized in the flame of the pyrotechnic reaction and exist for a short time before decomposing. The flame temperature must be carefully adjusted so that these emitters do not decompose too rapidly.

  

  Which of the following bonds has the greatest ionic character?

  A. Ba–Cl

  B. Sr–Cl

  C. N–N

  D. O=CO

  Correct Answer: A

  Covalent bonds are formed by the sharing of electrons between atoms. In bonds between the same elements, these electrons are shared evenly. In most bonds, the electrons are not shared evenly and one element draws electrons towards itself, imparting a partial negative charge on that element and a partial positive charge on the other element in the bond. This uneven sharing is defined as ionic character of the bond and depends on the relative electronegativities of the two elements involved in the bond. The greater the difference in electronegativities, the greater the ionic character. At the extreme end of the spectrum, when the electronegativities are vastly different, electron transfer is complete and the bond is considered ionic.According to periodic trends, electronegativity increases to the right and up in the periodic table. Chlorine has a very high electronegativity while Sr and Ba have very low electronegativities. Because Ba is lower on the periodic table, it has a lower electronegativity than Sr and thus, BaCl2 has more ionic character than SrCl2 because the difference in electronegativities is greater. It may help to remember that F is the most electronegative element while Cs is the least electronegative.Choice C is incorrect because the bond between two atoms of the same species is completely covalent.Choice D is incorrect because carbon and oxygen are closer together on the periodic table than are Ba and Cl.

• Question 204:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  All of the following are resonance structures of N3- EXCEPT:

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  The formal charges are assigned incorrectly: their values should be as indicated in choice B. Resonance structures vary only in their arrangement of electrons. The connectivity of nuclei is preserved. The other resonance structures for N3are valid and contribute to the resonance hybrid structure that is more stable than any of the individual resonance structures alone.

• Question 205:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  A researcher wishes to make the decomposition of sodium azide (Reaction 1) less favorable. Which of the following adjustments to the reaction would NOT drive it to the left?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  Only choice C would drive the reaction towards the right, according to Le Chatelier's principle. The products include 3 moles of gas (N2). The reaction is exothermic (although not excessively so according to the passage); increasing the temperature, therefore, would make the reactants more favorable thermodynamically.

• Question 206:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  Decomposition of which of the following transition metal complexes would produce the highest theoretical yield of carbon monoxide per gram of reactant?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  This pure stoichiometry problem requires you to figure out, either quantitatively or qualitatively, that the more CO a formula contains and the less a mole of the compound weighs, the more CO per gram of the compound will be produced. Since chromium has the smallest atomic mass of any of the metals offered, and since Cr(CO)6 has six moles of CO per formula mole, choice A is the best choice.Quantitatively, choice A produces 6 mol CO for every 220 g of Cr(CO)6, choice B produces 5 mol CO per 195 g Mn(CO)5, choice C produces 4 mol CO per 208 g Mo(CO)4 , and choice D produces 6 mol CO per 274 g Pd(CO)6. Since 6/220 is greater than any of the other ratios, choice A is quantitatively proven correct.

• Question 207:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  A sodium azide air bag inflates to a volume of 45 Liters at STP. According to the information contained in the passage, what is the mass of NaN3 (Mol. Wt. = 65) that is required to inflate the bag?

  A. 76 grams

  B. 81 grams

  C. 87 grams

  D. 130 grams

  Correct Answer: B

  According to the passage, each 2 moles of NaN3 ultimately lead to the production of 3.2 moles of N2, 3 moles from Reaction 1 and 1/5 moles from Reaction 2.

  

  A single mole of an ideal gas at STP occupies 22.4 L. Thus, we require 2 moles of gas to occupy about 45L. Calculating for x, the number of moles of NaN3 required to produce 2 moles of N2:

  

  Now we determine the mass of 1.25 moles by multiplying by the mass of NaN3 (from the periodic table):1.25 moles ?(23 + 3(14)) grams/mole = 81.25 grams

• Question 208:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  Potassium chlorate, KClO3(s), decomposes when heated, yet it is unsuitable as an airbag inflatant. All of the following characteristics of KClO3 make it a poor candidate for an air bag inflatant EXCEPT:

  A. the decomposition requires a steady supply of energy due to its high activation energy and low exothermicity.

  B. the oxygen gas formed upon decomposition creates a combustion hazard during an automobile accident.

  C. the potassium chloride formed upon decomposition is a dense solid.

  D. the oxygen gas formed upon decomposition leads to rapid expansion of the reaction mixture.

  Correct Answer: D

  D is correct because rapid expansion is a key requirement of any airbag inflatant. The airbag must inflate before the driver and passenger hit the windshield. All the other statements are true and are reasons why potassium chlorate is not suitable as an airbag inflatant.

• Question 209:

  The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteria. Researchers thus turned their attention to chemical alternatives.

  The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements. The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.

  

  Reaction 1

  The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).

  

  Reaction 2

  

  Reaction 3

  

  In order for the decomposition reaction to spread throughout the sodium azide after ignition, the H and G for Reaction 1 must be, respectively:

  A. positive, negative

  B. positive, positive

  C. negative, negative

  D. negative, positive

  Correct Answer: C

  The decomposition reactions require an activation energy. Initially, this is supplied by the electrical impulse. In order for the reaction to continue following this ignition, however, the reaction must release sufficient energy to overcome the activation barrier of subsequent decompositions. In other words, in order for the reaction to spread throughout the sample, it must produce enough heat to provide the activation energy for adjacent reactions. The reaction must be exothermic and spontaneous.

• Question 210:

  Historically, two different methods have been used to estimate the fluid pressure in capillary beds.

  Method 1 A glass pipette is inserted into the capillary. The level of blood rising in the pipette is measured and used to calculate the pressure. Alternatively, an inert fluid of density can be placed in the pipette and its height h can be measured. The pressure in the capillary is given by gh, where g is the acceleration due to gravity.

  

  Figure 1 Method 2

  The pressure can be measured indirectly in the following way. A section of gut tissue is removed from a specimen and placed on a beam balance. Blood is circulated through the tissue by a pump. The arterial pressure is then decreased. This leads to a decrease in the capillary hydrostatic pressure in the gut capillaries. The constant osmotic pressure of plasma proteins in the capillary causes absorption of fluid from the gut section which will decrease its weight. To prevent a change in the weight of the gut section, the venous pressure is increased. This tends to increase the capillary pressure, reducing the flow of fluid from the gut tissue into the capillaries. The capillary pressure is thus held constant (and the balance kept level) as the arterial pressure is decreased and the venous pressure increased. The arterial and venous pressures meet at the capillary pressure being measured.

  ( = MRT, where is the osmotic pressure, M the molarity of the solutes, R the universal gas constant, and T the temperature in Kelvin.)

  

  Figure 2

  Assume that the beam balance of Method 2 is initially level. If the arterial pressure is decreased to a lower level while everything else is held constant, which graph best represents the change in the mass of the gut following the decrease in arterial pressure?

  

  A. A

  B. B

  C. C

  D. D

  Correct Answer: C

  The passage states that if the arterial pressure is decreased fluid will leave the gut section and its mass will decrease. The mass will decrease until a certain amount of fluid is lost and a new equilibrium between hydrostatic pressure and osmotic pressure in the capillaries is reached. Choice C best shows the initially rapid decrease in mass and the gradual leveling out as a new equilibrium is reached. Choice A is incorrect because the mass of the gut will not decrease to zero. Also, the rate of mass loss will not be linear. There will be rapid mass loss followed by a gradual leveling out. Choice B is incorrect because the mass of the gut will decrease as fluid is lost from the gut section to the capillaries. Choice D is incorrect because the mass loss will occur rapidly at first, followed by a gradual leveling out as a new equilibrium is reached. Choice D indicates a gradual increase in the rate of mass loss which is the opposite of what actually happens.