Admission Tests Admission Tests Certifications PCAT Questions & Answers

• Question 161:

  Under what conditions do gases exhibit ideal behavior?

  A. Low pressures and low temperatures

  B. Low pressures and high temperatures

  C. High pressures and high temperatures

  D. High pressures and low temperatures

  Correct Answer: B

  At high pressures, gas particles are forced close together; therefore, intermolecular forces between the particles become significant, and the gas deviates from ideality. On the other hand, at low pressures, the gas particles are far apart from each other, so they cannot interact, and intermolecular forces are insignificant. Ideal gas behavior is also observed at high temperatures because the gas particles possess a large amount of kinetic energy ?large enough for the particles to overcome their attractive forces towards each other. As the temperature is lowered, the kinetic energy of the particles decreases, so the particles do not have enough energy to break away from each other, and attractive forces become significant.

• Question 162:

  200 mL of an ideal gas is placed in a piston and is held at a pressure of 500 torr. If the temperature is held constant and the pressure is increased to 650 torr, what is the new volume of the gas?

  A. 75 mL

  B. 154 mL

  C. 220 mL

  D. 300 mL

  Correct Answer: B

  When the pressure of a gas increases at constant temperature, the volume decreases. Therefore, before calculation, choices [220 mL] and [300 mL] can be eliminated. Since PV / nT is constant, and the temperature and number of moles of gas are kept constant, we can derive the relationship, P1V1 = P2V2. Rearranging this equation to find the final volume of gas, we get: V2 = P1V1/P2. By substituting numbers into this equation, we get: V2 = (500 ?200) / 650, or 154 mL, choice B. Another way to arrive at the correct answer is to notice that the original pressure (500 mL) divided by the new pressure (650 mL) is approximately 3/4, so the new volume is around 3/4 of 200 mL or 150 mL.

• Question 163:

  Large unreactive molecules such as di-isooctyl terephthalate, used to add flexibility to polymers, are called:

  A. adjuvants

  B. block copolymers

  C. stabilizers

  D. plasticizers

  Correct Answer: D

  Plasticizers are incorporated into the physical structure of a polymer and act to keep the polymer molecules separated. This provides flexibility or "plasticity" to the polymer. Stabilizers are compounds added to prevent destructive reactions such as oxidation from occurring. Adjuvants are typically unreactive materials added to a polymer in order to impart certain properties such as color. Block copolymers are polymers that are composed of a long section, or "block", of one type of monomer and then joined to another block composed of a different type of monomer.

• Question 164:

  Polymers are formed by:

  A. successive addition reactions

  B. successive condensation reactions

  C. cross-linking between molecules

  D. all of the above

  Correct Answer: D

  Polyethylene and similar thermoplastic polymers are formed by addition reactions. Polyesters and polyamides such as nylon are formed by condensation reactions. Molecules with more than one functional reactive site can polymerize in a three-dimensional crosslinked manner instead of a strictly linear manner.

• Question 165:

  Separation of compounds by distillation is more efficient when:

  A. carried out under reduced pressure

  B. more condensation-vaporization cycles occur

  C. carried out quickly

  D. the condenser is very cold

  Correct Answer: B

  Separation of compounds in distillation occurs because of their differential rates of evaporation and condensation equilibria. The more often the vapors in the apparatus can condense and evaporate, the more distinct the separation of the components will be. The more quickly a distillation process is carried out, the less distinct the separation of the components will be, making the process less efficient. A very cold condenser temperature has the effect of forcing the vapor through the system before it can condense and evaporate again, decreasing both the separation of the components and the efficiency of the process. Reduced pressure decreases the temperatures at which the components will distill, without affecting the separation behavior.

• Question 166:

  What product results from treating ethanol with pyridinium chlorochromate (PCC)?

  A. acetaldehyde

  B. formaldehyde

  C. acetone

  D. acetic acid

  Correct Answer: A

  PCC is a mild oxidizing reagent used to oxidize primary alcohols to aldehydes. Ethanol, a primary alcohol, treated with PCC results in a 2 carbon aldehyde (because of the 2 carbons initially present in ethanol) called ethanal, or acetaldehyde.

• Question 167:

  A 50mL solution of HCl is diluted to 250mL at 10M. What was the initial concentration of the HCl?

  A. 500M

  B. 50M

  C. 5M

  D. 0.5M

  Correct Answer: B

  The product of the initial concentration and the initial volume is equal to the product of the final concentration and the final volume of the solution. This can be remembered as: c1 ?v1 = c2 ?v2 Substitute the known quantities and solve for the unknown: c1 : c1 ?50mL = 10M ?250mL ?gt; c1 = 50M

• Question 168:

  Which molecule is nonpolar?

  A. carbon monoxide

  B. acetone

  C. carbon tetrachloride

  D. ethanol

  Correct Answer: C

  Carbon tetrachloride is the only nonpolar compound because it is the only compound listed in which all of

  the electrons are shared equally across the four same bonds; each chlorine atom bonds with the carbon

  atom with the same force.

  Consequently, carbon tetrachloride exhibits a tetrahedral structure with geometric symmetry.

• Question 169:

  The viscosity of a liquid:

  A. increases with decreasing temperature

  B. increases with increasing temperature

  C. decreases with decreasing temperature

  D. is independent of temperature

  Correct Answer: A

  A liquid's viscosity, or its resistance to flow, changes with a change in temperature. As a liquid's temperature increases, bonds within it break and the molecules in the liquid can move more freely and easily, decreasing its viscosity. Likewise, if the temperature decreases, molecules have more of an opportunity to form bonds which then decreases the liquid's fluidity, increasing its viscosity.

• Question 170:

  Which of the following reactions creates an alkene?

  A. addition

  B. substitution

  C. hydration

  D. dehydration

  Correct Answer: D

  Dehydration involves the loss of an H2O molecule from the reacting molecule. Recall that the general structure of alcohols is: CNH2n + 1OH. The removal of H2O from an alkyl group results in a molecule of the general form: CNH2nOH, also known as an alkene.