MCAT General Chemistry Practice Test 8: Acids and Bases

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Blood pH homeostasis is the result of several systems operating within the bloodstream. They collectively maintain blood plasma pH at 7.4, since a drop in pH below 6.8 or rise above 7.8 may result in death.

One component of this system is the enzyme carbonic anhydrase, which catalyzes the conversion of CO2 in the blood to carbonic acid. Carbonic acid, in turn, ionizes to form the carbonic acid-bicarbonate buffer. The interdependence of these reactions is shown below in Equation 1.

CO2(g) + H2O(l) H2CO3(aq) H+(aq) + HCO-3(aq)

Equation 1

Uncatalyzed blood CO2 and H+ can be found binding to hemoglobin after oxygen liberation in peripheral tissues. As the blood reaches the lungs these actions reverse themselves; hemoglobin binds with oxygen, releasing the CO2 and H+ ions. The exchange of gases between the lungs and the blood and other tissues in the body is a physiologic process known as respiration.

A second system, the phosphoric acid buffer, plays a minor role compared to the carbonic acid-bicarbonate buffer. Phosphoric acid (H3PO4), the primary reactant of this system, is a triprotic acid, which can ionize three protons. This three-step process is illustrated below:

1. Carbonic acid is best described as:

  • A. amphoteric.
  • B. polyprotic.
  • C. a strong acid.
  • D. the conjugate acid for CO2.

2. If CO2 gas is bubbled continuously in a beaker of water to form carbonic acid, which of the following would be true?

I. Addition of carbonic anhydrase will increase the Keq of the reaction.

II. Carbonic acid will increase in concentration until Keq is reached.

III. Addition of bicarbonate will increase the pH of the system.

  • A. I only
  • B. II only
  • C. III only
  • D. II and III

3. All of the following statements are true regarding human respiration EXCEPT:

  • A. when a person's breathing is hampered by conditions such as asthma or emphysema, the blood [H+] increases.
  • B. exercise stimulates deeper and more rapid breathing, which increases blood plasma pH.
  • C. slow, shallow breathing allows CO2 to accumulate in the blood.
  • D. hyperventilation can result in the loss of too much CO2, causing the accumulation of bicarbonate ions.

4. In the dissociation of phosphoric acid, the trend Ka1 > Ka2 > Ka3 is predominantly due to:

  • A. an equilibrium shift towards the reactants side in Reactions 2 and 3 due to the release of H+ in Reaction 1.
  • B. a smaller radius in the H+ liberated in Reaction 1 compared to that in Reactions 2 and 3.
  • C. a slower rate of reaction after subsequent ionizations.
  • D. an increasing influence of the anion after subsequent ionizations.

5. What is the relationship between the Ka1 value for phosphoric acid and the Kb1 value for dihydrogen phosphate?

  • A. Ka1 and Kb1 are inversely related through the dissociation constant for water, Kw.
  • B. Ka1 and Kb1 are directly related through the dissociation constant for water, Kw.
  • C. The Ka1 is less than the Kb1.
  • D. There is no relationship between Ka1 and Kb1.

6. What would be the pH of a solution made from combining 50 mL of 0.030 M acetic acid (Ka = 1.8 × 10- 5) and 10 mL of 0.15 M sodium acetate?

  • A. pH = 1.6
  • B. pH = 2.5
  • C. pH = 3.3
  • D. pH = 4.7