AP Chemistry Equilibrium Test Potential Questions

When a strong acid is titrated with a strong base using phenolphthalein as an indicator, the color changes abruptly at the endpoint of the titration and can be switched back and forth by the addition of only one drop of acid or base. The reason for the abruptness of this color change is that

*(A) a large change in pH occurs near the endpoint of the titration*

When 200. mL of 2.0 M NaOH(aq) is added to 500. mL of 1.0 M HCl(aq), the pH of the resulting mixture is closest to

*(A) 1.0*

2 H2S(g) + CH4(g) <--> CS2(g) + 4 H2(g) Kc = 3.4 x 10^-4 A 0.10 mol sample of each of the four species in the reaction represented above is injected into a rigid, previously evacuated 1.0 L container. Which of the following species will have the highest concentration when the system reaches equilibrium?

*(A) H2S(g)*

HX(aq) + Y-(aq) <--> HY(aq) + X-(aq) Keq > 1 Based on the information given above, which of the following is the strongest acid?

*(A) HX(aq)*

Ni2+(aq) + 6 NH3(aq) -> [Ni(NH3)6] 2+(aq) The reaction represented above is best classified as

*(A) a Lewis acid-base reaction*

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated. After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below. Which of the following is the balanced net-ionic equation for the reaction between Li(s) and water?

*(B) 2 Li(s) + 2 H2O(l) → 2 Li+(aq) + 2 OH−(aq) + H2(g)*

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated. After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below. On the basis of the pH curve, the value of the acid is closest to

*(B) 5*

HC3H5O3(aq) -- Ka: 8.3 × 10−4 CH3NH3+(aq) -- Ka: 2.3 × 10−11 The acid-dissociation constants of HC3H5O3(aq) and CH3NH3+(aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 3 ?

*(B) A mixture of 100. mL of 0.1 M HC3H5O3 and 50. mL of 0.1 M NaOH*

Which of the following accounts for the observation that the pH of pure water at 37°C is 6.8?

*(B) At 37C the autoionization constant for water, Kw, is larger than it is at 25C.* .

The pH of a 0.01 M HNO2(aq) solution is in which of the following ranges? (For HNO2(aq), Ka = 4.0 × 10^-4)

*(B) Between 2 and 3*

NH3(aq) + HCl(aq) <--> NH4+(aq) + Cl -(aq) The Brønsted-Lowry bases in the reaction represented above are

*(B) NH3(aq) and Cl -(aq)* (

Solution -- Acid -- Ka 1 -- CH3CO2H -- 1.75^10-5 2 -- CF3CO2H -- 1.0^0 Acid-dissociation constants of two acids are listed in the table above. A 20. mL sample of a 0.10 M solution of each acid is titrated to the equivalence point with 20. mL of 0.10 M NaOH. Which of the following is a true statement about the pH of the solutions at the equivalence point?

*(B) Solution 1 has a higher pH at the equivalence point because

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated. After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below. What will be the effect on the amount of gas produced if the experiment is repeated using 0.35 g of K(s) instead of 0.35 g of Li(s)?

*(B) Some gas will be produced but less than the amount of gas produced with Li(s).*

HX(aq) + Y−(aq) <--> HY(aq) + X−(aq) Keq > 1 A solution of a salt of a weak acid HY of another weak acid HX. Based on the information given above, which of the following species is the strongest base?

*(B) Y−(aq)*

CO(g) + 2 H2(g) <--> CH3OH(g) DH < 0 The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5. A 1.0 mol sample of CO(g) and a 1.0 mol sample of H2(g) are pumped into a rigid, previously evacuated 2.0 L reaction vessel at 483 K. Which of the following is true at equilibrium?

*(B) [H2] < [CO]*

FeF2(s) <--> Fe2+(aq) + 2 F-(aq) K1 = 2 × 10^−6 F−(aq) + H+(aq) <--> HF(aq) K2 = 1 × 10^3 FeF2(s) + 2 H+(aq) <--> Fe2+(aq) + 2 HF(aq) K3 = ? On the basis of the information above, the dissolution of FeF2(s) in acidic solution is

*(B) thermodynamically favorable, because K3 > 1*

A student performs an acid-base titration and plots the experimental results in the graph above. Which of the following statements best explains the experimental findings? (S shaped curve, vertical asymptote from ph=6 to ph=12)

*(C) A weak acid was titrated with a strong base, as evidenced by the equivalence point at pH > 7.*

A solution containing HCl and the weak acid HClO2 has a pH of 2.4. Enough KOH(aq) is added to the solution to increase the pH to 10.5. The amount of which of the following species increases as the KOH(aq) is added?

*(C) ClO2-(aq)*

A solution is prepared by adding 100 mL of 1.0 M HC2H3O2(aq) to 100 mL of 1.0 M NaC2H3O2(aq). The solution is stirred and its pH is measured to be 4.73. After 3 drops of 1.0 M HCl are added to the solution, the pH of the solution is measured and is still 4.73. Which of the following equations represents the chemical reaction that accounts for the fact that acid was added but there was no detectable change in pH?

*(C) H3O+(aq) + C2H3O2−(aq) -> HC2H3O2(aq) + H2O(l)*

CO(g) + 2 H2(g) <--> CH3OH(g) DH < 0 The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5. Which of the following explains the effect on the equilibrium constant, Kc , when the temperature of the reaction system is increased to 650 K?

*(C) Kc will decrease because the reaction is exothermic.*

CO(g) + 2 H2(g) <--> CH3OH(g) DH < 0 The synthesis of CH3OH(g) from CO(g) and H2(g) is represented by the equation above. The value of Kc for the reaction at 483 K is 14.5. Which of the following statements is true about bond energies in this reaction?

*(C) The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed.*

PCl3(g) + Cl2(g) <--> PCl5(g) Kc = 6.5 At a certain point in time, a 1.00 L rigid reaction vessel contains 1.5 mol of PCl3(g), 1.0 mol of Cl2(g), and 2.5 mol of PCl5(g). Which of the following describes how the measured pressure in the reaction vessel will change and why it will change that way as the reaction system approaches equilibrium at constant temperature?

*(C) The pressure will decrease because Q < Kc .*

A 0.35 g sample of Li(s) is placed in an Erlenmeyer flask containing 100 mL of water at 25°C. A balloon is placed over the mouth of the flask to collect the hydrogen gas that is generated. After all of the Li(s) has reacted with H2O(l), the solution in the flask is added to a clean, dry buret and used to titrate an aqueous solution of a monoprotic acid. The pH curve for this titration is shown in the diagram below. Which of the following changes will most likely increase the rate of reaction between Li(s) and water?

*(C) Using a 0.35 g sample of Li(s) cut into small pieces*

CO(g) + H2O(g) <--> CO2(g) + H2(g) Kc = 1.5 103 A 2.0 mol sample of CO(g) and a 2.0 mol sample of H2O(g) are introduced into a previously evacuated 100. L rigid container, and the temperature is held constant as the reaction represented above reaches equilibrium. Which of the following is true at equilibrium?

*(C) [CO2] > [CO]*

An acetate buffer solution is prepared by combining 50. mL of 0.20 M acetic acid, HC2H3O2(aq), and 50. mL of 0.20 M sodium acetate, NaC2H3O2(aq). A 5.0 mL sample of 0.10 M NaOH(aq) is added to the buffer solution. Which of the following is a correct pairing of the acetate species present in greater concentration and of the pH of the solution after the NaOH(aq) is added? (The pKa of acetic acid is 4.7.)

*(D) C2H3O2 -- > 4.7*

Which of the following is a weak acid in aqueous solution?

*(D) H2S*

Acid - Concentration - pH X - 0.005 M - 2.3 Y - 2.0 M - 2.8 Z - 3.0 M - 2.8 Which of the following correctly ranks the three monoprotic acids listed in the table above from the weakest to the strongest?

*(D) Z < Y < X*

Sr 2+(aq) + F -(aq) <--> SrF +(aq) At 25°C, the equilibrium constant for the reaction represented above has a value of 1.3. At 50°C, the value of the equilibrium constant is less than 1.3. Based on this information, which of the following must be correct?

*(E) At 25°C, Δ H° for the reaction is negative.*

A 0.10 M solution of which of the following salts is most basic?

*(E) K2CO3*