Buffers (College Board AP® Chemistry): Exam Questions

  N₂ (g) + O₂ (g)  2NO (g)

At high temperatures, N₂ (g) and O₂ (g) can react to produce nitrogen monoxide, NO (g), as represented  by the equation above.

Write the expression for the equilibrium constant, K_P , for the forward reaction.

A student injects N₂ (g) and O₂ (g) into a previously evacuated, rigid vessel and raises the temperature of the vessel to 2000° At this temperature the initial partial pressures of N₂ (g) and O₂ (g) are 6.01 atm and 1.61 atm, respectively. The system is allowed to reach equilibrium. The partial pressure of NO (g)  at equilibrium is 0.122 atm. Calculate the value of K_P.

Nitrogen monoxide, NO (g), can undergo further reactions to produce acids such as HNO₂, a weak acid with a K_a of 4.0 x 10⁻⁴ and a pK_a of 3.40.

A student is asked to make a buffer solution with a pH of 3.40 by using 0.100 M HNO₂ (aq) and 0.100 M NaOH (aq).

i) Explain why the addition of 0.100 M NaOH (aq)  to 0.100 M HNO₂ (aq) can result in the formation of a buffer solution. Include the net ionic equation for the reaction that occurs when the student adds the NaOH (aq)  to the HNO₂ (aq).

ii) Determine the volume, in mL, of 0.100 M NaOH (aq)  the student should add to 100. mL of 0.100 M HNO₂ (aq) to make a buffer solution with a pH of 3.40. Justify your answer.

A second student makes a buffer by dissolving 0.100 mol of NaNO₂ (s) in 100. mL of 1.00 M HNO₂ (aq). Which is more resistant to changes in pH when a strong acid or a strong base is added, the buffer made by the second student or the buffer made by the first student in part (c)? Justify your answer.

A new buffer is made using HNO₂(aq) as one of the A particulate representation of a small representative portion of the buffer solution is shown below. (Cations and water molecules are not shown.)

Is the pH of the buffer represented in the diagram greater than, less than, or equal to 3.40 ? Justify your answer.