Buffers for AP Chemistry

Practice problems for these concepts can be found at:

• Equilibrium Multiple Choice Review Questions for AP Chemistry
• Equilibrium Free-Response Questions for AP Chemistry

Buffers are solutions that resist a change in pH when an acid or base is added to them. The most common type of buffer is a mixture of a weak acid and its conjugate base. The weak acid will neutralize any base added, and the weak base of the buffer will neutralize any acid added to the solution. The hydronium ion concentration of a buffer can be calculated using an equation derived from the K_a expression:

Taking the negative log of both sides yields the Henderson–Hasselbalch equation, which can be used to calculate the pH of a buffer:

The weak base K_b expression can also be used giving:

These equations allow us to calculate the pH or pOH of the buffer solution knowing K of the weak acid or base and the concentrations of the conjugate weak acid and its conjugate base. Also, if the desired pH is known, along with K, the ratio of base to acid can be calculated. The more concentrated these species are, the more acid or base can be neutralized and the less the change in buffer pH. This is a measure of the buffer capacity, the ability to resist a change in pH.

Let's calculate the pH of a buffer. What is the pH of a solution containing 2.00 mol of ammonia and 3.00 mol of ammonium chloride in a volume of 1.00 L?

There are two ways to solve this problem.

Assume x small

Alternate solution:

Practice problems for these concepts can be found at:

• Equilibrium Multiple Choice Review Questions for AP Chemistry
• Equilibrium Free-Response Questions for AP Chemistry