Question

A solution has 0.100 M HC7H5O2 and 0.200 M Ca(C7H5O2)2. The solution volume is 5.00 L. What is the pH of the solution after 2.00 g NaOH is added? The Ka for HC7H5O2 is 6.3 × 10-5.

Answer 1

The initial moles of the buffer components are calculated:

n HA = M * V = 0.1 M * 5 L = 0.5 mol

n A- = 2 * M * V = 2 * 0.2 M * 5 L = 2 mol

The added moles of NaOH are calculated:

n NaOH = g / MM = 2 g / 40 g / mol = 0.05 mol

NaOH reacts with the acid (decreasing it) and forms salt ion (increasing it).

The pH of the solution is calculated:

pH = - log Ka + log (n A- / n HA) = - log (6.3x10 ^ -5) + log (2 + 0.05 / 0.5 - 0.05) = 4.86

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