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.
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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