Calculate the ph of a buffer solution that is 0.250 M HCN and 0.170 M KCN. Ka=4.9x10^-10 for HCN. A) add 0.099 mol NaOH to calculate the new ph B) add 10 ml of 1.00 M HCl and calculate the new ph

Answer 1

A. The reaction between HCN + NaOH ===> NaCN + H2O, so for every mole of NaOH added reduces the moles of HCN and increases the moles of NaCN by the same amount. The ICE table is

HCN NaCN

Initial 0.250 0.170

Change -0.099 +0.099

Equilibrium 0.151 + 0.269

The equation for a buffer of a weak acid and salt of the acid is

pH = pKa + log [salt] / [acid] = 9.31 + log 0.269 / 0.151 = 9.56

B. Adding 10 mL of 1.0 M HCl is adding 0.01 moles of HCl. We have to assume that the volume of the overall solution doesn't change significantly.

HCN NaCN

Initial 0.151 0.269

Change +0.01 -0.01

Equilibrium +0.161 0.259

pH = 9.31 + log 0.259/0.161 = 9.52