Question

For the next three problems, consider 1.0 L of a solution which is 0.3 M NH4Cl and 0.2 M NH3 (Ka for NH4Cl = 5.6 x 10-10). Assume 2 significant figures in all of the given concentrations so that you should calculate all of the following pH values to two decimal places. 1. Calculate the pH of this solution. 2. Calculate the pH after 0.10 mol of HCl has been added to the original solution. Assume no volume change on addition of HCl. 3. Calculate the pH after 0.20 mol of NaOH has been added to the original buffer solution. Assume no volume change on addition of NaOH.

Answer 1

a)

pH of the solution = pKa + log[salt]/[base]

pH = -log(5.6 * 10^-10) + log(0.3/0.2)

pH = 9.4279

b)

when 0.1 mole of HCl is added then,

pH = pKa + log[salt + HCl]/[Base - HCl]

pH = -log(5.6 * 10^-10) + log((0.3 + 0.1)/(0.2 - 0.1)) ( since volume of the solution does not change)

pH = 9.8538

c)

when 0.2 mole of NaOH is added then,

pH = pKa + log[salt - NaOH]/[Base + NaOH]

pH = -log(5.6 * 10^-10) + log((0.3 - 0.2)/(0.2 + 0.2)) ( since volume of the solution does not change)

pH = 8.649