What is the pH?

Question

A 175.0 mL sample of 0.20 M HF is titrated with 0.10 M CsOH. Determine the pH of the solution after the addition of 700.0 mL of CsOH. The K_a of HF is 6.8 × 10^-4.

Answer 1

The reaction is:

HF + CsOH ==> H2O + CsF

mols HF present = 0.1750 L x 0.20 mol/L = 0.035 mols HF

mols CsOH present = 0.700 L x 0.10 mol/L = 0.070 mols CsOH

Since they react in a 1:1 mol ratio, all the HF will react leaving 0.035 mols CsOH

The final volume = 175 ml + 700 ml = 875 mls = 0.875 L

Final [OH^-] = 0.035 mols / 0.875 L = 0.04 M OH^-

pOH = -log [OH-] = -log 0,04 = 1.398

pH = 14 - 1.398

pH = 12.6

The Ka of HF is not needed in this problem since there is no HF remaining after the reaction with CsOH.