Question

a) The pKa of HF is 3.17. Calculate the pH of a 1.00 L solution that is 1.00 M HF and 1.50 M NaF.

b) What is the pH of this solution after addition of 50.0 mL of 10.0 M HCl?

Answer 1

a)

use:

pH = pKa + log {[conjugate base]/[acid]}

= 3.17+ log {1.5/1}

= 3.346

Answer: 3.35

b)

mol of HCl added = 10.0M *0.05 L = 0.5 mol

F- will react with H+ to form HF

Before Reaction:

mol of F- = 1.5 M *1.0 L

mol of F- = 1.5 mol

mol of HF = 1.0 M *1.0 L

mol of HF = 1 mol

after reaction,

mol of F- = mol present initially - mol added

mol of F- = (1.5 - 0.5) mol

mol of F- = 1 mol

mol of HF = mol present initially + mol added

mol of HF = (1 + 0.5) mol

mol of HF = 1.5 mol

since volume is both in numerator and denominator, we can use mol instead of concentration

use:

pH = pKa + log {[conjugate base]/[acid]}

= 3.17+ log {1/1.5}

= 2.994

Answer: 2.99