pH problem?

Question

Calculate the pH of a solution formed by mixing 152.0 mL of 0.100 M NaF and 209.0 mL of 0.025 M HCl.

Answer 1

HF is a weak acid with pKa=3.15
So when you mix NaF and HCl, some HF will form. Let's find out how much

mole NaF= M1V1= 0.1*0.152=1.52*10^-2
mole HCl = M2V2= 0.025*0.209 =5.2*10^-3

The stoichiometry of the reaction
NaF + HCl -> HF + NaCl
is 1:1:1
so NaF is in excess thus
mole NaF reacting = mole HCl= 5.2*10^-3
mole HF forming = mole HCl= 5.2*10^-3
mole NaF remaining= mole NaF- mole HCl= 1.52*10^-2 -5.2*10^-3 = 0.01

use the Henderson-Hasselbalch equation

pH=pKa+log[F-]/[HF]= pKa+ log ((mole F- /V) /(mole HF/V)) =
=pKa+log (mole F-/mole HF)=
=3.15+ log (0.01/0.0052) = 3.43