Question

1) A 17.0 mL sample of a 0.446 M aqueous hydrofluoric acid solution is titrated with a 0.314 M aqueous sodium hydroxide solution. What is the pH at the start of the titration, before any sodium hydroxide has been added?

2)When a 24.4 mL sample of a 0.304 M aqueous acetic acid solution is titrated with a 0.445 M aqueous potassium hydroxide solution, what is the pH after 25.0 mL of potassium hydroxide have been added?

Answer 1

Sol 1.

As initial conc. of hydrofluoric acid , HF = c = 0.446 M

Ka of HF = 6.6 × 10^-4  

So , initial pH = - 1/2 × log(Ka × c)

= - 1/2 × log(6.6 × 10^-4 × 0.446 )

=   1.76

Sol 2.

initial moles of acetic acid , CH3COOH

= 0.304 × 24.4 / 1000 = 0.00742 mol

initial moles of potassium  hydroxide , KOH

= initial moles of OH-

= 0.445 × 25 / 1000

= 0.01112 mol

So , moles of OH- left = initial moles of OH- - initial moles of CH3COOH

= 0.01112 - 0.00742 = 0.0037 mol

As Total Volume = 24.4 + 25 = 49.4 mL = 0.0494 L

So , Conc. of OH- left = [OH-]

= 0.0037 / 0.0494 = 0.07489 M

Therefore , pOH = - log[OH-] = - log(0.07489) = 1.12

and , pH = 14 - pOH = 14 - 1.12 =   12.88