Question

What volume of $0.1 \,M\, HCOONa$ solution should be added to $50 \,ml$ of $0.05\, M$ formic acid to produce a buffer solution of $pH =4.0, p K_{ a }$ of formic acid $=3.7 $ ?

• A. 50 ml
• B. 40 ml
• C. 30 ml
• D. 60 ml

Answer 1

Answer: (A)

Let $V$ ml of $0.1\, M\, HCOONa$ be mixed to $50 \,ml$ of $0.05\,M \,HCOOH$

in mixture

$[ HCOONa ]=\frac{0.1 \times V}{(V+50)}$

$[ HCOOH ]=\frac{50 \times 0.05}{(V+50)}$

$\therefore pH =-\log K_{ a }+\log \frac{[\text { Salt }]}{[\text { Acid }]}$

$ 4.0 =3.7+\log \frac{(0.1 \times V) /(V+50)}{2.5 /(V+50)}$

$ V=50 \,ml $