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  4. pH of a buffer solution decreases by 0.02 units when 0.12 g of acetic acid is added to 250 mL of a buffer solution of acetic acid and potassium acetate at 27° C. The buffer capacity of the solution is

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Q. $pH$ of a buffer solution decreases by $0.02$ units when $0.12\,g$ of acetic acid is added to $250\,mL$ of a buffer solution of acetic acid and potassium acetate at $27^{\circ} C$. The buffer capacity of the solution is

EAMCETEAMCET 2009

Solution:

Buffer capacity, $\beta=\frac{d C_{ HA }}{d_{ pH }}$,

where, $d C_{ HA }=$ no. of moles of acid added per litre

$\therefore d_{ pH } =$ change in $pH$

$d C_{ HA } =\frac{\text { moles of acetic acid }}{\text { volume }} $

$=\frac{0.12 / 60}{250 / 1000}=\frac{1}{125} $

$\therefore \beta =\frac{1 / 125}{0.02}=\frac{1}{2.5}=0.4$