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  4. A student needs to prepare a buffer solution of propanoic acid and its sodium salt with pH 4. The ratio of ([ CH 3 CH 2 COO -]/[ CH 3 CH 2 COOH ]) required to make buffer is .......... Given: K a ( CH 3 CH 2 COOH )=1.3 × 10-5

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Q. A student needs to prepare a buffer solution of propanoic acid and its sodium salt with $pH 4$. The ratio of $\frac{\left[ CH _{3} CH _{2} COO ^{-}\right]}{\left[ CH _{3} CH _{2} COOH \right]}$ required to make buffer is ..........
Given : $K _{ a }\left( CH _{3} CH _{2} COOH \right)=1.3 \times 10^{-5}$

JEE MainJEE Main 2022Equilibrium

Solution:

$pH = pK _{ a }+\log \frac{[\text { Salt }]}{[\text { Acid }]}$
$4=5-\log 1.3+\log \frac{\left[ CH _{3} CH _{2} COO ^{-}\right]}{\left[ CH _{3} CH _{2} COOH \right]}$
$\log \frac{\left[ CH _{3} CH _{2} COO ^{-}\right]}{\left[ CH _{3} CH _{2} COOH \right]}=\log 1.3-1=\log \frac{1.3}{10}$
$\frac{\left[ CH _{3} CH _{2} COO ^{-}\right]}{\left[ CH _{3} CH _{2} COOH \right]}=0.13$