Question

Lemon juice normally has a pH of $2$ If all the acid in the lemon juice is citric acid and there are no citrate salts present, then what will be the citric acid concentration $[HCit]$ in the lemon juice ? (Assume that only the first hydrogen of citric acid is important)
$HCit \rightleftharpoons H ^{+}+ Cit ^{-}, K_{ a }=8.4 \times 10^{-4} \,mol\, L ^{-1}$

• A. $8.4 \times 10^{-4} \,M$
• B. $4.2 \times 10^{-4} \,M$
• C. $16.8 \times 10^{-4} \,M$
• D. $12.0 \times 10^{-2} \,M$

Answer 1

Answer: (D)

$\left[ H ^{+}\right]=\sqrt{K_{ a } C}, pH =2$,

$\therefore \left[ H ^{+}\right]=10^{-2}\,M$

$10^{-2}=\sqrt{K_{ a } C}$

$10^{-4}=8.4 \times 10^{-4} \times C$

$\therefore C=11.9 \times 10^{-2} \,m$