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 (H.Cit) in the lemon juice?

Question

Lemon juice normally has a $\mathrm{pH}$ 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 (H.Cit) in the lemon juice?

(Assume that only the first hydrogen of citric acid is important). (H.Cit $\rightleftharpoons \mathrm{H}^{+}+\mathrm{Cit}^{-} ; K_a=8.4 \times 10^{-4} \mathrm{~mol} \mathrm{lit}^{-1}$ )

(A) $8.4 \times 10^{-4} \mathrm{M}$

(B) $4.2 \times 10^{-4} \mathrm{M}$

(C) $16.8 \times 10^{-4} \mathrm{M}$

(D) $12.0 \times 10^{-2} \mathrm{M}$

Answer 1

Correct Option : (D).

SOLUTION —

$\begin{array}{l}\ K_a=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{cit}^{-}\right]}{[\mathrm{H} . \mathrm{cit}]} \\8.4 \times 10^{-4}=\frac{\left[10^{-2}\right]\left[10^{-2}\right]}{[\text { H. cit }]} \\8.4 \times 10^{-4}=\frac{10^{-4}}{[\text { H. cit }]} \\\text { [H. cit] }=\frac{10^{-4}}{8.4 \times 10^{-4}}=0.119 \\=11.9 \times 10^{-2} \mathrm{M} \simeq 12 \times 10^{-2}\mathrm{M} \\\end{array}$