Here is a $0.200 M$ solution of$H_3C_6H_5O_7$
(citric acid, $K_a$ $=$ $3.5 \cdot 10^{-4}$)
Hydrolysis equilibrium:
$H_3C_6H_5O_7$ $+$ $H_2O$
$H_3O^+$ $+$ $H_2C_6H_5O_7^-$
Now substitute the equilibrium concentrations in the expression of the acid constant $ K_a $! Which of the following is the correct substitution ?
a) $3.5 \cdot 10^{-4}$ $=$ $\frac{x(0.200-x)}{ x^2}$ b) $3.5 \cdot 10^{-4}$ $=$ $\frac{0.200-x }{ x^2}$ c) $3.5 \cdot 10^{-4}$ $=$ $\frac{x^2 }{ 0.200-x}$ d) $3.5 \cdot 10^{-4}$ $=$ $\frac{x^2 }{ x(0.200-x)}$
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Answer c) is correct, $3.5 \cdot 10^{-4} = \frac{x^2 }{ 0.200-x}$