Question

What is the equilibrium constant of citric acid?

Answer 1

Hint : The molecular formula of citric acid is ${C_6}{H_8}{O_7}$. It is a weak organic acid. It is commonly found in citrus fruits like lemon. It comes under the category of triprotic acids. Citric acid has three carbon atoms in its structure and also three carboxylic acid groups and one hydroxyl group.

Complete Step By Step Answer:
Citric acid is a polyprotic acid. Polyprotic acids contain more than one acidic hydrogen that can react with water and produce hydronium ions as a result. Citric acid is a triprotic acid as it has three acidic hydrogen atoms in its structure as you can observe below:
The following steps occur when citric acid is placed in water:
${C_6}{H_8}{O_7} + {H_2}O \rightleftharpoons {C_6}{H_7}O_7^ - + H_3^ + O{\text{ }}.........{\text{(1)}}$
${C_6}{H_7}O_7^ - + {H_2}O \rightleftharpoons {C_6}{H_6}O_7^{2 - } + H_3^ + O{\text{ }}..........{\text{(2)}}$
${C_6}{H_6}O_7^{2 - } + {H_2}O \rightleftharpoons {C_6}{H_5}O_7^{3 - } + H_3^ + O{\text{ }}..........{\text{(3)}}$
We will get different values of acid dissociation constants for each step. So,
Step (1): ${{\text{K}}_{a1}} = 7.5 \times {10^{ - 4}}$
Step (2): ${{\text{K}}_{a2}} = 1.7 \times {10^{ - 5}}$
Step (3): ${{\text{K}}_{a3}} = 4.0 \times {10^{ - 7}}$

Note :
Observe that all the three dissociation constants are less than one. This proves that citric acid is a weak acid. The dissociation constant for the third step is very very less. This means the number of molecules that undergo ionization in this step are almost zero. Citric acid is a weak acid and has a $pH$between $3 - 6$. The main purpose of acid dissociation constant is to distinguish between strong acids and weak acids. Citric acid is also used as a $pH$adjuster. It alters the $pH$of a substance to improve the stability of the substance. The dissociation constant is called acid ionization constant for aqueous solution of a weak acid.