Boric Acid Ion Release Explained
$H_3BO_3$, known as boric acid, has a specific behavior when dissolved in water.
Nature of Boric Acid: It acts as a weak Lewis acid. Instead of donating a proton ($H^+$) directly like Brønsted-Lowry acids, it accepts a hydroxide ion ($OH^-$) from water ($H_2O$).
Reaction Mechanism
The dissociation reaction in water is represented as:
$ H_3BO_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+ $
This equation shows that one molecule of $H_3BO_3$ reacts with water to yield one borate ion ($[B(OH)_4]^-$) and one proton ($H^+$).
Proton Release Analysis
Boric acid is a very weak acid, indicated by its small acid dissociation constant ($K_a \approx 5.8 \times 10^{-10}$). Consequently, the extent of dissociation is low.
This means that, on average, only a fraction of the $H_3BO_3$ molecules in solution actually release an $H^+$ ion. The number of $H^+$ ions released per molecule is therefore less than one.
The description "The correct value lies between 1 and 1" signifies that while the reaction mechanism involves the potential release of one proton (characteristic of a monoprotic acid), the actual average number released per molecule is less than 1 due to weak dissociation.
Important Questions from Chemical Biology {p}K_a Henderson Hasselbalch
- The equilibrium dissociation constant of acetic acid is $1.74 \times 10^{-5}$ $M$. The $pK_a$ of acetic acid (rounded off to one decimal place) is ________.
- The ionic product of water at $40$ °C is $2.92 \times 10^{-14}$ $M^2$. The pH of water at $40$ °C is ________ (rounded off to 2 decimal places).
- In a lactic acid solution at pH $4.8$, the concentrations of lactic acid and lactate are $0.01$ M and $0.087$ M, respectively. The calculated pKa of lactic acid is ________ (Round off to one decimal place)
An aqueous solution of aspirin (HA) is prepared at pH 7.4. The ratio of concentration of $A^-$ and HA at equilibrium is ________ (round off to the nearest integer).
Given: $K_a$ of aspirin is $3.98 \times 10^{-4}$
- If 50 mL of 0.02 M HCl is added to 950 mL of $H_2O$, then the pH of the final solution will be _________
