Question

The equivalent mass of ${H_3}B{O_3}$ in its reaction with NaOH to form $N{a_2}{B_4}{O_7}$ is equal to:
                  (A) Molar mass/4
                   (B) Molar mass/3
                   (C) Molar mass/2
                   (D) Molar mass

Answer 1

Hint: Boric acid (${H_3}B{O_3}$) is a weak, monobasic Lewis acid of Boron. It is not a protonic acid but accepts electrons from hydroxyl ions and acts as Lewis acid.

Formula used:
-Equivalent weight = molar mass / acidity or basicity (1)

Complete step by step answer:
-${H_3}B{O_3}$ is a weak monobasic Lewis acid. It is a weakly acidic hydrate of boric oxide.
-A monobasic acid is an acid which has only 1 acidic H atom in its molecule. Example: HCL, $HN{O_3}$, $C{H_3}COOH$, etc.
Although boric acid (${H_3}B{O_3}$) has 3 hydroxyl groups but still it behaves as a monobasic Lewis acid and not a tribasic acid. This is so because boric acid does not behave as a proton donor instead it accepts a pair of electrons from $O{H^ - }$ ions. Each molecule of boric acid produces one ${H_3}{O^ + }$ ion.
            $B{(OH)_3} + 2{H_2}O \to {\left[ {B{{(OH)}_4}} \right]^ - } + {H_3}{O^ + }$
-Being weakly acidic ${H_3}B{O_3}$reacts with NaOH giving a double displacement reaction as shown below:
                          $4{H_3}B{O_3} + 2NaOH \to 7{H_2}O + N{a_2}{B_4}{O_7}$
-Since boric acid is a monobasic acid its basicity would be =1.
Now we can calculate the equivalent weight of boric acid using equation (1):
                                       Equivalent weight = molar mass / acidity or basicity
                                                                        = molar mass / 1
                                                                        = molar mass
-It has mild antiviral, antifungal and antiseptic properties. It is also used as an insecticide, neutron absorber, etc. It also treats boron deficiency in plants and is used to preserve rice and grains.

So, the correct option is: (D) Molar mass

Note: Even though boric acid has 3 hydroxyl groups it does not act like a tribasic acid. It behaves as a monobasic Lewis acid and so its basicity will be = 1.