Borax (Na2B4O7.10H2O, 382 g/mol) reacts with acid as B4O7^2- + 2H+ + 5H2O -> 4H3BO3. What volume of 0.500 M HCl is needed to react completely with 1.91 g of borax?

JEE Advanced levelSingle correct optionDifficulty 3 of 3eq145

Borax ( $Na X_{2} B X_{4} O X_{7} \cdot 10 H X_{2} O$ , 382 g/mol) reacts with acid as $B X_{4} O X_{7} X^{2 -} + 2 H X^{+} + 5 H X_{2} O 4 H X_{3} BO X_{3}$ . What volume of 0.500 M HCl is needed to react completely with 1.91 g of borax?

Moles of borax = 1.91/382 = 5.00 mmol. Each

B X_{4} O X_{7} X^{2 -}

takes 2

H X^{+}

(n = 2), so HCl needed = 10.0 mmol. Volume = 10.0/0.500 = 20.0 mL.