20 mL of calcium hydroxide was consumed when it was reacted with 10 mL of unknown solution of . Also 20 mL standard solution of 0.5 M HCl containing 2 drops of phenolphthalein was titrated with calcium hydroxide, the mixture showed pink colour when burett

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Solution

Q.
20 mL of calcium hydroxide was consumed when it was reacted with 10 mL of unknown solution of $H_{2} {SO}_{4}$ . Also 20 mL standard solution of 0.5 M HCl containing 2 drops of phenolphthalein was titrated with calcium hydroxide, the mixture showed pink colour when burette displayed the value of 35.5 mL whereas the burette showed 25.5 mL initially.
The concentration of $H_{2} {SO}_{4}$ is ______ M. (Nearest integer) [2023]

Ans.
(1)

(i) ${Ca(OH)}_{2} + 2 HCl \to {CaCl}_{2} + 2 H_{2} O$

Millimole $M_{1} \times 10 (2 \times M_{1}) 10$

Millimole of HCl = $(2 \times M_{1}) 10 = 0.5 \times 20$

$M_{{Ca(OH)}_{2}} of HCl = 0.5 M$

(ii) With $H_{2} {SO}_{4}$ ${⇒Ca(OH)}_{2} + H_{2} {SO}_{4} \to {CaSO}_{4} + 2 H_{2} O$

$0.5 \times 2 (M_{2} \times 10)$

$M_{2} \times 10 = 0.5 \times 20$

$M_{2} = 1$

${Molarity of H}_{2} {SO}_{4} = 1$

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