Question

20 mL of calcium hydroxide was consumed when it was reacted with 10 mL of unknown solution of H₂SO4. Also 20 mL standard solution of 0.5 M HCI 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 H₂SO₄ is_____M (Nearest integer)

Answer 1

Correct Answer: 1

First, let us solve for the concentration of HCl solution used in the titration.
Given:
- Volume of HCl = \( 20 \, \text{mL} \)
- Molarity of HCl = \( 0.5 \, \text{M} \)
- Volume of HCl used in titration = \( 35.5 - 25.5 = 10 \, \text{mL} \) The number of moles of HCl reacted is: \[ \text{Moles of HCl} = M \times V = 0.5 \, \text{M} \times \frac{10}{1000} \, \text{L} = 0.005 \, \text{mol} \] The balanced equation between calcium hydroxide and HCl is: \[ \text{Ca(OH)}_2 + 2 \, \text{HCl} \rightarrow \text{CaCl}_2 + 2 \, \text{H}_2\text{O} \] Thus, 1 mole of \( \text{Ca(OH)}_2 \) reacts with 2 moles of HCl The number of moles of calcium hydroxide required for the titration is: \[ \text{Moles of Ca(OH)}_2 = \frac{0.005}{2} = 0.0025 \, \text{mol} \] Now, for the unknown solution of H₂SO₄, the balanced equation is: \[ \text{Ca(OH)}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + 2 \, \text{H}_2\text{O} \] 1 mole of \( \text{Ca(OH)}_2 \) reacts with 1 mole of H₂SO₄ The moles of calcium hydroxide required for 10 mL of unknown H₂SO₄ solution is 0.0025 mol (from the titration) Finally, the concentration of the unknown H₂SO₄ solution is: \[ \text{Molarity of H}_2\text{SO}_4 = \frac{0.0025 \, \text{mol}}{0.01 \, \text{L}} = 0.25 \, \text{M} \] The concentration is 0.25 M