The calcium in a 100 mL sample of hard water is precipitated as CaC2O4, filtered, dissolved in dilute H2SO4 and titrated with 0.0100 M KMnO4, which needs 12.4 mL. Express the hardness as ppm (mg per litre) of CaCO3 (molar mass 100), assuming all Ca^2+ is converted to oxalate. Give the answer to the nearest integer.

JEE Main levelNumerical answerDifficulty 2 of 3rx189

The calcium in a 100 mL sample of hard water is precipitated as $CaC X_{2} O X_{4}$ , filtered, dissolved in dilute $H X_{2} SO X_{4}$ and titrated with 0.0100 M $KMnO X_{4}$ , which needs 12.4 mL. Express the hardness as ppm (mg per litre) of $CaCO X_{3}$ (molar mass 100), assuming all $Ca X^{2 +}$ is converted to oxalate. Give the answer to the nearest integer.

Numerical answer type, answer in ppm, tolerance 1.

mmol

MnO X_{4} X^{-}

12.4 \times 0.0100 = 0.124

; mmol oxalate =

0.124 \times 2.5 = 0.310

= mmol

Ca X^{2 +}

= mmol

CaCO X_{3}

equivalent. Mass =

0.310 \times 100 = 31.0

mg in 0.100 L, so hardness = 310 mg/L.