Question

The limit for discharge of hexachromium salts is 0.50 mg/L. What is the molar concentration of chromium ion in this solution? What is the concentration in ppm?

Answer 1

The limit for discharge of hexachromium salts is 0.50 mg/L.

potassium dichromate (K₂Cr₂O₇) is used as hexavalent chromium salt.

Hence 1 L solution can maximum contain 0.5 mg of K₂Cr₂O₇

Molar mass of K₂Cr₂O₇ = 294.185 g/mol

atomic mass of Cr = 52 g/mol

Hence, 294.185 g of K₂Cr₂O₇ contains Cr(VI) = 2 x 52 g = 104 g

294.185 mg of K₂Cr₂O₇ contains Cr(VI) = 104 mg

0.50 mg of K₂Cr₂O₇ contains Cr(VI) = (104/294.185) x 0.5 = 0.18 mg

Hence, the concentration of chromium ion in this solution = 0.18 mg/L = 0.18 ppm

52 g of Cr(IV) 1 mole

0.18 mg = 0.18 x 10^-3 g (1/52) x 0.18 x 10^-3 = 3.46 x 10^-6 mol

Hence, the molar concentration of chromium ion in this solution = 3.46 x 10^-6 mol/L