The chemical oxygen demand of waste water is estimated as follows. 20.0 mL of the sample is refluxed with 10.0 mL of 0.0250 M K2Cr2O7 in acid. The unreacted dichromate is titrated with 0.100 M Fe^2+ solution, needing 12.4 mL. Express the oxygen demand as mg of O2 per litre (O = 16; O2 takes 4 electrons per molecule, so 1 meq corresponds to 8 mg of O2). Give the answer to the nearest integer.

JEE Advanced levelNumerical answerDifficulty 3 of 3rx211

The chemical oxygen demand of waste water is estimated as follows. 20.0 mL of the sample is refluxed with 10.0 mL of 0.0250 M $K X_{2} Cr X_{2} O X_{7}$ in acid. The unreacted dichromate is titrated with 0.100 M $Fe X^{2 +}$ solution, needing 12.4 mL. Express the oxygen demand as mg of $O X_{2}$ per litre (O = 16; $O X_{2}$ takes 4 electrons per molecule, so 1 meq corresponds to 8 mg of $O X_{2}$ ). Give the answer to the nearest integer.

Numerical answer type, answer in mg/L, tolerance 1.

Dichromate taken =

10.0 \times 0.0250 \times 6 = 1.50

meq. Excess dichromate equals

Fe X^{2 +}

used =

12.4 \times 0.100 = 1.24

meq. Dichromate consumed by the sample = 0.26 meq, equivalent to

0.26 \times 8 = 2.08

O X_{2}

in 20.0 mL. COD = 2.08/0.0200 = 104 mg/L.