Chemistry Question

[匿名]

2008-12-14 5:41 am

2.00g of impure oxalic acid crystals were carefully weighed out, dissovled in distilled water, and made up to 250cm^3 of solution. 25.0cm^3 of this solution were acidified with dilute sulphuric acid and found to react completely with 30.0cm^3 of a solution of potassium permanganate containing 3.00g of KMnO4 per dm^3. Calculate the percentage by mass of oxalic acid dihydrate, H2C2O4.2H2O, in the crystals, assuming that the impuity has no reaction with potassium permanganate

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回答 (1)

Uncle Michael

2008-12-18 10:48 am

✔ 最佳答案

Molar mass of KMnO4 = 39.01 + 54.94 + 16x4 = 157.95 g mol-1
Molar mass of H2C2O4•2H2O = 1.008x6 + 12.01x2 + 16x6 = 126.068 g mol-1

2MnO42- + 16H+ + 5C2O42- → 2Mn2+ + 8H2O + 10CO2
Mole ratio MnO42- : C2O42- = 2 : 5
Mole ratio KMnO4 : H2C2O4•2H2O = 2 : 5

Consider the titration:
No. of moles of KMnO4 used = (3/157.95) x (30/1000) = 0.0005698 mol
No. of moles of H2C2O4•2H2O used = 0.0005698 x (5/2) = 0.001425 mol

In the impure crystals:
No. of moles of H2C2O4•2H2O = 0.001425 x 10 = 0.01425 mol
Mass of H2C2O4•2H2O = 0.01425 x 126.068 = 1.796 g
% by mass of H2C2O4•2H2O = (1.796/2) x 100% = 89.8%
=

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