F.5 Chemistry calculation

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2009-07-19 10:57 pm

2.17 g of oxalic acid crystals H2C2O4 ⋅2 H2O were made up to 250 cm3 of solution. 25 cm3 of the solution required 29.3 cm3 of 0.119 M sodium hydroxide solution for complete neutralization. Find the basicity of the acid.

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老爺子

2009-07-20 2:33 am

✔ 最佳答案

Consider the preparation of H2C2O4 solution:
Molar mass of H2C2O4•2H2O = 1x6 + 12x2 + 16x6 = 126 g/mol
No. of moles of H2C2O4•2H2O used = 2.17/126 = 0.0172 mol
Volume of the solution = 250 cm3 = 0.25 dm3
Molarity of H2C2O4 solution = 0.0172/0.25 = 0.0688 M

Let n be the basicity of H2C2O4.
Hence, denote the acid as HnA.

Consider the neutralization between oxalic acid and sodium hydroxide solution:
HnA + nNaOH → NanA + nH2O
Mole ratio H2C2O4 : NaOH = 1 : n
No. of moles of H2C2O4 used = 0.0688 x (25/1000) = 0.00172 mol
No. of moles of NaOH used = 0.119 x (29.3/1000) = 0.00349 mol
Hence, n = 0.00349/0.00172 ≈ 2

Basicity of the acid = 2
(dibasic)

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