A Chemistry question!!~HELP~~~~

2008-05-22 1:50 am

A student dissolves 10.86 g of crystalline sodium acetate trihydrate (CH3COONa.3H2O) in a little water, adds 4 g of glacial (10%) acetic acid, and then distilled water to make the total volume 500ml.Calculate the molar concentration to the buffer and its pH.
唔識計...CH3COONA.3H2O...入面三個水洗唔洗計??
Please help me~~thank you

回答 (1)

Uncle Michael

2008-05-23 1:21 am

✔ 最佳答案

Glacial acetic acid means pure acetic acid. Therefore, I suppose it is 「glacial (100%) acetic acid.」
Ka(CH3COOH) = 1.8 x 10-5 M at 25oC

Molar mass of CH3COONa•3H2O = 12.01x2 + 1.008x9 + 16x5 + 22.99 = 136.082 g mol-1
No. of moles of CH3COONa•3H2O = mass/(molar mass) = 10.86/136.082 = 0.0798 mol
No. of moles of CH3COO- = 0.0798 mol
Molar mass of CH3COOH = 12.01x2 + 1.008x4 + 16x2 = 60.052 g mol-1
No. of moles of CH3COOH = mass/(molar mass) = 4/60.052 = 0.06661 g mol-1
Volume of the solution = 500 mL = 0.5 L (0.5 dm3)

[CH3COO-] = mol/volume = 0.0798/0.5 = 0.1596 M
[CH3COOH] = mol/volume = 0.06661/0.5 = 0.1332 M

pH = pKa + log([CH3COO-]/[CH3COOH]
pH = -log(1.8 x 10-5) + log(0.1596/0.1332)
pH = 4.82

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