Question

What volumes of 0.1 M acetic acid, pK_a 4.76, and 0.1 M sodium acetate are required to make 1 L of acetate buffer, pH 5.30?

		776 mL acetate, 224 mL acetic acid
		351 mL acetate, 649 mL acetic acid
		540 mL acetate, 460 mL acetic acid
		712 mL acetate, 288 mL acetic acid

Answer 1

answer : 776 mL acetate, 224 mL acetic acid

solution :

acetic acid volume = V L

moles of acetic acid = 0.1 x V

total volume = 1 L

aceitate volume = 1 - V

moles of acetate = 0.1 x (1-V)

the pH of acidic buffer

pH = pKa + log [salt/acid]

5.30 = 4.76 + log (sodium acetate / acetic acid)

5.30 = 4.76 + log (0.1 x (1-V) / 0.1 V)

0.54 = log (0.1 x (1-V) / 0.1 V)

(0.1 x (1-V) / 0.1 V) = 3.467

0.1 - 0.1 V = 0.3467 V

0.1 = 0.4467 V

V = 0.224 L = 224ml

acetic acid volume = V = 224 ml

sodium acetate volume = 1 - V = 1 - 0.224 L

                                      = 0. 776 L

                                      = 776 ml