I need help with an equilibrium problem and pH. I don\'t get how you get b. A 1.

Question

I need help with an equilibrium problem and pH. I don't get how you get b.

A 1.000 L sample of a buffer solution contains 0.100 moles of acetic acid (CH3COOH), a weak acid, and 0.100 moles of sodium acetate (NaCH3COO) a soluble ionic compound. 1.00 mL of a 1.00 M solution of sodium hydroxide (NaOH), a strong soluble base, is added to the solution. The pH of the solution will increase by a large amount (more than 0.50 pH units). will increase by a small amount (less than 0.10 pH units). will decrease by a large amount (more than 0.50 pH units). will decrease by a small amount (less than 0.10 pH units). will not change.

Explanation / Answer

pH = pka + log [ CH3COOna]/[CH3COOH]

pH = 4.745 + log ( 0.1/0.1) = 4.745 initially

moles of NaOH added = MV = 1 x 1/1000 = 0.001

now CH3COOH moles = 0.1-0.001 = 0.099

CH3COOna moles = 0.1+0.001 = 0.101

pH = 4.745 + log ( 0.101/0.099) =4.745 + 0.0087

hence pH is increased less than 0.1 pH units

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