Question

A chemistry graduate student is given 500. mL of a 0.20 M chloroacetic acid (HCH2ClCO2) solution. Chloroacetic acid is a weak acid with Ka= 1.3 x 10^-3. What mass of KCH2ClCO2 should the student dissolve in the HCH2ClCO2 solution to turn it into a buffer with pH = 3.01? You may assume that the volume of the solution doesn't change when the KCH2ClCO2 is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

12 g of choloracetic acid

Step-by-step explanation:

The buffer equilibrium is:

HCH₂ClCO₂ ⇄ CH₂ClCO₂⁻ + H⁺

pka= -log ka =

Ka: 1,3x10⁻³ = [CH₂ClCO₂⁻] [H⁺] / [HCH₂ClCO₂]

By Henderson-Hasselbalch equation:

pH = pka + log₁₀ [A⁻] / [HA]

3,01 = 2,89 + log₁₀ [A⁻] / [HA]

1,318 = [A⁻] / [HA]

As molar concentration of chloroacetic acid (HA) is 0,20M

[A⁻] = 0,26 mol/L

The volume is 500 mL ≡ 0,5 L

0,26mol/L × 0,5 L = 0,13 moles of chloroacetic acid. In grams:

0,13 mol × (94,5g / 1mol) = 12 g of choloracetic acid

I hope it helps!