BridgeAcids and basesFree

Titration curve for ethanoic acid with sodium hydroxide

Weak acid–strong base titration curve and buffers

20.0 mL of 0.120 mol L

^{-1}

ethanoic acid,

\ce{CH3COOH}

, is titrated with 0.100 mol L

^{-1}

NaOH.

K_a(\ce{CH3COOH}) = 1.74 \times 10^{-5}

K_a

4.76);

K_w = 1.00 \times 10^{-14}

Facts you may need

$[\ce{H3O+}] = \sqrt{K_a c}$
buffer pH from the acid : base ratio
$K_aK_b = K_w$
n = cV

(a)

Calculate the pH before any NaOH is added.

(b)

Calculate the volume of NaOH needed to reach the equivalence point, and the pH after 6.00 mL and after 12.0 mL of NaOH have been added.

(c)

Calculate the pH at the equivalence point and choose a suitable indicator from: methyl orange (3.1–4.4), bromothymol blue (6.0–7.6), phenolphthalein (8.3–10.0).

(d)

Calculate the pH after 30.0 mL of NaOH has been added.

(e)

Use the curve to explain where the solution is the most effective buffer, and why the buffering fails close to the equivalence point.