Question

please I need help with this question;

7. Titration of Acetic Acid: using the equation [H^+] = (ka [HA])^1/2 for weak acids, calculate the theoretical starting pH of the titration based on the concentration of acetic acid notated on the bottle and the final concentration of the solution after preparing it for the reaction. calculate the percent error relative to the pH that was recorded in the lab.

(the concentration of the acetic acid is 0.10M and the pH recorded in the lab is 3.7)

Answer 1

K_a of acetic acid, CH₃COOH is K_a = 1.8*10^-5.

Write down the ionization of CH₃COOH as below.

CH₃COOH (aq) <======> H⁺ (aq) + CH₃COO^- (aq)

The acid ionization constant is expressed as

K_a = [H⁺][CH₃COO^-]/[CH3COOH]

======> 1.8*10^-5 = (x)(x)/(0.1 – x)

where x is the equilibrium concentration of H⁺ = equilibrium concentration of CH₃COO^- due to 1:1 nature of ionization of acetic acid.

Assume x << 0.1 M (since K_a is small).

Therefore,

1.8*10^-5 = x²/(0.1)

=====> x² = 1.8*10^-5*0.1

=====> x² = 1.8*10^-6

=====> x = 1.3416*10^-3

Therefore,

[H⁺] = 1.3416*10^-3 M

and

pH = -log [H⁺]

= -log (1.3416*10^-3 M)

= 2.8723 ≈ 2.87

The initial pH of the acetic acid solution is 2.87 (ans).