Question

Calculate the pH at 25°C of a 0.73M solution of potassium acetate KCH3CO2. Note that acetic acid HCH3CO2 is a weak acid with a pKa of 4.76 . Round your answer to 1 decimal place.

Answer 1

The pH of a 0.73M potassium acetate solution is approximately 8.9, determined by calculating the hydrolysis of acetate ions in the solution and using the pKa of acetic acid.

Step-by-step explanation:

To calculate the pH of a 0.73M potassium acetate solution, we need to consider that potassium acetate is a salt of a weak acid (acetic acid) and a strong base (potassium hydroxide). In water, it will dissociate completely to give the acetate ions, CH3COO-, and potassium ions, K+. The acetate ion will partially react with water in a hydrolysis reaction, which increases the pH of the solution.

The hydrolysis reaction is as follows:

CH3COO- + H2O → CH3COOH + OH-

Because acetic acid is a weak acid, its conjugate base, the acetate ion, is a weak base. To find the hydroxide ion concentration [OH-], we use the equilibrium equation for the reaction of the acetate ion with water:

Kb = (Kw/Ka) = ([OH-]2)/[CH3COO-]

Since pKa = 4.76, Ka = 10-4.76. The Kw value is 1 × 10-14 at 25°C, so Kb becomes 10-14 / 10-4.76 which is approximately 5.75 × 10-10. Considering the initial concentration of acetate ions is 0.73M and neglecting the contribution of water's autoionization:

[OH-] = √(Kb × [CH3COO-]) = √(5.75 × 10-10 × 0.73) ≈ 7.04 × 10-6M

To find the pOH:

pOH = -log[OH-] = -log(7.04 × 10-6) ≈ 5.15

And finally, we can calculate the pH:

pH = 14 - pOH = 14 - 5.15 ≈ 8.9

Therefore, the pH of the 0.73M potassium acetate solution is approximately 8.9.

Answer 2

The pH of a 0.73M solution of potassium acetate at 25°C is 4.76, which is the pKa of acetic acid.

Step-by-step explanation:

To calculate the pH of a 0.73M solution of potassium acetate at 25°C, you can use the formula pH = pKa + log([A-]/[HA]), where pKa is the negative log of the Ka value of acetic acid, [A-] is the concentration of the acetate ion, and [HA] is the concentration of acetic acid. Potassium acetate dissociates completely in water, so the concentration of acetate ion is the same as the initial concentration of potassium acetate, and since the acetic acid is not present initially, we can assume [HA] to be zero. The pKa of acetic acid is given as 4.76, so the pH of the potassium acetate solution is simply its pKa since the log of [A-]/[HA] tends toward infinity when [HA] approaches zero.

The calculated pH is thus 4.76, corresponding to the pKa of acetic acid, assuming that the presence of acetate ion does not significantly shift the pH beyond the pKa value.