Question

A 100.0 mL sample of 0.20 M HF is titrated with 0.10 M KOH. Determine the pH of the solution before the addition of any KOH. The Ka of HF is 3.5 x 10-4.

Answer 1

2.77

Step-by-step explanation:

To determine the pH of the solution before the addition of any KOH, we need to calculate the initial concentration of H+ ions in the solution.

HF is a weak acid and will partially dissociate in water, according to the following equation:

⇒ HF + H2O ⇌ H3O+ + F-

The equilibrium constant for this reaction is given by the acid dissociation constant, Ka, which is 3.5 x 10^-4 for HF.

The initial concentration of HF is 0.20 M, so we can use an ICE table to determine the concentrations of the other species in the solution:

HF H3O+ F-

Initial 0.20 M 0 0

Change -x +x +x

Equilibrium 0.20 - x x x

At equilibrium, the concentration of H3O+ is equal to the concentration of F- because HF is a monoprotic acid.

Using the equation for Ka, we can write:

Ka = [H3O+][F-]/[HF]

Substituting the equilibrium concentrations from the ICE table and the value of Ka, we get:

3.5 x 10^-4 = x^2 / (0.20 - x)

This is a quadratic equation that can be solved for x, which represents the concentration of H3O+ at equilibrium:

x = 1.7 x 10^-3 M

Therefore, the pH of the solution before the addition of any KOH is:

pH = -log[H3O+] = -log(1.7 x 10^-3) = 2.77

Answer 2

Sure! Here's a step-by-step guide on how to calculate the pH of a solution of 100.0 mL of 0.20 M HF before any KOH is added:

Step 1: Write down the balanced chemical equation for the dissociation of HF in water:

HF (aq) ⇌ H+ (aq) + F- (aq)

Step 2: Calculate the concentration of H+ ions in the solution using the initial concentration of HF:

[H+] = [HF]

Since the initial concentration of HF is 0.20 M, the concentration of H+ ions is also 0.20 M.

Step 3: Calculate the pOH of the solution using the concentration of OH- ions, which can be assumed to be zero initially:

pOH = -log10[OH-]

Since there are no OH- ions initially, the pOH is 0.

Step 4: Calculate the pH of the solution using the equation:

pH = 14 - pOH

Since the pOH is 0, the pH is 14.

Step 5: Verify your result by checking if it falls within the acidic range (pH < 7). In this case, the pH is 14, which is greater than 7, indicating that the solution is not acidic.

So, the pH of the solution of 100.0 mL of 0.20 M HF before any KOH is added is 14, indicating that it is basic.

Please note that this calculation assumes that HF is a strong acid and completely dissociates in water. If HF is a weak acid, additional calculations involving the acid dissociation constant (Ka) would be necessary.