If a solution of HF (Ka=6.8×10−4) has a pH of 2.90, calculate the concentration of hydrofluoric acid.

To calculate the concentration of hydrofluoric acid (HF) when the pH is given, follow these steps:

1. Convert pH to H+ concentration: pH = -log[H+] 2.90 = -log[H+] [H+] = 10^-2.90 [H+] = 1.26×10^-3 M (molarity)

2. Write down the dissociation equation for HF: HF ↔ H+ + F-

3. Write the expression for the acid dissociation constant, Ka: Ka = [H+][F-]/[HF]

Since we have the pH, we also have the concentration of H+ ions. At the start of the reaction, let's assume the concentration of HF to be 'x'. Therefore, the change in concentration for H+ and F- would also be 'x' due to the stoichiometry of the reaction (1:1 ratio).

But, from the pH, we know that [H+] = 1.26×10^-3 M. Let's assume that x is so small that when we subtract it from the initial concentration of HF, the change is negligible. This assumption is often valid for weak acids, like HF.

4. Substitute the values in the Ka expression: 6.8×10^-4 = (1.26×10^-3)(1.26×10^-3)/[HF]

5. Solve for [HF]: [HF] = (1.26×10^-3)(1.26×10^-3) / 6.8×10^-4 [HF] ≈ (1.58×10^-6) / 6.8×10^-4 [HF] ≈ 2.32×10^-3 M