Frequently Asked Question

HA Dissociation Constant (Ka):
The dissociation constant Ka for a weak acid HA in aqueous solution is given by the equation:
Ka = [H+][A-] / [HA]
where [H+] is the molar concentration of hydrogen ions, [A-] is the molar concentration of the conjugate base, and [HA] is the molar concentration of the weak acid.

pH Calculations for Weak Acids:
The pH of a solution containing a weak acid can be calculated using the following equation:
pH = -log[H+]

Given Information:
- HA is a weak acid with Ka = 3.7 x 10^-13
- The initial concentration of NaA (the salt of HA) is 0.077 M
- Temperature: 25.0oC

Steps to Calculate pH:
1. Dissociation of NaA:
NaA dissociates in water to form HA and Na+ ions:
NaA(aq) + HOH(l) ⇌ HA(aq) + Na+(aq)

2. Initial Concentration of HA:
The initial concentration of HA is equal to the initial concentration of NaA, which is 0.077 M.
[HA]initial = 0.077 M

3. Ice Table:
Initialize an ICE table to determine the equilibrium concentrations of the species:

| | HA | + HOH ⇌ | H+ | + | A- |
|---|---|---|---|---|
| Initial | 0.077 | | 0 | | 0 |
| Change | -x | | +x | | +x |
| Equilibrium | (0.077 - x) | | x | | x |

4. Dissociation Constant Expression:
Substitute the equilibrium concentrations into the dissociation constant expression:
Ka = [H+][A-] / [HA]
3.7 x 10^-13 = x^2 / (0.077 - x)

5. Solving for x:
Since Ka is very small, we can assume that x is much less than 0.077. Therefore, we can simplify the equation to:
Ka ≈ x^2 / 0.077
x ≈ √(Ka * 0.077) ≈ 1.7 x 10^-6 M

6. Calculating pH:
The equilibrium concentration of hydrogen ions is x, so:
[H+] = x = 1.7 x 10^-6 M
pH = -log[H+] = -log(1.7 x 10^-6) ≈ 5.77

Therefore, the pH of the aqueous solution of NaA is approximately 5.77.