Weak Acid
1.) The pH of a 0.060M weak monoprotic acid HA is 3.44. Calculate the Ka of the acid.
2.) The pH of 0.100M solution of weak monoprotic acid HA is 2.85. What is the Ka of the acid?
I have shown you how to do these below.
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2.21x10^-6
To calculate the Ka of a weak acid, we can use the formula:
Ka = 10^(-pH) * [HA] / [A-]
where [HA] is the concentration of the undissociated acid, [A-] is the concentration of the conjugate base, and pH is the given pH value.
Let's solve each question step-by-step:
1.) The pH of a 0.060M weak monoprotic acid HA is 3.44. Calculate the Ka of the acid.
Step 1: Identify the given values:
- pH = 3.44
- [HA] = 0.060M
- [A-] (conjugate base concentration) is not given directly, but we can assume it is negligible compared to [HA].
Step 2: Calculate Ka using the formula:
Ka = 10^(-pH) * [HA] / [A-]
= 10^(-3.44) * 0.060M / [A-]
≈ 3.56 * 10^(-4) / [A-]
Therefore, the Ka of the acid is approximately 3.56 * 10^(-4).
2.) The pH of 0.100M solution of weak monoprotic acid HA is 2.85. What is the Ka of the acid?
Step 1: Identify the given values:
- pH = 2.85
- [HA] = 0.100M
- [A-] (conjugate base concentration) is not given directly, but we can assume it is negligible compared to [HA].
Step 2: Calculate Ka using the formula:
Ka = 10^(-pH) * [HA] / [A-]
= 10^(-2.85) * 0.100M / [A-]
≈ 7.08 * 10^(-3) / [A-]
Therefore, the Ka of the acid is approximately 7.08 * 10^(-3).
To find the Ka value of a weak acid, you can use the pH and concentration of the acid. The Ka expression for a weak monoprotic acid HA is:
Ka = [H+][A-] / [HA]
1.) To calculate the Ka of the acid when the pH is 3.44 and the concentration is 0.060M:
Step 1: Determine the concentration of H+ ions.
Since pH is the negative logarithm of the concentration of H+ ions, you can calculate the concentration of H+ ions using the formula 10^(-pH). Therefore, [H+] = 10^(-3.44).
Step 2: Determine the concentration of A- ions.
Since the acid HA is weak, it will dissociate very little. So, we can assume that the concentration of A- ions is the same as the concentration of the acid HA, which is 0.060M.
Step 3: Calculate the value of Ka.
Using the Ka expression: Ka = [H+][A-] / [HA], substitute the values we obtained:
Ka = (10^(-3.44))(0.060) / 0.060
2.) To find the Ka value when the pH is 2.85 and the concentration is 0.100M:
Repeat Steps 1 to 3 as described above, substituting the given pH and concentration values.
By following these steps, you can calculate the Ka value of the weak monoprotic acid HA for both scenarios.