How do I do a question like this please help and explain it!!
Calculate the pH of a buffer solution that contains 0.22 M benzoic acid (C6H5CO2H) and 0.24 M sodium benzoate (C6H5COONa). [Ka = 6.5 × 10-5 for benzoic acid] Round your answer to two places past the decimal.
You use the Henderson-Hasselbalch equation. It's a simple matter of plugging in the numbers and punching into the calculator.
To calculate the pH of a buffer solution, you need to understand the concept of a buffer and how it maintains a stable pH. A buffer is a solution that resists changes in pH when small amounts of acid or base are added to it. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.
In this case, the buffer solution contains benzoic acid (C6H5CO2H) and sodium benzoate (C6H5COONa). Benzoic acid is a weak acid, while sodium benzoate is the conjugate base of benzoic acid.
The pH of a buffer solution mainly depends on the ratio of the concentrations of the weak acid (C6H5CO2H) and the conjugate base (C6H5COONa). The pH can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log([conjugate base]/[weak acid])
Given:
- Concentration of benzoic acid (C6H5CO2H) = 0.22 M
- Concentration of sodium benzoate (C6H5COONa) = 0.24 M
- Ka (acid dissociation constant) for benzoic acid = 6.5 × 10-5
First, we need to calculate the ratio [conjugate base]/[weak acid]:
[conjugate base]/[weak acid] = [C6H5COONa]/[C6H5CO2H]
Next, we can substitute the values in the Henderson-Hasselbalch equation:
pH = pKa + log([C6H5COONa]/[C6H5CO2H])
where pKa = -log(Ka).
Now let's calculate the pH step by step:
1. Calculate pKa:
pKa = -log(6.5 × 10-5)
2. Calculate [conjugate base]/[weak acid]:
[conjugate base]/[weak acid] = 0.24/0.22
3. Substitute the values into the Henderson-Hasselbalch equation:
pH = pKa + log(0.24/0.22)
4. Calculate the logarithm of the ratio:
pH = pKa + log(0.24/0.22)
Finally, calculate the pH using a calculator or logarithmic tables, and round the answer to two decimal places.
Note: The pKa value for benzoic acid mentioned in the question should be used. If you have a different pKa value, substitute that value instead.
That's it! By following these steps, you can calculate the pH of the given buffer solution.