# How to find pKa based on pH and concentration?

## I'm given the concentration of lactic acid as 0.025 M and its pH as 2.74. Here's what I did: $\left[{H}^{+}\right] = {10}^{- 2.74} \setminus \approx 1.82 \setminus \times {10}^{- 3} M$ ${K}_{a} = \setminus \frac{{x}^{2}}{0.025 - x} = \setminus \frac{{\left(1.82 \setminus \times {10}^{- 3}\right)}^{2}}{0.025 - \left(1.82 \setminus \times {10}^{- 3}\right)} \setminus \approx 1.4 \setminus \times {10}^{- 4}$ And then I found pKa by taking $- \log \left(1.4 \setminus \times {10}^{-} 4\right) \setminus \approx 3.9$. Have I done this correctly?

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Dave Share
Mar 11, 2018

The setup is correct.

#### Explanation:

I didn't recalculate your numbers, but your setup is right. Well done.

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