Question

# At a pH of 7.40, what is the ratio of the molar concentrations of H$_{2}$PO$_{4}^{-}$ to H$_{3}$PO$_{4}$?

Solution

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Given data:

$\text{pH}=7.40$

$K_{a}(\text{H}_{3}\text{PO}_{4})=7.1 \times 10^{-3} \ \text{mol/dm}^{3}$

We have to determine the molar ratio of hydrogenphosphate anion to phosphate acid.

Buffer solutions consists of a week acid and its conjugate base. Since the hydrogenphosphate anion is a conjugated base of phosphate acid, we can assume that we have a buffer solution here.

Buffer solutions follow the Handerson's Hasselbach equation:

$\text{pH}=\text{pK}_{a}+ \log\dfrac{[\text{A}^{-}]}{[\text{HA}]} \tag{1}$

So, knowing the constant dissociation at the pH value, we can determine the molar ratio of acid and its conjugate base.

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