Question

What will be the pH of a solution formed by mixing 10 mL 0.1 M $NaH_{2}PO_{4}$ and 15 mL 0.1 M $Na_{2}HPO_{4}$ ?

[Given: for $H_{3}PO_{4}$ $pK_{a_{1}}=2.12,pK_{a_{2}}=7.2$ ]

• A. 7.0
• B. 6.9
• C. 7.4
• D. 7.5

Answer 1

Answer: (C)

$pH=pK_{a}+log \frac{\left[c o n j u g a t e \text{ base}\right]}{\left[a c i d\right]}$

Number of moles of $NaH_{2}PO_{4}=\frac{10}{1000}L\times 0.1 \, mol \, \text{L}^{- 1}$

$=0.001$ moles

Number of moles of $Na_{2}HPO_{4}=\frac{15}{1000}\times L\times 0.1\text{ mol }\text{L}^{- 1}$

$=0.0015$ moles

$pH=7.2+log \frac{\left[0.0015\right]}{\left[0.001\right]}$

$=7.37\approx 7.4$