Question

Assuming equal concentrations of conjugate base and acid, which one of the following mixtures is suitable for making a buffer solution with an optimum pH of 9.2−9.3? CH3COONa/CH3COOH (Ka = 1.8 x 10^−5) NH3/NH4Cl (Ka = 5.6 x 10^−10) NaOCl/HOCl (Ka = 3.2 x 10^−8) NaNO2/HNO2 (Ka = 4.5 x 10^−4) NaCl/HCl

Answer 1

Answer:

NH₃/NH₄Cl

Explanation:

We can calculate the pH of a buffer using the Henderson-Hasselbalch's equation.

$pH=pKa+log\frac{[base]}{[acid]}$

If the concentration of the acid is equal to that of the base, the pH will be equal to the pKa of the buffer. The optimum range of work of pH is pKa ± 1.

Let's consider the following buffers and their pKa.

• CH₃COONa/CH3COOH (pKa = 4.74)

• NH₃/NH₄Cl (pKa = 9.25)

• NaOCl/HOCl (pKa = 7.49)

• NaNO₂/HNO₂ (pKa = 3.35)

• NaCl/HCl Not a buffer

The optimum buffer is NH₃/NH₄Cl.