Question

1a) Which of the following buffer systems is the best choice to create a buffer with pH = 7.20?

CH3COOH/CH3COOK

HClO2/KClO2

NH3/NH4Cl

HClO/KClO --> ANS B) For the best system, calculate the ratio of the masses of the buffer components required to make the buffer. For HClO, Ka=4.0×10−8.

Answer 1

The given pH of buffer is 7.20

1.) CH₃COOH / CH₃COOK

Here CH₃COOH is the acid with acid ionization constant Ka = 1.8 x 10^-5

pKa = -log(Ka) = -log(1.8 x 10^-5) = 4.74

For a good buffer, pKa must be close to given pH. But here pKa (4.74) < pH (7.20). Hence this is not a good choice for buffer.

2.) HClO₂ / KClO₂

Here HClO₂ is the acid with acid ionization constant Ka = 1.1 x 10^-2

pKa = -log(Ka) = -log(1.1 x 10^-2) = 1.96

For a good buffer, pKa must be close to given pH. But here pKa (1.96) < pH (7.20). Hence this is not a good choice for buffer.

3.) NH₃ / NH₄Cl

Here NH₃ is the acid with base ionization constant K_b = 1.8 x 10^-5

pK_b = -log(K_b) = -log(1.8 x 10^-5) = 4.74

pKa for conjugate acid = 14 - pK_b = 14 - 4.74 = 9.26

For a good buffer, pKa must be close to given pH. But here pKa (9.26) > pH (7.20). Hence this is not a good choice for buffer.

4.) HClO / KClO

Here HClO is the acid with acid ionization constant Ka = 4.0 x 10^-8

pKa = -log(Ka) = -log(4.0 x 10^-8) = 7.34

For a good buffer, pKa must be close to given pH. Here pKa (7.34) is very close to the given pH (7.20). Hence this is the best choice for buffer.