Question

Suppose you want to prepare a hypochlorous acid (Ka = 3.0x10-8) buffer by mixing hypochlorous acid and sodium hypochlorite. If you want to prepare 1.0 L of buffer with a pH of 7.0 and a total buffer concentration of 0.10 M, how many moles of hypochlorous acid should be added? moles how many moles of sodium hypochlorite should be added? moles

Answer 1

0.0768 moles of hypochlorous acid, 0.0232 moles of sodium hypochlorite.

Step-by-step explanation:

It is possible to answer this question using Henderson-Hasselbalch formula:

pH = pka + log₁₀ [A⁻] / [HA] (1)

Where A⁻ is conjugate base, in this case sodium hypochlorite and HA wek acid (hypochlorous acid).

pka is -log ka; -log 3.0x10⁻⁸ = 7.52 and pH is desire pH, 7.0

As you want to prepare 1.0L of a buffer 0.10M, you need 0.10mol of buffer, that means:

[A⁻] + [HA] = 0.10 (2)

Replacing (2) in (1):

7.0 = 7.52 + log₁₀ [0.10 - HA] / [HA]

0.302[HA] = [0.10 - HA]

1.302[HA] = 0.10

[HA] = 0.0768 moles of hypochlorous acid

Replacing in (2):

[A⁻] + 0.0768 = 0.10

[A⁻] = 0.10 - 0.0768 = 0.0232 moles of sodium hypochlorite

I hope it helps!