Question

How many grams of sodium hypochlorite (NaClO)is required to make abuffer with pH= 7.35 if you had 500 mL of a 0.1M solution of hypochlorous acid with pKa=7.54.(Assume the addition of NaClO does not change the volume.)

Answer 1

2.4g of NaClO must be added

Step-by-step explanation:

To find the pH of a buffer we must use H-H equation:

pH = pKa + log [A⁻] / [HA]

Where pH is the pH of the buffer, 7.35

pKa of the HClO buffer is 7.54

[A⁻] could be taken as moles of NaClO

And [HA] moles of the weak acid: 500mL = 0.500L * (0.1mol/L) = 0.05 moles

Replacing:

pH = pKa + log [A⁻] / [HA]

7.35 = 7.54 + log [A⁻] / [0.05moles]

0.6457 = [A⁻] / [0.05moles]

0.03228 moles = Moles of NaClO we need to make the pH = 7.35 buffer.

To convert these moles to grams we use the molar mass of NaClO (74.44g/mol):

0.03228 moles * (74.44g/mol) =

2.4g of NaClO must be added