If you have 500 mL of a 0.10 M solution of the acid, what mass of the corresponding sodium salt of the conjugate base do you need to make the buffer with a pH of 7.74 (assuming no change in volume)

5.90g of NaClO are needed

Explanation:

Assuming the weak acid is HClO pKa = 7.54

To solve this question we must use the H-H equation for acids:

pH = pKa + log [A-] / [HA]

Where pH is the pH we want = 7.74

pKa is pKa of the weak acid = 7.54

[A-] could be taken as the moles of the conjugate base (Moles NaClO) and [HA] moles weak acid = 0.500L * (0.10mol/L) = 0.0500 moles HClO

Replacing:

7.74 = 7.54 + log [NaClO] / [0.0500 moles]

0.20 = log [NaClO] / [0.0500 moles]

1.5849 = [NaClO] / [0.0500 moles]

Moles NaClO = 0.0792 moles

The mass is: -Molar mass NaClO = 74.44g/mol-

0.0792 moles * (74.44g / 1mol) =

the answer is d. 74.02