Question

How many grams of sodium acetate (NaC₂H₃O₂) must be added to 2.0 L of a 0.500M acetic acid solution to produce a buffer of pH=5.00?

At what pH would this buffer have the highest capacity?

Answer 1

1)
Ka of CH3COOH = 1.8*10^-5

pKa = - log (Ka)
= - log(1.8*10^-5)
= 4.745

use formula for buffer
pH = pKa + log ([CH3COONa]/[CH3COOH])
5.0 = 4.745 + log ([CH3COONa]/[CH3COOH])
log ([CH3COONa]/[CH3COOH]) = 0.255273
[CH3COONa]/[CH3COOH] = 1.8
[CH3COONa] = 0.9
volume , V = 2.0 L


number of mol,
n = Molarity * Volume
= 0.9*2
= 1.8 mol

Molar mass of CH3COONa,
MM = 2*MM(C) + 3*MM(H) + 2*MM(O) + 1*MM(Na)
= 2*12.01 + 3*1.008 + 2*16.0 + 1*22.99
= 82.034 g/mol

mass of CH3COONa,
m = number of mol * molar mass
= 1.8 mol * 82.034 g/mol
= 147.7 g

Answer: 148 g

2)
Any buffer has its highest caapcity at Ph equal to pKa
pka here is 4.745
So, pH at maximum capacity is 4.745
Answer: 4.745