To make a solution with a pH = 4.20 a student used the following procedure :?

Question

To make a solution with a pH = 4.20 a student used the following procedure :
a certain amount of sodium acetate along with 0.250 moles of acetic acid is added to enough water to make a solution of 1.00 L
How many grams of sodium acetate were added ?

Ka = 1.80 x 10-5

Answer 1

Use the Henderson-Hasselbach equation and solve for the concentration of the base.

pH = pKa + log ([base]/[acid])
pH - pKa = log ([base]/0.250)
4.20 - 4.74 = log ([base]/0.250)
10^-0.54 = [base]/0.250
0.288 * 0.250 = [base] = 0.072 M

Since it's one liter, that's 0.072 moles of sodium acetate, which is 5.90 grams (MW NaC2H3O2 = 82 g/mol).