please help with this chem problem?

Question

To make a solution with a pH = 3.80 a student used the following procedure :
a certain amount of sodium acetate along with 0.280 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 definition of the ka constant for buffer solutions:
if you know the pH, you can calculate the H+ concentration (10^pH).
then use the value of ka into the equation and reagrange to find [ acetate ions]:

ka = [H+][acetate ions - what u want to find] / [0.280/1]

[acetate ions] = moles/volume (where volume is 1.00L)
rearange to calculate moles