Question

Calculate the mass of sodium sulfate (Na2SO4) used to prepare a 0.5 L of 1000 mg/L SO42- solution.

Answer 1

mass of SO42- = density * volume

= 1000 mg/L * 0.5 L

= 500 mg

Molar mass of SO4 2-,

MM = 1*MM(S) + 4*MM(O)

= 1*32.07 + 4*16.0

= 96.07 g/mol

mass(SO4 2-)= 0.500 g

use:

number of mol of SO4 2-,

n = mass of sulfate/molar mass of sulfate

= (0.5 g)/(96.07 g/mol)

= 5.205*10^-3 mol

1 molecule of Na2SO4 has 1 molecule of SO42-

So,

mol of Na2SO4 = 5.205*10^-3 mol

Molar mass of Na2SO4,

MM = 2*MM(Na) + 1*MM(S) + 4*MM(O)

= 2*22.99 + 1*32.07 + 4*16.0

= 142.05 g/mol

use:

mass of Na2SO4,

m = number of mol * molar mass

= 5.205*10^-3 mol * 142.05 g/mol

= 0.739 g

Answer: 0.739 g