How to calculate a solutions molarity

A solution is prepared by dissolving 8.83 g of Na2SO4 in enough water to make 225 mL of solution. Calculate the solution’s molarity.

M (molarity) = mol/L

Convert 8.83 g Na2SO4 -> mol

Given grams x 1 mol/molar mass

Molar mass Na2SO4:

2 Na x 22.99 = 45.98
1 S x 32.07 = 32.07
4 O x 16.00 = 64.00

Add them up, we get 142.05 g/mol

8.83 g Na2SO4 x 1 mol Na2SO4/142.05 g Na2SO4 = 0.0622 mol Na2SO4


Convert 225 mL -> L

Given mL x 1 L/1000 mL

225 mL x 1 L/1000 mL = 0.225 L


0.0622 mol Na2SO4/0.225 L Na2SO4 = 0.27644 mol/L Na2SO4


Sig figs... 0.276 M Na2SO4

First calculate the molar mass of Na2SO4. Go to the periodic table.

Na = 22.989768
S = 32.064
O = 15.9994

Mm = 2(22.989758) + 32.064 + 4(15.9994) = 142.041136g/mol

Divide the grams (8.83) by the Mm (142.041136)
8.83 / 142.041136 = 0.0622mol

Divide the mol by the volume to get the molarity
0.0622 divide by 225 / 1000 = 2.76M

Notice I divided by 1000, this is because we had to change the volume from ml to L. Molarity is measured in L.

8.83g/Molar Mass (g/mol) of Na2SO4 = number of mol in solution.

Number of mol in solution / .225L = Concentration of mol (molarity).