Total mass of the Element  (Part)   

Total mass of compound (Whole)

Calculate the percent by weight of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)

• Calculate the molecular mass (MM):
  MM = 22.99 + 35.45 = 58.44
• Calculate the total mass of Na present:
  1 Na is present in the formula, mass = 22.99
• Calculate the percent by weight of Na in NaCl:
  %Na = (mass Na ÷ MM) x 100 = (22.99 ÷ 58.44) x 100 = 39.34%
• Calculate the total mass of Cl present:
  1 Cl is present in the formula, mass = 35.45
• Calculate the percent by weight of Cl in NaCl:
  %Cl = (mass Cl ÷ MM) x 100 = (35.45 ÷ 58.44) x 100 = 60.66%

The answers above are probably correct if %Na + %Cl = 100, that is,
39.34 + 60.66 = 100.

Calculate the percent by weight of each element present in sodium sulfate (Na₂SO₄).

• Calculate the molecular mass (MM):
  MM = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04
• Calculate the total mass of Na present:
  2 Na are present in the formula, mass = 2 x 22.99 = 45.98
• Calculate the percent by weight of Na in Na₂SO₄:
  %Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) x 100 = 32.37%
• Calculate the total mass of S present in Na₂SO₄:
  1 S is present in the formula, mass = 32.06
• Calculate the percent by weight of S present:
  %S = (mass S ÷ MM) x 100 = (32.06 ÷ 142.04) x 100 = 22.57%
• Calculate the total mass of O present in Na₂SO₄:
  4 O are present in the formula, mass = 4 x 16.00 = 64.00
• Calculate the percent by weight of O in Na₂SO₄:
  %O = (mass O ÷ MM) x 100 = (64.00 ÷ 142.04) x 100 = 45.06%

The answers above are probably correct if %Na + %S + %O = 100, that is,
32.37 + 22.57 + 45.06 = 100