How To Work Out Percentage Mass

Formula: Percentage mass = (Mass of element / Total mass of compound) x 100%

Steps:

• Determine the mass of the element in the compound.

• Find the total mass of the compound.

• Substitute values into the formula and calculate.

Examples:

Compound: H2O

Mass of H = 1g

Mass of H = 1g

Mass of O = 16g

Total mass of H2O = 18g

Percentage mass of H = (2 / 18) x 100 = 11.11%

Percentage mass of O = (16 / 18) x 100 = 88.89%

1. Oxygen (O) in magnesium oxide (MgO):

   • Molar mass of MgO = Molar mass of Mg + Molar mass of O

   • Mg = 24 g/mol, O = 16 g/mol

   • Molar mass of MgO = 24 + 16 = 40 g/mol

   • % O in MgO = (mass of O / molar mass of MgO) × 100

   • % O in MgO = (16 / 40) × 100 = 40%

2. Sodium (Na) in sodium hydroxide (NaOH):

   • Molar mass of NaOH = Molar mass of Na + Molar mass of O + Molar mass of H

   • Na = 23 g/mol, O = 16 g/mol, H = 1 g/mol

   • Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol

   • % Na in NaOH = (mass of Na / molar mass of NaOH) × 100

   • % Na in NaOH = (23 / 40) × 100 = 57.5%

3. Hydrogen (H) in water (H₂O):

   • Molar mass of H₂O = 2 × Molar mass of H + Molar mass of O

   • H = 1 g/mol, O = 16 g/mol

   • Molar mass of H₂O = 2 × 1 + 16 = 18 g/mol

   • % H in H₂O = (mass of H / molar mass of H₂O) × 100

   • % H in H₂O = (2 / 18) × 100 = 11.1%