Question

Calculate the percentage of oxygen in calcium hydroxide.

Answer 1

• The percentage composition of a compound is the percentage weight of each component element present in it.
• Mathematically the percentage of an element in a compound

= $\frac{\mathrm{Total}\mathrm{weight}\mathrm{of}\mathrm{element}\mathrm{present}\mathrm{in}\mathrm{the}\mathrm{compound}}{\mathrm{Molecular}\mathrm{weight}\mathrm{of}\mathrm{the}\mathrm{compound}}$ × 100

• To find the percentage of oxygen(O) in calcium hydroxide(Ca(OH)₂),

First, determine the atomic mass of component elements in calcium hydroxide(Ca(OH)₂).

The components present in 1 mole of Ca(OH)₂ is, 1atom of calcium(Ca), 2 atoms of oxygen(O), and, 2 atoms of hydrogen(H).

1atom of calcium(Ca) with atomic mass = 40g

1 atom of oxygen(O) with atomic mass = 16g, since there are 2 atoms of oxygen, atomic weight = 2×16 = 32g

1 atom of hydrogen(H) with atomic mass =1 g, since there are 2 atoms of hydrogen, atomic weight = 2×1 = 2g

Therefore the relative molecular mass of calcium hydroxide(Ca(OH)₂) = Total weight of component elements

= 40 + 32 + 2 = 74g/mol

• The percentage of Oxygen(O) in Calcium hydroxide(Ca(OH)₂) = $\frac{\mathrm{Total}\mathrm{weight}\mathrm{of}\mathrm{element}\mathrm{present}\mathrm{in}\mathrm{the}\mathrm{compound}}{\mathrm{Molecular}\mathrm{weight}\mathrm{of}\mathrm{the}\mathrm{compound}}$ × 100

= $\frac{\mathrm{Total}\mathrm{weight}\mathrm{of}\mathrm{Oxygen}}{\mathrm{Molecular}\mathrm{weight}\mathrm{of}\mathrm{Calcium}\mathrm{hydroxide}}$ × 100

= $\frac{32}{74}$ × 100 = 43.24%

Hence the percentage of Oxygen(O) in Calcium hydroxide(Ca(OH)₂) is 43.24%.