Let's first calculate a molar mass (M) of HClO₃. Molar mass is a mass of 1 mole of a substance. It can be expressed as the sum of relative atomic masses (Ar), which are masses of atoms of the elements: M(HClO₃) = Ar(H) + Ar(Cl) + 3Ar(O)
Ar(H) = 1 u Ar(Cl) = 35.5 u Ar(O) = 16 u M(HClO₃) = Ar(H) + Ar(Cl) + 3Ar(O) = 1 + 35.5 + 3 · 16 = 84.5 u
Now, we just need to calculate mass percent of each element using the proportion: Ar : x = M : 100% ⇒ x = Ar ÷ M · 100%
Hydrogen: x = Ar(H) ÷ M · 100% = 1 ÷ 84.5 · 100% = 1.2% ≈ 1% Chlorine: x = Ar(Cl) ÷ M · 100% = 35.5 ÷ 84.5 · 100% = 42% Oxygen: x = 3Ar(O) ÷ M · 100% = 48 ÷ 84.5 · 100% = 56.8% ≈ 57%