Question

The percent composition of all elements in HClO3 is:

a. 2% H, 20% Cl, 78% O
b. 1% H, 42% Cl, 57% O
c. 2% H, 68% Cl, 30% O
d. None of the above

Answer 1

The answer is b. 1% H, 42% Cl, 57% O.

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%