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

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%