Percentage Composition | MCQs

Explanation

Water (H₂O) is composed of 2 hydrogen atoms (1g each) and 1 oxygen atom (16g) → total molar mass = 18g.

Percentage of oxygen = (16/18) × 100 = 88.8% by mass.

Explanation

Molar mass of nitrogen (N): 14.01 g/mol 

Molar mass of hydrogen (H): 1.01 g/mol (3 H atoms in NH3) 

Molar mass of ammonia (NH3): 14.01 + (3 * 1.01) = 17.04 g/mol 

Percentage of nitrogen in NH3 = (14.01 / 17.04) * 100 = 82.23% 

This is very close to the provided option of 82.35%, which can be expressed as 14/17 * 100. 

Explanation

The percentage by mass of an element in a compound is calculated using the formula:

Percentage by mass = (Mass of element in 1 mole of compound / Molar mass of compound) x 100