Percentage composition; empirical and molecular formulae — NEET & JEE Questions

Question 1 · difficulty L1 · recall

The percentage by mass of an element in a compound is calculated as

• A. (mass of element in one mole of compound / molar mass of compound) × 100
• B. (molar mass of compound / mass of element in one mole) × 100
• C. mass of element × molar mass of compound
• D. moles of compound / moles of element

Question 2 · difficulty L2 · understanding

A compound contains carbon and oxygen in the mole ratio C:O = 1:2. What is its empirical formula?

• A. CO
• B. CO2
• C. C2O
• D. C2O4

Question 3 · difficulty L2 · understanding

The empirical formula of a compound is $CH_2O$. Which information is additionally required to determine its molecular formula?

• A. Its colour
• B. Its molar mass
• C. Its melting point only
• D. Its physical state

Question 4 · difficulty L2 · understanding

A compound has empirical formula $NO_2$ and molar mass $92\,\text{g mol}^{-1}$. What is its molecular formula?

• A. NO2
• B. N2O4
• C. N3O6
• D. N4O8

Question 5 · difficulty L3 · application

What is the mass percentage of oxygen in water, $H_2O$? Use H = 1 and O = 16.

• A. 11.1%
• B. 16.0%
• C. 88.9%
• D. 94.1%

Question 6 · difficulty L3 · application

A compound contains 40.0% carbon, 6.67% hydrogen and 53.33% oxygen by mass. What is its empirical formula? Use C = 12, H = 1 and O = 16.

• A. CHO
• B. CH2O
• C. C2H4O2
• D. CH4O

Question 7 · difficulty L3 · application

A compound has empirical formula $CH_2$ and molar mass $56\,\text{g mol}^{-1}$. What is its molecular formula?

• A. CH2
• B. C2H4
• C. C3H6
• D. C4H8

Question 8 · difficulty L3 · application

What is the mass percentage of nitrogen in $NH_4NO_3$? Use N = 14, H = 1 and O = 16.

• A. 17.5%
• B. 28.0%
• C. 35.0%
• D. 70.0%

Question 9 · difficulty L4 · application

An oxide contains 69.9% iron and 30.1% oxygen by mass. Which empirical formula is closest? Use Fe = 55.85 and O = 16.

• A. FeO
• B. Fe2O3
• C. Fe3O4
• D. FeO2

Question 10 · difficulty L4 · application

A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its molar mass is $56\,\text{g mol}^{-1}$. What is its molecular formula? Use C = 12 and H = 1.

• A. CH2
• B. C2H4
• C. C3H6
• D. C4H8