Empirical and molecular formulae from analytical data — NEET & JEE Questions

Question 1 · difficulty L1 · recall

To obtain an empirical formula from elemental analysis, the elemental masses are first converted into

• A. moles
• B. volumes only
• C. atomic numbers
• D. densities

Question 2 · difficulty L2 · understanding

The molecular formula of a compound is

• A. an integer multiple of its empirical formula
• B. always identical to its empirical formula
• C. independent of molar mass
• D. obtained from mass percentages without mole conversion

Question 3 · difficulty L2 · understanding

In combustion analysis of a compound containing only C, H and O, oxygen in the original sample is often found by

• A. mass difference after determining C and H
• B. using all oxygen present in the produced $CO_2$ and $H_2O$
• C. weighing nitrogen gas
• D. counting carbon atoms only

Question 4 · difficulty L2 · understanding

If the mole ratio obtained from analysis is approximately C:H:O = 1:1.5:1, the next step is to

• A. multiply all ratios by 2 to get whole numbers
• B. round 1.5 to 2 only
• C. discard hydrogen
• D. divide all ratios by 2

Question 5 · difficulty L3 · application

A compound contains 40.0% C, 6.67% H and 53.33% O. What is its empirical formula?

• A. CHO
• B. $CH_2O$
• C. $C_2H_4O_2$
• D. $CH_4O$

Question 6 · difficulty L3 · application

A compound has empirical formula $CH_2O$ and molar mass 180 g mol$^{-1}$. What is its molecular formula?

• A. $CH_2O$
• B. $C_2H_4O_2$
• C. $C_6H_{12}O_6$
• D. $C_{12}H_{24}O_{12}$

Question 7 · difficulty L3 · application

Combustion of a compound gives 0.44 g $CO_2$ and 0.18 g $H_2O$. How many moles of carbon atoms and hydrogen atoms were present?

• A. 0.010 mol C and 0.020 mol H
• B. 0.010 mol C and 0.010 mol H
• C. 0.020 mol C and 0.010 mol H
• D. 0.44 mol C and 0.18 mol H

Question 8 · difficulty L3 · application

A 0.30 g compound containing only C, H and O gives 0.44 g $CO_2$ and 0.18 g $H_2O$. What is its empirical formula?

• A. CHO
• B. $CH_2O$
• C. $C_2H_2O$
• D. $CH_4O$

Question 9 · difficulty L4 · application

A compound contains 27.27% C and 72.73% O by mass. What is its empirical formula?

• A. CO
• B. $CO_2$
• C. $C_2O$
• D. $C_2O_3$

Question 10 · difficulty L4 · application

An empirical formula is $NO_2$ and the vapour-density-based molar mass is 92 g mol$^{-1}$. What is the molecular formula?

• A. $NO_2$
• B. $N_2O_4$
• C. $N_3O_6$
• D. $N_4O_8$