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Some Basic Concepts of Chemistry MCQs
Class 11 Chemistry — questions with answers and worked explanations.
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These questions are drawn from the Pariksha Sutra question bank for Some Basic Concepts of Chemistry,
part of the Class 11 Chemistry syllabus. Each one shows the correct answer and,
where a method helps, the working behind it.
Read the question, decide your answer before looking, then check the explanation — that is what turns practice
into marks. If a question catches you out, the explanation is the part worth re-reading.
Questions
Question 1
The law of conservation of mass states that in a chemical reaction:
- Mass is neither created nor destroyed
- Mass is converted into energy
- Mass increases with temperature
- Mass of products is always greater than reactants
Answer: Mass is neither created nor destroyed
Question 2
The number of molecules in 2 moles of CO2 is:
- 12.044 x 10^24
- 3.011 x 10^23
- 6.022 x 10^23
- 1.2044 x 10^24
Answer: 1.2044 x 10^24
Question 3
The molar mass of ammonium sulphate (NH4)2SO4 is:
- 98 g/mol
- 114 g/mol
- 150 g/mol
- 132 g/mol
Answer: 132 g/mol
Question 4
The empirical formula of a compound containing C:H:O in mole ratio 1:2:1 is:
- CH4O
- CHO
- CH2O
- C2H4O2
Answer: CH2O
Question 5
In N2 + 3H2 -> 2NH3, if 3 mol N2 react with 3 mol H2, the limiting reagent is:
- N2
- NH3
- H2
- Both
Answer: H2
Question 6
The number of atoms in exactly 12 g of carbon-12 is:
- 3.011 x 10^23
- 12 x 10^23
- 1.204 x 10^24
- 6.022 x 10^23
Answer: 6.022 x 10^23
Question 7
The mass of 0.2 mole of NaOH (molar mass 40 g/mol) is:
- 20 g
- 8 g
- 40 g
- 4 g
Answer: 8 g
Question 8
A compound has empirical formula CH2 and molar mass 42 g/mol. Its molecular formula is:
- C3H6
- C4H8
- CH2
- C2H4
Answer: C3H6
Question 9
In CaCO3 -> CaO + CO2, the mass of CaO obtained by decomposing 100 g of CaCO3 is:
- 44 g
- 100 g
- 40 g
- 56 g
Answer: 56 g
Question 10
The sum of mole fractions of all components in a solution is always:
- 100
- 1
- Equal to molarity
- 0
Answer: 1
Question 11
Which law is illustrated by CO and CO2, where masses of oxygen combining with a fixed mass of carbon are in the ratio 1:2?
- Law of definite proportions
- Gay-Lussac's law
- Law of multiple proportions
- Law of conservation of mass
Answer: Law of multiple proportions
Question 12
The number of moles in 11.2 L of any ideal gas at STP is:
- 22.4 mol
- 0.5 mol
- 2 mol
- 1 mol
Answer: 0.5 mol
Question 13
The number of moles present in 8 g of oxygen gas (O2) is:
- 0.5 mol
- 1 mol
- 0.25 mol
- 8 mol
Answer: 0.25 mol
Question 14
Benzene has molecular formula C6H6. Its empirical formula is:
- CH2
- C2H2
- CH
- C6H6
Answer: CH
Question 15
For the reaction 2Mg + O2 -> 2MgO, the moles of O2 needed to react completely with 0.5 mol Mg are:
- 0.25 mol
- 0.125 mol
- 0.5 mol
- 1 mol
Answer: 0.25 mol
Question 16
The law of definite proportions states that a given compound always contains elements in a fixed:
- Number of atoms per gram
- Energy content
- Ratio by volume
- Ratio by mass
Answer: Ratio by mass
Question 17
The total number of atoms in 1 mole of NH3 is:
- 3 x 6.022 x 10^23
- 6.022 x 10^23
- 4 x 6.022 x 10^23
- 2 x 6.022 x 10^23
Answer: 4 x 6.022 x 10^23
Question 18
The gram atomic mass concept means 1 mole of sodium atoms weighs:
- 1 g
- 23 g
- 46 g
- 11 g
Answer: 23 g
Question 19
A compound has empirical formula HO and molar mass 34 g/mol. Its molecular formula is:
- H2O
- H2O2
- H4O2
- HO
Answer: H2O2
Question 20
In the reaction 2H2 + O2 -> 2H2O, the volume of O2 at STP needed to form 36 g of water is:
- 22.4 L
- 11.2 L
- 5.6 L
- 44.8 L
Answer: 22.4 L
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