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Some Basic Concepts of Chemistry: questions with answers

Class 11 Chemistry, Chapter 1 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Chemistry, matter, states, and classification

Question 1

Air has a uniform but variable composition. How is it classified?

  1. (a)Element
  2. (b)Heterogeneous mixture
  3. (c)Pure compound
  4. (d)Homogeneous mixture
Show answer

(d) Homogeneous mixture

Uniform distribution makes air homogeneous, while variable composition makes it a mixture.

Question 2

O₂ contains only oxygen atoms joined in pairs. How is O₂ classified?

  1. (a)Homogeneous mixture
  2. (b)Element
  3. (c)Heterogeneous mixture
  4. (d)Compound because each particle has two atoms
Show answer

(b) Element

An element contains one kind of atom even when its particles are diatomic molecules.

Measurement, SI units, uncertainty, and dimensional analysis

Question 3

Which unit factor converts 3.0 in to centimetres using 1 in = 2.54 cm?

  1. (a)2.54 in/1 cm
  2. (b)1 cm/2.54 in
  3. (c)2.54 cm/1 in
  4. (d)1 in/2.54 cm
Show answer

(c) 2.54 cm/1 in

The inch unit must be in the denominator so it cancels.

Question 4

Using 1 L = 10⁻³ m³, what is 2.0 L in cubic metres?

  1. (a)2.0 m³
  2. (b)2.0 × 10⁻⁶ m³
  3. (c)2.0 × 10⁻³ m³
  4. (d)2.0 × 10³ m³
Show answer

(c) 2.0 × 10⁻³ m³

Multiply 2.0 L by 10⁻³ m³/L; litres cancel.

Laws of chemical combination and Dalton's atomic theory

Question 5

Which Dalton postulate directly supports conservation of mass in a chemical reaction?

  1. (a)Atoms change into different elements
  2. (b)Each reactant keeps its original mass
  3. (c)Atoms are rearranged but are neither created nor destroyed
  4. (d)Equal gas volumes have equal masses
Show answer

(c) Atoms are rearranged but are neither created nor destroyed

Rearrangement without creation or destruction preserves the total number of each type of atom.

Question 6

Which observation was not explained by Dalton's original atomic theory?

  1. (a)Multiple proportions by mass
  2. (b)Simple whole-number ratios of reacting gas volumes
  3. (c)Fixed composition of a compound
  4. (d)Conservation of total mass
Show answer

(b) Simple whole-number ratios of reacting gas volumes

Dalton's postulates explained the mass laws but not gaseous-volume relationships.

Atomic, average atomic, molecular, and formula mass

Question 7

An element has isotopes of mass 35 u (75%) and 37 u (25%). What is its average atomic mass?

  1. (a)35.5 u
  2. (b)35.0 u
  3. (c)36.0 u
  4. (d)72.0 u
Show answer

(a) 35.5 u

Use the abundance-weighted sum: 35(0.75) + 37(0.25) = 35.5 u.

Question 8

An element has isotopes of mass 10 u and 11 u, and its average atomic mass is 10.2 u. Which isotope is more abundant?

  1. (a)Abundance cannot affect average atomic mass
  2. (b)The 10 u isotope
  3. (c)Both isotopes are equally abundant
  4. (d)The 11 u isotope
Show answer

(b) The 10 u isotope

The weighted average lies much nearer 10 u, so the 10 u isotope is more abundant.

Mass percent, mole fraction, molarity, molality, and dilution

Question 9

A solution contains 2.0 g solute and 18.0 g water. What is the solute mass percent?

  1. (a)11.1%
  2. (b)90%
  3. (c)0.10%
  4. (d)10%
Show answer

(d) 10%

Mass percent = 2.0/(2.0 + 18.0) × 100 = 10%.

Question 10

A solution contains 2 mol A and 3 mol B. What is the mole fraction of A?

  1. (a)2.00
  2. (b)0.67
  3. (c)0.60
  4. (d)0.40
Show answer

(d) 0.40

xA = nA ÷ (nA + nB) = 2 ÷ 5 = 0.40.

Mole, Avogadro constant, entities, and molar mass

Question 11

How many moles are in 36 g H₂O if its molar mass is 18 g mol⁻¹?

  1. (a)2.0 mol
  2. (b)18 mol
  3. (c)0.50 mol
  4. (d)648 mol
Show answer

(a) 2.0 mol

n = m ÷ M = 36 ÷ 18 = 2.0 mol.

Question 12

What mass is 0.50 mol CO₂ if its molar mass is 44 g mol⁻¹?

  1. (a)0.011 g
  2. (b)44 u
  3. (c)22 g
  4. (d)88 g
Show answer

(c) 22 g

m = nM = 0.50 × 44 = 22 g.

Mass composition, empirical formula, and molecular formula

Question 13

Using H = 1 and O = 16, what is the mass percent of hydrogen in H₂O?

  1. (a)11.1%
  2. (b)88.9%
  3. (c)12.5%
  4. (d)5.56%
Show answer

(a) 11.1%

Hydrogen contributes 2 of the 18 mass units: (2 ÷ 18) × 100 = 11.1%.

Balanced equations and stoichiometric calculation

Question 14

What coefficient of O₂ balances P₄(s) + O₂(g) → P₄O₁₀(s)?

  1. (a)10
  2. (b)4
  3. (c)1
  4. (d)5
Show answer

(d) 5

Five O₂ molecules supply the ten oxygen atoms in P₄O₁₀.

Limiting reagent, excess reagent, and product yield

Question 15

For 2A + B₂ → 2AB, a mixture contains 3 mol A and 2 mol B₂. Which reactant is limiting?

  1. (a)A
  2. (b)Neither; coefficients cannot compare moles
  3. (c)B₂ because 2 is less than 3
  4. (d)Both reactants are fully consumed
Show answer

(a) A

Two moles B₂ would require 4 mol A, but only 3 mol A is available.

Common misconception in Some Basic Concepts of Chemistry

Open full misconception guide →

Do equal masses contain equal numbers of molecules?

Wrong

18 g of water and 18 g of CO₂ contain the same number of molecules.

Why it feels right. The samples have the same mass on a scale.

Right

The number of molecules depends on the amount in moles, which is mass divided by molar mass.

Worked example

H₂O6.02 × 10²³
CO₂2.46 × 10²³
Both samples weigh 18 g. The water sample has about 2.4 times as many molecules.
  1. 18 g of water is about 1 mol, or about 6.02 × 10²³ molecules.
  2. 18 g of CO₂ is about 18 ÷ 44 = 0.41 mol, or about 2.46 × 10²³ molecules.