Chapter 9: Atomic Foundations of Matter
Q1. A student says, "An atom of oxygen and a molecule of oxygen are exactly the same because both represent oxygen." Which statement correctly identifies the error?
A. An oxygen atom contains two oxygen atoms.
B. An oxygen molecule contains two oxygen atoms chemically bonded together.
C. An oxygen atom is heavier than an oxygen molecule.
D. Oxygen molecules contain different elements.
Q2. Which of the following statements about atoms is scientifically correct?
A. All atoms of different elements have different masses.
B. Atoms of the same element always have identical masses.
C. Atoms are the smallest particles that retain the chemical identity of an element.
D. Atoms can be divided into smaller atoms through ordinary chemical reactions.
Q3. A sample of pure water is obtained from three different sources: a river, a glacier and a laboratory. Which statement must be true?
A. All three samples must have the same colour.
B. All three samples must contain the same mass of hydrogen per gram of water.
C. All three samples must contain the same number of water molecules.
D. All three samples must have identical temperatures.
Q4. Two compounds are formed using elements X and Y.
Compound A contains 12 g of X combined with 16 g of Y.
Compound B contains 12 g of X combined with 32 g of Y.
Which law explains this observation?
A. Law of conservation of mass
B. Law of constant proportions
C. Law of multiple proportions
D. Law of conservation of energy
Q5. A chemical reaction is carried out in an open container. The initial mass is 50 g, but after the reaction, the measured mass is 46 g. Which is the most scientifically reasonable explanation?
A. Four grams of matter were destroyed.
B. The law of conservation of mass is incorrect.
C. A gaseous product may have escaped into the surroundings.
D. Atoms lost their masses during the reaction.
Q6. Which statement about Dalton's atomic theory requires modification in the light of modern discoveries?
A. Matter is made up of atoms.
B. Atoms combine in simple whole-number ratios.
C. Atoms are indivisible and cannot be created or destroyed under any circumstances.
D. Chemical reactions involve rearrangement of atoms.
Q7. Why is the chemical formula of water H₂O rather than H₂O₂?
A. Water contains two hydrogen atoms and one oxygen atom per molecule.
B. Water contains twice as much oxygen as hydrogen by mass.
C. Hydrogen is always present in pairs in every compound.
D. Oxygen cannot form compounds with two hydrogen atoms.
Q8. Which of the following represents a molecule of an element but not a molecule of a compound?
A. H₂O
B. CO₂
C. O₃
D. NH₃
Q9. A student claims that one mole of oxygen atoms and one mole of oxygen molecules have equal masses. Is the claim correct?
A. Yes, because both contain oxygen.
B. Yes, because both contain the same number of particles.
C. No, because an oxygen molecule contains two oxygen atoms.
D. No, because oxygen atoms have different atomic masses in a molecule.
Q10. Which statement correctly distinguishes an element from a compound?
A. An element contains only one type of atom, whereas a compound contains two or more different elements chemically combined.
B. An element always exists as single atoms, whereas a compound always exists as molecules.
C. Elements can be separated by physical methods, whereas compounds cannot.
D. Every compound must contain exactly two elements.
Section 2: Chemical Formulae, Valency and Atomicity
Q11. An element X has valency 3, while element Y has valency 2. What will be the correct formula of their compound?
A. XY
B. X₂Y₃
C. X₃Y₂
D. XY₃
Q12. Which pair has the same total number of atoms in one formula unit or molecule?
A. H₂SO₄ and Ca(OH)₂
B. HNO₃ and H₂O
C. NH₃ and CO₂
D. CH₄ and H₂O₂
Q13. How many total atoms are present in three formula units of Al₂(SO₄)₃?
A. 15
B. 39
C. 51
D. 57
Q14. Which of the following contains the greatest number of oxygen atoms?
A. One molecule of O₃
B. One molecule of CO₂
C. One formula unit of CaCO₃
D. One formula unit of Al₂(SO₄)₃
Q15. An element forms an oxide with the formula X₂O₃. What is the valency of X?
A. 1
B. 2
C. 3
D. 6
Q16. Which of the following is correctly matched?
A. Sodium – Na₂
B. Magnesium – Mg₂
C. Chlorine – Cl₂
D. Helium – He₂
Q17. A compound has the formula Ca₃(PO₄)₂. How many phosphorus atoms and oxygen atoms are present in five formula units?
A. 10 phosphorus and 20 oxygen
B. 5 phosphorus and 20 oxygen
C. 10 phosphorus and 40 oxygen
D. 15 phosphorus and 40 oxygen
Q18. Which statement about polyatomic ions is correct?
A. They always contain atoms of only one element.
B. They are groups of atoms carrying an overall charge.
C. They cannot participate in chemical reactions.
D. They are always electrically neutral.
Section 3: Atomic Mass and Molecular Mass
Q19. The molecular mass of a compound X₂Y₃ is 100 u. If the atomic mass of X is 20 u, what is the atomic mass of Y?
A. 10 u
B. 15 u
C. 20 u
D. 30 u
Q20. Which of the following has the greatest molecular or formula mass?
A. H₂SO₄
B. CaCO₃
C. Na₂CO₃
D. MgSO₄
Q21. Calculate the molecular mass of glucose (C₆H₁₂O₆).
A. 120 u
B. 150 u
C. 180 u
D. 198 u
Q22. A molecule of a compound has a molecular mass of 44 u. Which of the following could be its molecular formula?
A. CO₂
B. CH₄
C. H₂O
D. NH₃
Q23. Which statement is correct about the atomic mass unit (u)?
A. It is defined as the mass of one hydrogen atom.
B. It is one-twelfth of the mass of one carbon-12 atom.
C. It is equal to the mass of one oxygen atom.
D. It is the mass of one electron.
Section 4: Advanced Numerical and Application-Based Questions
Q24. A sample of water contains 18 g of water molecules. Calculate the mass of hydrogen and oxygen present in it.
A. 2 g hydrogen and 16 g oxygen
B. 16 g hydrogen and 2 g oxygen
C. 9 g hydrogen and 9 g oxygen
D. 1 g hydrogen and 17 g oxygen
Q25. In a reaction, 12 g of carbon reacts completely with 32 g of oxygen to form carbon dioxide. What mass of carbon dioxide will be produced?
A. 20 g
B. 32 g
C. 44 g
D. 48 g
Q26. Two samples of carbon dioxide are collected. Sample A contains 3 g carbon and Sample B contains 9 g carbon. What is the ratio of oxygen mass in sample A to sample B?
A. 1 : 2
B. 1 : 3
C. 2 : 3
D. 3 : 1
Q27. A compound contains 40% carbon, 6.67% hydrogen and 53.33% oxygen by mass. What is its simplest empirical formula?
A. CHO
B. CH₂O
C. C₂H₄O₂
D. CH₄O
Q28. A 24 g sample of magnesium reacts completely with oxygen to form magnesium oxide. If 16 g of oxygen combines with it, what will be the mass of magnesium oxide formed?
A. 8 g
B. 24 g
C. 40 g
D. 48 g
Q29. Consider the following statements:
1. One molecule of H₂O contains three atoms.
2. One molecule of O₂ contains two atoms.
3. One formula unit of NaCl contains two atoms.
4. One molecule of CO₂ contains three atoms.
Which option contains only correct statements?
A. 1 and 2 only
B. 1, 2 and 3 only
C. 1, 2 and 4 only
D. All four statements
Q30. A student has two samples: Sample A contains 36 g of water (H₂O) and Sample B contains 44 g of carbon dioxide (CO₂). Which statement is correct?
A. Both samples contain equal numbers of molecules.
B. Sample A contains twice as many molecules as Sample B.
C. Sample B contains twice as many molecules as Sample A.
D. Both samples contain equal masses of oxygen.
SOLUTIONS
Answer Key
Q1 – B | Q2 – C | Q3 – B | Q4 – C | Q5 – C
Q6 – C | Q7 – A | Q8 – C | Q9 – C | Q10 – A
Q11 – B | Q12 – No correct option | Q13 – C | Q14 – D | Q15 – C
Q16 – C | Q17 – C | Q18 – B | Q19 – C | Q20 – D
Q21 – C | Q22 – A | Q23 – B | Q24 – A | Q25 – C
Q26 – B | Q27 – B | Q28 – C | Q29 – D | Q30 – A
Section 1: Detailed Solutions (Q1–Q10)
Q1. Answer: B
An oxygen atom (O) is a single atom. An oxygen molecule (O₂) consists of two oxygen atoms chemically bonded together.
Q2. Answer: C
An atom is the smallest unit of an element that retains its chemical identity. Atoms contain protons, neutrons and electrons.
Q3. Answer: B
According to the law of constant proportions, pure water always contains hydrogen and oxygen in a fixed mass ratio of 2:16, or 1:8.
Q4. Answer: C
The law of multiple proportions states that when two elements form multiple compounds, the masses of one element combining with a fixed mass of the other are in simple whole-number ratios.
16:32 = 1:2.
Q5. Answer: C
A gaseous product may escape from an open container. Matter is not destroyed. In a closed system, total mass remains conserved.
Q6. Answer: C
Atoms contain subatomic particles. Nuclear reactions can also change one type of atom into another. Therefore, Dalton's original statement about absolute indivisibility requires modification.
Q7. Answer: A
One water molecule contains two hydrogen atoms and one oxygen atom. Its formula is H₂O. The hydrogen-to-oxygen mass ratio is 2:16 = 1:8.
Q8. Answer: C
O₃ is ozone. It contains only oxygen atoms, so it is a molecule of an element, not a compound.
Q9. Answer: C
One mole of oxygen atoms has a mass of 16 g, whereas one mole of oxygen molecules (O₂) has a mass of 32 g.
Q10. Answer: A
An element contains only one type of atom. A compound contains two or more different elements chemically combined in a fixed proportion.
Section 2: Detailed Solutions (Q11–Q18)
Q11. Answer: B
Valency of X = 3 and Y = 2. Applying the criss-cross method gives X₂Y₃.
Q12. Answer: No correct option
H₂SO₄ = 2 + 1 + 4 = 7 atoms.
Ca(OH)₂ = 1 + 2 + 2 = 5 atoms.
HNO₃ = 5 atoms; H₂O = 3 atoms.
NH₃ = 4 atoms; CO₂ = 3 atoms.
CH₄ = 5 atoms; H₂O₂ = 4 atoms.
Therefore, none of the given pairs has an equal number of atoms. The question needs correction.
Q13. Answer: C
Al₂(SO₄)₃ contains 2 aluminium, 3 sulphur and 12 oxygen atoms.
Total = 2 + 3 + 12 = 17 atoms.
For three formula units = 17 × 3 = 51 atoms.
Q14. Answer: D
O₃ contains 3 oxygen atoms, CO₂ contains 2, CaCO₃ contains 3, and Al₂(SO₄)₃ contains 12 oxygen atoms.
Q15. Answer: C
In X₂O₃, oxygen has valency 2. Therefore, the valency of X is 3.
Q16. Answer: C
Chlorine normally exists as a diatomic molecule, Cl₂. Sodium is Na, magnesium is Mg and helium is He.
Q17. Answer: C
Ca₃(PO₄)₂ contains 2 phosphorus and 8 oxygen atoms.
In five formula units:
Phosphorus = 2 × 5 = 10.
Oxygen = 8 × 5 = 40.
Q18. Answer: B
Polyatomic ions are groups of atoms carrying an overall electrical charge. Examples include OH⁻, SO₄²⁻, NO₃⁻ and NH₄⁺.
Section 3: Detailed Solutions (Q19–Q23)
Q19. Answer: C
Given: X₂Y₃ = 100 u and X = 20 u.
Let atomic mass of Y = x.
2(20) + 3x = 100
40 + 3x = 100
3x = 60
x = 20 u.
Q20. Answer: D
H₂SO₄ = 2 + 32 + 64 = 98 u.
CaCO₃ = 40 + 12 + 48 = 100 u.
Na₂CO₃ = 46 + 12 + 48 = 106 u.
MgSO₄ = 24 + 32 + 64 = 120 u.
Therefore, MgSO₄ has the greatest formula mass.
Q21. Answer: C
Glucose = C₆H₁₂O₆.
Molecular mass = 6(12) + 12(1) + 6(16).
= 72 + 12 + 96 = 180 u.
Q22. Answer: A
CO₂ = 12 + 2(16) = 44 u.
The other compounds have masses of 16 u, 18 u and 17 u respectively.
Q23. Answer: B
One atomic mass unit (u) is defined as one-twelfth of the mass of one carbon-12 atom.
Section 4: Detailed Solutions (Q24–Q30)
Q24. Answer: A
Molecular mass of H₂O = 2(1) + 16 = 18 u.
Therefore, 18 g water contains 2 g hydrogen and 16 g oxygen.
Q25. Answer: C
C + O₂ → CO₂
Mass of carbon = 12 g.
Mass of oxygen = 32 g.
Mass of CO₂ = 12 + 32 = 44 g.
Q26. Answer: B
Carbon dioxide has a carbon-to-oxygen mass ratio of 12:32 = 3:8.
Sample A: 3 g carbon combines with 8 g oxygen.
Sample B: 9 g carbon combines with 24 g oxygen.
Ratio = 8:24 = 1:3.
Q27. Answer: B
Assume 100 g of compound.
Moles of carbon = 40/12 = 3.33.
Moles of hydrogen = 6.67/1 = 6.67.
Moles of oxygen = 53.33/16 = 3.33.
Divide by the smallest value:
C:H:O = 1:2:1.
Empirical formula = CH₂O.
Q28. Answer: C
According to the law of conservation of mass:
Mass of magnesium oxide = Mass of magnesium + Mass of oxygen.
= 24 + 16 = 40 g.
Q29. Answer: D
H₂O = 3 atoms.
O₂ = 2 atoms.
NaCl = 2 ions in one formula unit, representing two atoms in the simplest ionic formula ratio.
CO₂ = 3 atoms.
All four statements are correct.
Q30. Answer: A
Molar mass of water = 18 g/mol.
Moles of water = 36/18 = 2 mol.
Molar mass of CO₂ = 44 g/mol.
Moles of CO₂ = 44/44 = 1 mol.
Correction: Sample A contains twice as many molecules as Sample B. Therefore, the correct answer is B.
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