Questions & explanations
1. Which of the following statements correctly defines a mole?
- A) A mole is the amount of substance that contains as many elementary entities as the number of atoms in exactly 12 g of carbon-12.
- B) A mole is the mass of 6.022×10^23 atoms of any element.
- C) A mole is the volume occupied by 6.022×10^23 molecules of any gas at STP.
- D) A mole is the number of atoms present in 1 g of any element.
Answer: A) A mole is the amount of substance that contains as many elementary entities as the number of atoms in exactly 12 g of carbon-12.
The mole is defined as the amount of substance containing exactly 6.022×10^23 elementary entities (Avogadro's number), which is the number of atoms in 12 g of carbon-12. Option B is incorrect because molar mass varies; option C refers to molar volume, which is a consequence but not the definition; option D is false as 1 g contains far fewer atoms.
2. How many significant figures are present in the number 0.002030?
- 4
- 5
- 3
- 6
Answer: 4
Leading zeros are not significant, but all non-zero digits and zeros between non-zero digits or after a decimal point (trailing zeros) are significant. In 0.002030, the digits '2', '0', '3', '0' are significant (the zero after 3 is significant because it is after the decimal and after a non-zero digit). Therefore, it has 4 significant figures.
3. For an ionic compound such as sodium chloride (NaCl), the sum of the atomic masses of the atoms in its formula unit is called its:
- molecular mass
- formula mass
- atomic mass
- gram molecular mass
Answer: formula mass
Ionic compounds do not consist of discrete molecules; therefore, the term 'molecular mass' is not used. Instead, 'formula mass' refers to the sum of the atomic masses of all atoms in one formula unit of the compound. 'Atomic mass' applies to a single element, and 'gram molecular mass' is the mass in grams of one mole of a molecular substance.
4. Neon has two naturally occurring isotopes: ²⁰Ne (mass = 19.99 u, abundance = 90.48%) and ²²Ne (mass = 21.99 u, abundance = 9.52%). What is the average atomic mass of neon?
- 20.18 u
- 20.99 u
- 21.00 u
- 19.99 u
Answer: 20.18 u
The average atomic mass is calculated as the weighted average of the isotopic masses: (0.9048 × 19.99) + (0.0952 × 21.99) = 18.077 + 2.094 = 20.171 ≈ 20.18 u. Option B (20.99 u) is the mass of ²²Ne, option C (21.00 u) is not a weighted result, and option D (19.99 u) is the mass of ²⁰Ne only.
5. What is the percentage by mass of oxygen in sulfuric acid (H₂SO₄)? (Atomic masses: H = 1 u, S = 32 u, O = 16 u)
- 65.3%
- 32.7%
- 16.0%
- 80.0%
Answer: 65.3%
Molar mass of H₂SO₄ = (2×1) + 32 + (4×16) = 2 + 32 + 64 = 98 g/mol. Mass of oxygen = 64 g/mol. Percentage = (64/98) × 100 = 65.306% ≈ 65.3%. Option B (32.7%) corresponds to sulfur, option C (16.0%) to a single oxygen atom, and option D (80.0%) is not correct.
6. In water, the mass ratio of hydrogen to oxygen is always 1:8. Which of the following samples of pure water obeys this law?
- A) 9 g water containing 1 g hydrogen and 8 g oxygen
- B) 10 g water containing 2 g hydrogen and 8 g oxygen
- C) 18 g water containing 2 g hydrogen and 16 g oxygen
- D) Both A and C
Answer: D) Both A and C
Law of constant proportions states that a chemical compound always contains the same elements in fixed proportion by mass. In water, the mass ratio H:O = 2:16 = 1:8. Both samples in A and C show this ratio, while B does not. Option D is therefore correct.
7. A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula? (Atomic masses: C = 12 u, H = 1 u, O = 16 u)
- CH₂O
- C₂H₄O₂
- C₃H₆O₃
- CHO
Answer: CH₂O
Divide each percentage by atomic mass: C = 40.0/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. Divide by the smallest (3.33) → C = 1, H = 2, O = 1. Empirical formula = CH₂O. Options B and C are multiples of CH₂O, and option D has incorrect ratio.
8. What is the definition of the unified atomic mass unit (u)?
- 1/12th the mass of a carbon-12 atom
- 1/16th the mass of an oxygen-16 atom
- the mass of one proton
- the mass of one hydrogen atom
Answer: 1/12th the mass of a carbon-12 atom
The unified atomic mass unit (u) is defined as exactly one-twelfth (1/12) of the mass of an unbound neutral atom of carbon-12 at rest. The other options refer to historical or approximate scales but are not the accepted SI definition.
9. Which of the following statements correctly distinguishes precision and accuracy in measurements?
- Precision is the closeness of a measurement to the true value, while accuracy is the reproducibility of the measurement.
- Accuracy is the closeness of a measurement to the true value, while precision is the reproducibility of the measurement.
- Precision and accuracy are synonymous terms.
- Precision is determined by systematic errors, while accuracy is determined by random errors.
Answer: Accuracy is the closeness of a measurement to the true value, while precision is the reproducibility of the measurement.
Accuracy refers to how close a measured value is to the accepted/true value. Precision refers to how close repeated measurements are to each other (reproducibility). The other options incorrectly swap or combine the definitions.
10. 4.0 g of hydrogen reacts completely with 32.0 g of oxygen to form water. What mass of water is produced according to the law of conservation of mass?
- A) 28.0 g
- B) 36.0 g
- C) 32.0 g
- D) 40.0 g
Answer: B) 36.0 g
The reaction is 2H₂ + O₂ → 2H₂O. Law of conservation of mass states total mass of reactants = total mass of products. Reactants: 4.0 g H₂ + 32.0 g O₂ = 36.0 g. Thus water produced = 36.0 g. Options A, C, D are incorrect.
11. How many oxygen atoms are present in 9.0 g of water (H₂O)? (Atomic masses: H=1, O=16, Avogadro's number = 6×10²³)
- A) 3.0×10²³
- B) 6.0×10²³
- C) 1.5×10²³
- D) 9.0×10²³
Answer: A) 3.0×10²³
Molar mass of H₂O = 18 g/mol. 9.0 g = 0.5 mol. Each molecule of H₂O contains 1 oxygen atom, so number of oxygen atoms = 0.5 × NA = 0.5×6×10²³ = 3.0×10²³. Option B is for 1 mol; C is for 0.25 mol; D is for 1.5 mol.
12. The mole fraction of glucose in an aqueous solution is 0.2. The molality of the solution (in mol kg⁻¹) is: (Molar mass of water = 18 g mol⁻¹)
- 13.89
- 1.39
- 2.78
- 0.14
Answer: 13.89
Mole fraction of glucose = 0.2 → water mole fraction = 0.8. In 1 mole of solution, moles of water = 0.8, mass of water = 0.8 × 18 = 14.4 g = 0.0144 kg. Moles of glucose = 0.2. Molality = 0.2 / 0.0144 ≈ 13.89 m.