Some Basic Concepts of Chemistry

11 NEET-UG practice questions on Some Basic Concepts of Chemistry, part of Chemistry. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 10 mL mixture of CH₄ and C₂H₆ burns completely in excess oxygen and gives 14 mL of CO₂. All volumes are at the same temperature and pressure. Find the volume of O₂ consumed.
1. A35 mL
2. B26 mL
3. C20 mL
4. D28 mL
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Answer: B · 26 mL
Let CH₄ = x and C₂H₆ = y: x + y = 10 and x + 2y = 14, so y = 4 mL and x = 6 mL. Oxygen used = 6 × 2 + 4 × 3.5 = 26 mL. Check by atoms: 14 CO₂ need 28 O, and 48 H give 24 H₂O needing 24 O, a total of 52 O = 26 O₂.
- A: This treats the whole mixture as C₂H₆.
- C: This treats the whole mixture as CH₄.
- D: This counts the oxygen in CO₂ only and forgets the oxygen in the water formed.
Remember this
At the same temperature and pressure, gas volumes combine in the mole ratio of the equation (Gay-Lussac's law).
Question 2 · Medium
Ethanol (23 g) is dissolved in water (54 g). What is the mole fraction of ethanol in this solution? (C₂H₅OH = 46, H₂O = 18)
1. A0.143
2. B0.167
3. C0.857
4. D0.299
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Answer: A · 0.143
Ethanol = 23/46 = 0.5 mol and water = 54/18 = 3 mol. Mole fraction of ethanol = 0.5/(0.5 + 3) = 1/7 = 0.143.
- B: This divides by the moles of water alone, not the total moles.
- C: This is the mole fraction of water.
- D: This is the mass fraction of ethanol, 23/77.
Remember this
Mole fraction divides by the total moles of all components, and the fractions in a solution add up to 1.
Question 3 · Easy
Which concentration term of a solution changes with temperature?
1. AMolality
2. BMolarity
3. CMass per cent
4. DMole fraction
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Answer: B · Molarity
Molarity is moles per litre of solution, and the volume of a solution changes with temperature. The other three are built only from masses or moles, which do not change on heating.
- A: Molality uses the mass of solvent, which does not change with temperature.
- C: Mass per cent is a ratio of masses, which does not change with temperature.
- D: Mole fraction is a ratio of moles, which does not change with temperature.
Remember this
A concentration term that has volume in it depends on temperature; a term based only on mass or moles does not.
Question 4 · Medium
An organic liquid contains 54.5% C, 9.1% H and 36.4% O by mass, and its molar mass is 88 g/mol. Identify its molecular formula. (C = 12, H = 1, O = 16)
1. AC₅H₁₂O
2. BC₃H₄O₃
3. CC₄H₈O₂
4. DC₂H₄O
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Answer: C · C₄H₈O₂
Dividing by atomic masses gives C 4.54, H 9.1 and O 2.275, a ratio of 2:4:1, so the empirical formula is C₂H₄O with mass 44. Since 88/44 = 2, the molecular formula is C₄H₈O₂.
- A: This also has mass 88 but does not fit the 2:4:1 atom ratio.
- B: This has mass 88 but does not fit the 2:4:1 atom ratio.
- D: This is the empirical formula, with mass 44 rather than 88.
Remember this
Molecular formula = empirical formula × (molar mass / empirical formula mass).
Question 5 · Hard
An aqueous NaOH solution is 2 M and has a density of 1.08 g/mL. Calculate its molality. (NaOH = 40)
1. A1.85 m
2. B2.00 m
3. C1.72 m
4. D2.16 m
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Answer: B · 2.00 m
One litre of solution has a mass of 1080 g and contains 2 × 40 = 80 g of NaOH, so the water is 1000 g = 1 kg. Molality = 2 mol / 1 kg = 2.00 m. Check by formula: m = M/(d − M × molar mass/1000) = 2/(1.08 − 0.08) = 2.
- A: This divides by the solution mass, forgetting to remove the solute.
- C: This adds the solute mass to the solution mass instead of subtracting it.
- D: This multiplies molarity by density instead of finding the solvent mass.
Remember this
For molality, subtract the solute mass from the solution mass; the density alone does not give the solvent mass.
Question 6 · Medium
Arrange the following samples in increasing order of the total number of atoms present. (H = 1, He = 4, N = 14, O = 16) A. 4 g of He B. 9 g of H₂O C. 28 g of N₂ D. 8 g of O₃
1. AD, B, A, C
2. BC, B, A, D
3. CA, D, B, C
4. DD, A, B, C
Answer and explanation in LearnFree account
Question 7 · Easy
A sample of methane, CH₄, has a mass of 8 g. How many moles of hydrogen atoms does it contain? (C = 12, H = 1)
1. A2 mol
2. B0.5 mol
3. C1 mol
4. D4 mol
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Question 8 · Easy
Count the significant figures in the measured length 0.02040 m.
1. A6
2. B3
3. C5
4. D4
Answer and explanation in LearnFree account
Question 9 · Medium
Given below are two statements: Statement I: The mole fraction of a component in a solution has no units and does not depend on temperature. Statement II: The molarity of an aqueous solution increases when the solution is warmed. Choose the correct answer.
1. AStatement I is incorrect but Statement II is correct
2. BStatement I is correct but Statement II is incorrect
3. CBoth Statement I and Statement II are correct
4. DBoth Statement I and Statement II are incorrect
Answer and explanation in LearnFree account
Question 10 · Medium
Aluminium (10.8 g) is heated with chlorine (21.3 g) until the reaction stops: 2Al + 3Cl₂ → 2AlCl₃ What mass of AlCl₃ forms? (Al = 27, Cl = 35.5)
1. A26.7 g
2. B53.4 g
3. C32.1 g
4. D40.05 g
Answer and explanation in LearnFree account
Question 11 · Medium
Butane burns completely in oxygen: a C₄H₁₀ + b O₂ → c CO₂ + d H₂O For the balanced equation in the lowest whole numbers, what are a, b, c and d, in that order?
1. A2, 13, 8, 10
2. B2, 13, 8, 5
3. C1, 9, 4, 5
4. D1, 13, 4, 5
Answer and explanation in LearnFree account

Some Basic Concepts of Chemistry

A 10 mL mixture of CH₄ and C₂H₆ burns completely in excess oxygen and gives 14 mL of CO₂. All volumes are at the same temperature and pressure. Find the volume of O₂ consumed.

Ethanol (23 g) is dissolved in water (54 g). What is the mole fraction of ethanol in this solution? (C₂H₅OH = 46, H₂O = 18)

Which concentration term of a solution changes with temperature?

An organic liquid contains 54.5% C, 9.1% H and 36.4% O by mass, and its molar mass is 88 g/mol. Identify its molecular formula. (C = 12, H = 1, O = 16)

An aqueous NaOH solution is 2 M and has a density of 1.08 g/mL. Calculate its molality. (NaOH = 40)

Arrange the following samples in increasing order of the total number of atoms present. (H = 1, He = 4, N = 14, O = 16) A. 4 g of He B. 9 g of H₂O C. 28 g of N₂ D. 8 g of O₃

A sample of methane, CH₄, has a mass of 8 g. How many moles of hydrogen atoms does it contain? (C = 12, H = 1)

Count the significant figures in the measured length 0.02040 m.

Given below are two statements: Statement I: The mole fraction of a component in a solution has no units and does not depend on temperature. Statement II: The molarity of an aqueous solution increases when the solution is warmed. Choose the correct answer.

Aluminium (10.8 g) is heated with chlorine (21.3 g) until the reaction stops: 2Al + 3Cl₂ → 2AlCl₃ What mass of AlCl₃ forms? (Al = 27, Cl = 35.5)

Butane burns completely in oxygen: a C₄H₁₀ + b O₂ → c CO₂ + d H₂O For the balanced equation in the lowest whole numbers, what are a, b, c and d, in that order?