Unit 1: Some Basic Concepts in Chemistry

Metal M forms two oxides. Oxide I contains 60% M by mass and Oxide II contains 75% M by mass. For a fixed mass of M, in what ratio are the masses of oxygen in Oxide I and Oxide II?

10 mL of a gaseous hydrocarbon is exploded with 90 mL of oxygen, an excess. After cooling to room temperature the gas occupies 70 mL, and after this gas is passed through aqueous KOH, which absorbs carbon dioxide, 30 mL remains. All volumes are at the same temperature and pressure, and the water formed condenses to a liquid of negligible volume. What is the molecular formula of the hydrocarbon?

(Molar masses: Mg 24, O₂ 32, MgO 40 g mol⁻¹.) Given below are two statements: one is labelled Assertion A and the other is labelled Reason R. Assertion A: When 12 g of magnesium burns in 12 g of oxygen, 2 Mg + O₂ → 2 MgO, 24 g of magnesium oxide forms. Reason R: In a chemical reaction the total mass of the substances that react equals the total mass of the products. In the light of the above statements, choose the correct answer.

Natural chlorine contains only ³⁵Cl (35 u) and ³⁷Cl (37 u), and its average atomic mass is 35.5 u. If the two isotopes pair up at random to form Cl₂ molecules, what percentage of the Cl₂ molecules have a mass of 72 u?

(Atomic masses: C 12, N 14, O 16.) Given below are two statements. Statement I: For a fixed mass of nitrogen, the masses of oxygen combined with it in N₂O, NO, N₂O₃, NO₂ and N₂O₅ are in the ratio 1 : 2 : 3 : 4 : 5. Statement II: For a fixed mass of oxygen, the masses of carbon combined with it in CO and CO₂ are in the ratio 1 : 2. In the light of the above statements, choose the correct answer.

Read the following statements about Dalton's atomic theory and the atomic mass scale. (a) The existence of isotopes contradicts Dalton's idea that all atoms of an element have the same mass. (b) Dalton's theory explained why gases combine in simple ratios by volume. (c) In a chemical reaction, atoms are rearranged but not created or destroyed. (d) One atomic mass unit is one twelfth of the mass of one carbon-12 atom. (e) The mass of a single carbon-12 atom is exactly 12 g. How many of the above statements are correct?

A 10.0 g mixture of sodium carbonate and sodium hydrogencarbonate is heated until its mass is constant. Only the hydrogencarbonate decomposes, 2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂, and both gases escape, so the mass falls by 1.55 g. What is the percentage by mass of NaHCO₃ in the mixture? (Molar masses: NaHCO₃ 84, H₂O 18, CO₂ 44 g mol⁻¹.)

Suppose the atomic mass unit were redefined as one twenty-fourth (instead of one twelfth) of the mass of a carbon-12 atom. On this new scale, what would the relative molecular mass of water be? (On the usual scale, H₂O is 18.)

Given below are two statements: one is labelled Assertion A and the other is labelled Reason R. Assertion A: When magnesium ribbon burns inside a sealed flask, the total mass of the flask and its contents does not change. Reason R: In a chemical reaction the number of atoms of each element stays the same. In the light of the above statements, choose the correct answer.

Element X forms two oxides, A and B. A contains 50% X by mass and has the formula XO₂. B contains 40% X by mass. What is the formula of B?

5.4 g of aluminium is heated in 14.2 g of chlorine: 2 Al + 3 Cl₂ → 2 AlCl₃. What is the greatest mass of aluminium chloride that can form? (Molar masses: Al 27, Cl 35.5 g mol⁻¹.)

A compound contains 54.5% carbon, 9.1% hydrogen and 36.4% oxygen by mass, and its molar mass is 88 g mol⁻¹. What is its molecular formula? (Atomic masses: C 12, H 1, O 16.)

What is the percentage of nitrogen by mass in ammonium sulphate, (NH₄)₂SO₄? (Atomic masses: N 14, H 1, S 32, O 16.)

Which sample contains the greatest total number of atoms? (Molar masses in g mol⁻¹: H₂ 2, CH₄ 16, H₂O 18, O₂ 32.)

What mass of oxygen is needed to burn 2.6 g of ethyne completely? 2 C₂H₂ + 5 O₂ → 4 CO₂ + 2 H₂O (Molar masses: C₂H₂ 26, O₂ 32 g mol⁻¹.)

How many electrons are present in 3.2 g of sulphur dioxide, SO₂? (Atomic numbers: S 16, O 8. Molar mass of SO₂ 64 g mol⁻¹. Avogadro constant 6.0 × 10²³ mol⁻¹.)

Heating 25.0 g of calcium carbonate gives 11.2 g of calcium oxide: CaCO₃ → CaO + CO₂. What is the percentage yield of calcium oxide? (Molar masses: CaCO₃ 100, CaO 56 g mol⁻¹.)

Given below are two statements. Statement I: One mole of oxygen gas, O₂, contains 6.022 × 10²³ oxygen atoms. Statement II: The mass of 0.5 mol of N₂ is equal to the mass of 1 mol of nitrogen atoms. In the light of the above statements, choose the correct answer.

4 g of hydrogen and 40 g of oxygen react: 2 H₂ + O₂ → 2 H₂O. (Molar masses: H₂ 2, O₂ 32, H₂O 18 g mol⁻¹.) Read the following statements about the reaction when it is complete. (a) Hydrogen is the limiting reagent. (b) 36 g of water forms. (c) 8 g of oxygen is left unreacted. (d) The water and the leftover oxygen together have a mass of 44 g. (e) 45 g of water forms. How many of the above statements are correct?

Given below are two statements: one is labelled Assertion A and the other is labelled Reason R. Assertion A: 1 mol of water and 1 mol of carbon dioxide contain the same number of molecules. Reason R: A water molecule and a carbon dioxide molecule each contain three atoms. In the light of the above statements, choose the correct answer.

2.46 g of hydrated magnesium sulphate, MgSO₄·xH₂O, is heated until all the water is driven off, leaving 1.20 g of anhydrous MgSO₄. What is the value of x? (Molar masses: MgSO₄ 120, H₂O 18 g mol⁻¹.)

20 mL of a mixture of methane and ethene burns completely in excess oxygen and gives 30 mL of carbon dioxide, all volumes at the same temperature and pressure. CH₄ + 2 O₂ → CO₂ + 2 H₂O; C₂H₄ + 3 O₂ → 2 CO₂ + 2 H₂O. What volume of oxygen was used up?

10 mL of a gaseous alkane, CₙH₂ₙ₊₂, needs exactly 50 mL of oxygen for complete combustion to carbon dioxide and water, all volumes at the same temperature and pressure. What is the alkane?