Atoms and Molecules

This chapter explains that matter is made of tiny particles called atoms and molecules. You will study the two laws of chemical combination: the law of conservation of mass, established by Antoine Lavoisier, which states that mass can neither be created nor destroyed in a chemical reaction, and the law of constant proportions, stated by Joseph Proust, which says that a pure chemical compound always contains the same elements combined in the same proportion by mass, as in water, where hydrogen and oxygen combine in the ratio 1 : 8 by mass. You will see how John Dalton's atomic theory explained these laws. You will learn that atoms are extremely small, are represented by symbols such as H, O, Na and Fe, and that their masses are expressed in atomic mass units, where 1 u is one-twelfth of the mass of a carbon-12 atom. You will study molecules of elements and compounds, atomicity, ions and valency, and learn to write chemical formulae of compounds such as Al2(SO4)3 and to calculate molecular mass and formula unit mass.

Detailed Worksheet: Atoms and Molecules Section A - Definitions (10 marks) 1. State the law of conservation of mass and the law of constant proportions. (2 marks) 2. Define atomic mass unit. What is the atomic mass of oxygen in u? (2 marks) 3. What is a molecule? Define atomicity and give the atomicity of O2, O3 and P4. (2 marks) 4. What are ions? Distinguish between a cation and an anion with an example each. (2 marks) 5. What is valency? Write the valency of sodium, calcium, aluminium and the sulphate ion. (2 marks) Section B - Calculations and Applications (15 marks) 6. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium acetate. Show that these observations agree with the law of conservation of mass. (3 marks) 7. Hydrogen and oxygen combine in the ratio 1 : 8 by mass to form water. What mass of oxygen gas would be needed to react completely with 3 g of hydrogen gas? How much water is formed? (3 marks) 8. Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, NH3 and CH3OH. (Atomic masses: H = 1 u, C = 12 u, N = 14 u, O = 16 u, Cl = 35.5 u.) (3 marks) 9. Calculate the formula unit masses of ZnO, Na2O and K2CO3. (Atomic masses: Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, O = 16 u.) (3 marks) 10. When 3 g of carbon is burnt in 8 g of oxygen, 11 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3 g of carbon is burnt in 50 g of oxygen? Which law of chemical combination governs your answer? (3 marks) Section C - Diagrams (10 marks) 11. Draw diagrams to represent the molecules of hydrogen (H2), oxygen (O2), water (H2O), ammonia (NH3) and carbon dioxide (CO2) using circles for atoms. (4 marks) 12. Draw the criss-cross diagram to work out the formulae of aluminium sulphate and calcium hydroxide. (3 marks) 13. Draw a labelled diagram of the experiment to verify the law of conservation of mass using solutions of barium chloride and sodium sulphate in a conical flask with an ignition tube. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. State the postulates of Dalton's atomic theory. Which postulate explains the law of conservation of mass and which explains the law of constant proportions? (5 marks) 15. Write the chemical formulae of: magnesium chloride, calcium oxide, copper nitrate, aluminium chloride, calcium carbonate, sodium sulphide, ammonium sulphate, potassium nitrate, hydrogen sulphide and aluminium oxide. (5 marks) 16. Explain why it is not possible to see an atom with the naked eye. Explain why atoms of most elements do not exist independently, and how polyatomic ions such as sulphate and ammonium behave like a single unit when writing formulae. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write symbols and formulae carefully with correct capital and small letters.