Some Basic Concepts of Chemistry

Chemistry deals with the composition, structure and properties of matter and the changes it undergoes. The chapter begins with the states of matter and its classification into mixtures and pure substances, which are elements or compounds. It then covers the measurement of physical properties in the seven base SI units, the mole being the unit of amount of substance. You will learn how to handle uncertainty using scientific notation, significant figures, precision and accuracy, and dimensional analysis. You will also study the laws of chemical combination: conservation of mass (Lavoisier), definite proportions (Proust), multiple proportions (Dalton), Gay Lussac's law of gaseous volumes and Avogadro's law. The chapter then explains Dalton's atomic theory and atomic, molecular and formula masses on the unified mass scale. It introduces the mole concept, with one mole containing 6.022 x 10^23 particles, and molar mass. You will learn to calculate percentage composition, empirical and molecular formulae, and quantities in reactions using stoichiometry and the limiting reagent. You will also express the concentration of solutions as mass percent, mole fraction, molarity and molality.

Detailed Worksheet: Some Basic Concepts of Chemistry Section A - Definitions (10 marks) 1. State the law of multiple proportions with an example. (2 marks) 2. Define one mole. What is Avogadro's number? (2 marks) 3. Distinguish between empirical and molecular formulae. (2 marks) 4. Define molarity and molality. Which of these changes with temperature and why? (2 marks) 5. What is a limiting reagent? (2 marks) Section B - Calculations and Applications (15 marks) 6. A compound contains 4.07% hydrogen, 24.27% carbon and 71.65% chlorine. Its molar mass is 98.96 g/mol. Calculate its empirical formula and molecular formula (H = 1, C = 12, Cl = 35.5). (3 marks) 7. 50.0 kg of N2 reacts with 10.0 kg of H2 to form ammonia: N2 + 3H2 -> 2NH3. Identify the limiting reagent and calculate the mass of ammonia formed (N = 14, H = 1). (3 marks) 8. A 3.0 M solution of NaCl has a density of 1.25 g/mL. Calculate the molality of the solution (NaCl = 58.5 g/mol). (3 marks) 9. Calculate the number of moles, molecules and atoms in 4.4 g of carbon dioxide (C = 12, O = 16, NA = 6.022 x 10^23). (3 marks) 10. Calculate the mass percent of each element in sodium sulphate, Na2SO4 (Na = 23, S = 32, O = 16). (3 marks) Section C - Diagrams (10 marks) 11. Draw a flowchart showing the classification of matter into mixtures and pure substances, with subtypes and examples. (4 marks) 12. Draw labelled diagrams showing the arrangement of particles in solids, liquids and gases. (3 marks) 13. Draw a diagram showing the difference between precision and accuracy using a target with points. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain Dalton's atomic theory. Which of its postulates have been modified by later discoveries? (5 marks) 15. Explain the laws of chemical combination with one numerical example each, including Gay Lussac's law of gaseous volumes. How did Avogadro's law explain it? (5 marks) 16. Explain the rules for determining significant figures in a measurement and in calculations. Express the results of 2.5 x 1.25 and 12.11 + 18.0 + 1.012 with the correct number of significant figures. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all calculation steps with units and the correct number of significant figures.