The mole and the Avogadro constant, molar mass, converting between mass, moles and number of particles, percentage composition, empirical formula, Avogadro’s law, molar volume and the conservation of matter.
5 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
A chemist cannot count atoms one by one, so chemists count them in huge groups called moles. A mole of any substance has the same number of particles, but a different mass.
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Amount of substance containing 6.0 × 10²³ particles (Avogadro constant).
Mass of 1 mole in g/mol; numerically equals relative mass.
m = n × M; N = n × 6.0 × 10²³.
(mass of element in 1 mol ÷ M) × 100.
Simplest whole-number ratio of atoms.
Equal volumes of gases at the same T and P have the same number of molecules.
24 dm³ at rtp; 22.4 dm³ at stp.
Total mass is conserved in a reaction.
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8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better. Work each item on paper first.
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Question 1 [18 marks]
Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Mg = 24, Ca = 40. Molar volume of a gas = 24 dm³ at rtp. Avogadro constant = 6.0 × 10²³.
(a) (a) Define the mole. [1]
(b) (b) Calculate the molar mass of calcium carbonate, CaCO₃. [2]
(c) (c) Calculate the number of moles and of formula units in 5.0 g of CaCO₃. [3]
(d) (d) Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃. [3]
(e) (e) A compound contains 24 g of C, 4 g of H and 32 g of O. Calculate its empirical formula. [4]
(f) (f) Calculate the volume at rtp of 4.4 g of carbon dioxide, CO₂. [3]
(g) (g) State Avogadro’s law. [2]
Find these in your own past-paper booklets for extra timed practice. Items above are written in the style of these papers; any adapted from a specific question say so.