NJC 1 Mole Concept Stoichiometry

National Junior College SH1 Chemistry 1 (I) The Mole Concept and Stoichiometry Content • Relative masses of atoms and molecules • The mole, the Avogadro constant • The calculation of empirical and molecular formulae • Reacting masses and volumes (of solutions and gases) Learning Outcomes required for H2 (9729) and H1 (8873) Chemistry: [The term relative formula mass or Mr will be used for ionic compounds] Candidates should be able to (a) Define the terms relative atomic, isotopic, molecular and formula mass, based on the 12C scale; (b) Define the term mole in terms of the Avogadro constant; (c) Calculate the relative atomic mass of an element given the relative abundances of its isotopes ; (d) Define the terms empirical and molecular formula; (e) Calculate empirical and molecular formulae using combustion data or composition by mass; (f) Write and /or construct balanced equations; (g) Perform calculations, including use of the mole concept, involving: (i) reacting masses (from formulae and equations); (ii) volumes of gases (e.g. in the burning of hydrocarbons); (iii) volumes and concentrations of solutions; [when performing calculations, candidates’ answers should reflect the no. of significant figures given or asked for in the question] (h) Deduce stoichiometric relationships from calculations such as those in (g). THE MOLE CONCEPT AND STOICHIOMETRY All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any other information storage and retrieval system, without prior permission in writing from the copyright owner. Copyright © 2023 National Junior College

National Junior College SH1 Chemistry 2 1. RELATIVE MASSES OF ATOMS AND MOLECULES Success Criteria: • Able to define the terms relative atomic , isotopic, molecular and formula mass, based on the 12C scale. • Able to calculate the relative atomic mass of an element given the relative abundances of its isotopes. 1.1 Relative Atomic Mass Scale: The Carbon–12 scale Atoms are too small to be weighed directly and it is inconvenient to express masses of individual atoms in terms of kilograms , kg, or grams, g (e.g. mass of one H atom = 1.67 × 10–27 kg). It is more practical to consider its mass relative to that of another atom known as the standard. The most abundant isotope of carbon, 12C, is the chosen standard. In this scale, a n atom of carbon –12 is assigned a mass of exactly 12 atomic mass units (a.m.u). 1.1.1 Relative Isotopic Mass (NO UNITS) Isotopes are atoms of the same element whose nuclei have the same number of protons but different number of neutrons; e.g. C17 35 l and C17 37 l. Relative isotopic mass is the mass of one atom of the isotope relative to 1 12 the mass of one atom of carbon-12. Note: Values of Ar are to be given to 1 decimal place unless otherwise stated. 1.1.2 Relative Atomic Mass of an element (symbol: Ar, NO UNITS) Many elements consist of isotopes of varying abundances. The weighted average of their mass numbers and abundances need to be taken into consideration when calculating the relative atomic mass of an element. Relative atomic mass is the weighted average of the isotopic masses of one atom of an element relative to 1 12 the mass of one atom of carbon-12. Worked Example 1 Calculate the relative atomic mass of chlorine from the isotopic abundances data: Isotope Relative isotopic mass Natural abundance (%) 35Cl 34.9689 75.77 37Cl 36.9658 24.23 Ar of Cl = (34.9689 × 75.77 + 36.9658 × 24.23) 100 = 35.5

National Junior College SH1 Chemistry 3 Worked Example 2 Given the Ar of copper is 63.54, determine the relative abundances of the two isotopes of copper, 63Cu and 65Cu. Let % abundance of 63Cu be x and 65Cu be (100 – x). 63(x) + 65(100 – x) 100 = 63.54 x = 73.0% Relative abundances of 63Cu and 65Cu are 73.0 % and 27.0 % respectively. Checkpoint 1 1. Naturally occurring gallium, Ga, is a mixture of two isotopes, gallium -69 and gallium -71. Given that the relative atomic mass of gallium is 69.7, calculate the percentage abundance of each isotope. [69Ga: 65.0%, 71Ga: 35.0%] Let % abundance of 69Ga be x and 71Ga be (100 – x). 69(x) + 71(100 – x) 100 = 69.7 x = 65.0% Relative abundance of 69Ga and 71Ga are 65.0% and 35.0% respectively. 2. Calculate the relative atomic mass of neon using the following data. [20.2] Isotope Natural abundance (%) Neon-20 90.5 Neon-21 0.3 Neon-22 9.2 Ar of Ne = (20 × 90.5 + 21 × 0.3 + 22 × 9.2 ) 100 = 20.187 = 20.2 (1 d.p.) 3. [2016/P1/Q3] Shakudo is a Japanese alloy of copper and gold. The information in the table was obtained by mass spectrometry of a sample of shakudo. What Ar value for copper is given by these figures? A 59.8 B 63.5 C 63.6 D 71.6 Ar of gold = 197 Ar of Cu = (65 × 63 + 29 × 65 ) 65 + 29 = 63.62 = 63.6 Mass number 63 65 197 % abundance 65 29 6

National Junior College SH1 Chemistry 4 Note: Values of Ar and Mr are to be given to 1 decimal place unless otherwise stated. 1.1.3 Relative Molecular Mass (symbol: Mr, NO UNITS) Relative molecular mass is the weighted average of the masses of one molecule of a substance relative to 1 12 the mass of one atom of carbon-12. Worked Example 3 Calculate the relative molecular mass of glucose, C 6H12O6. Mr of C6H12O6 = 6(12.0) + 12(1.0) + 6(16.0) = 180.0 1.1.4 Relative Formula Mass (symbol: Mr, NO UNITS) Relative Formula Mass is used for compounds with giant lattices. Relative formula mass is the weighted average of the masses of one formula unit relative to 1 12 the mass of one atom of carbon-12. Worked Example 4 Calculate the relative formula mass of Na2SO4. Mr of Na2SO4 = 2(23.0) + 32.1 + 4(16.0) = 142.1 Unit for mole: mol Avogadro constant, L = 6.02 × 1023 mol–1 (This value is given in the Data Booklet) 2. THE MOLE, THE AVOGADRO CONSTANT Success Criteria: • Able to define the term mole in terms of the Avogadro constant. • Able to calculate the amount of particles present using Avogadro’s constant, L • Able to calculate the amount of gas using molar volumes at r.t.p. & s.t.p conditions • Able to express a quantity in terms of parts per million (ppm) One mole of a substance is the amount of that substance which contains the same number of particles as there are atoms in 12.0 g of 12C isotopes. The term “particles” could refer to atoms, molecules, ions or electrons etc. This number is known as Avogadro’s constant, L, and has a value of 6.02 × 1023.

National Junior College SH1 Chemistry 5 Note: The term “amount” is used to represent "number of moles” 2.1 Calculation of amount (i.e. no. of moles) from number of particles  23 –1 number of particlesamount of particles (in mol) = 6.02 10 (in mol )