Chapter 3: The Mole Concept, Chemical Formula and Equation
Form 4 Chemistry Bab 3: The Mole Concept, Chemical Formula and Equation
3.1 Relative Atomic Mass and Relative Molecular Mass
Relative atomic mass (RAM) of an element = average mass of one atom of the element ÷ (1/12 × mass of one carbon-12 atom); carbon-12 is the standard atom since 1961. (p.44)
Relative molecular mass (RMM) = average mass of one molecule ÷ (1/12 × mass of one carbon-12 atom) = sum of the RAM of all atoms in the molecule. (p.45)
Relative formula mass (RFM) applies the same concept to ionic substances, summing the RAM of all atoms shown in the formula. (p.48)
3.2 Mole Concept
Avogadro constant, NA = 6.02 × 1023 mol−1: the number of particles in one mole of substance. Number of moles = number of particles / NA. (p.50)
Molar mass = mass of one mole of substance (unit g mol−1), numerically equal to the relative mass (RAM/RMM/RFM). Number of moles = mass (g) / molar mass (g mol−1). (p.53)
Molar volume = volume occupied by 1 mole of gas: 22.4 dm3 mol−1 at STP (0°C, 1 atm) or 24 dm3 mol−1 at room conditions (25°C, 1 atm). Number of moles = volume of gas / molar volume. (p.54)
3.3 Chemical Formula
Chemical formula represents a substance using element symbols and subscript numbers for the number of each atom. (p.59)
Empirical formula shows the simplest whole-number ratio of atoms of each element in a compound. Molecular formula shows the actual number of atoms of each element in one molecule. (p.60)
Empirical formula is determined by finding the mole ratio of each element from experimental mass data, then simplifying to the smallest whole-number ratio. (p.61)
Molecular formula = (empirical formula)n, where n is found by equating the calculated RMM of (empirical formula)n to the given RMM. (p.65)
Ionic compound formulae are constructed by the cross-change method: cross the charge of the cation and anion to get the number of each ion, keeping the overall charge neutral. (p.67)
Naming: metal ions with multiple charges use Roman numerals (e.g. iron(II), iron(III)); simple molecular compounds use Greek prefixes (mono, di, tri, tetra…) to show atom numbers, with the second element ending in ‘-ide’. (p.68)
3.4 Chemical Equation
In a chemical equation, reactants are written on the left and products on the right; physical states (s, l, g, aq) are usually shown, and the equation must be balanced (law of conservation of mass) by adjusting coefficients. (p.70)
Stoichiometry is the quantitative study of the composition of substances involved in a chemical reaction; coefficients in a balanced equation give the mole ratio of substances. (p.72)