The atomic number tells us: mass number The atomic number tells us: the number of protons in the nucleus of an atom. The mass number tells us: the mass of the atom (protons + neutrons). atomic number Sometimes, scientists want to find out the total mass of an element or a compound. The total mass of an element is called the RELATIVE ATOMIC MASS (Ar). The total mass of a compound is called the RELATIVE FORMULA MASS (Mr).

Task – 3 minutes! 05/11/2019 Find the relative atomic masses from the periodic table (inside front cover of your workbook): C H O S Ca Al Cl Fe N 12 1 16 32 40 27 35.5 56 14

Relative formula mass, Mr The relative formula mass of a compound is the relative atomic masses of all the elements in the compound added together. Relative atomic mass of O = 16 E.g. water H2O: Relative atomic mass of H = 1 Therefore Mr for water = 16 + (2x1) = 18 Work out Mr for the following compounds: HCl NaOH MgCl2 H2SO4 K2CO3 H=1, Cl=35 so Mr = 36 Na=23, O=16, H=1 so Mr = 40 Mg=24, Cl=35 so Mr = 24+(2x35) = 94 H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138

More examples CaCO3 40 + 12 + (3x16) 100 HNO3 1 + 14 + (3x16) 63 2MgO 80 3H2O 3 x ((2x1) + 16) 54 4NH3 4x (14+(3x1)) 68 2KMnO4 2x (39+55+(16x4)) 316 3C2H5OH 3x((2x12)+(5x1)+16+1)) 138 Mg(OH)2 24+ ((16+1)x2) 58

Calculate the Percentage by mass CO2 (Oxygen) 72.7% Relative Formula Mass of CO2 = 12 + (2x16) = 44g/mol. In CO2 there’s 2x Oxygen: 2x16 = 32. To find percentage by mass: (32/44) x 100 = 72.7%

Calculate the Percentage by mass CO2 (Oxygen) 72.7% CaCl2 (Calcium) 36% H2SO4 (Sulfur) 32.7% Al2O3 (Aluminium) 52.9% Fe(NO3)3 (Oxygen) 59.5%

Conservation of mass in reactions In any reaction the total mass of products is the same as the total mass of the reactants Example 1 – Magnesium oxide and hydrochloric acid Mg O H Cl 1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms Also 1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms Example 2 – Burning methane C H O

Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? Step 1: READ the equation: 2Mg + O2 2MgO ignore Reacting ratio 2 Mg makes 2 MgO Step 2: WORK OUT the relative formula masses (Mr): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80 Step 3: LEARN and APPLY the following 3 points: 48g of Mg makes 80g of MgO 1g of Mg makes 80/48 = 1.66g of MgO 60g of Mg makes 1.66 x 60 = 100g of MgO

1g of CaCl2 produced by (73/111)= 0.66 2) What mass of hydrochloric acid is needed to produce 11.1g calcium chloride? Ca(OH)2 + 2HCl CaCl2 + 2H2O ignore ignore reacting ratio 2 HCl makes 1 CaCl2 Mr: 2HCl = 2 x (1 + 35.5) CaCl2 = 40 + (2 x 35.5) = 73 = 111 111g CaCl2 produced by 73g HCl 1g of CaCl2 produced by (73/111)= 0.66 so 11.1g produced by 0.66 x 11.1 = 7.3g of HCl

Mass of H2 and Cl2 needed to make 73g of HCl? 05/11/2019 Example… H2 + Cl2  HCl Mass of H2 and Cl2 needed to make 73g of HCl? Atom Number on Left Number on Right H 2 1 Cl

Mass of H2 and Cl2 needed to make 73g of HCl? 05/11/2019 Example from sheet H2 + Cl2  2HCl Mass of H2 and Cl2 needed to make 73g of HCl? Atom Number on Left Number on Right H 2 Cl

Example from sheet H2 + Cl2  2HCl 05/11/2019 Reactant/Product Rel. Formula Mass (Mr) Molar Ratio Mr x Molar Ratio H2 2x1 = 2g/mol 1 2g/mol Cl2 35.5x2 = 71g/mol 71g/mol HCl 1+35.5 = 36.5g/mol 2 2 x 36.5g/mol = 73g/mol What if I wanted to make only 7.3g of HCl?

Example from sheet H2 + Cl2  2HCl 05/11/2019 Reactant/Product Molar Ratio Mr x Molar Ratio H2 1 2g/mol 2 x 0.1 = 0.2g Cl2 71g/mol 71 x 0.1 = 7.1g HCl 2 2 x 36.5g/mol = 73g/mol 7.3/73 = 0.1 What if I wanted to make only 7.3g of HCl? Divide the mass you want by (Mr x Molar Ratio) to find a factor to multiply by!

Workbook Qs! 05/11/2019 Pg 63 - 67