Chemicals of the Natural Environment. Lesson 9

Learning objective: To calculate how much metal can be extracted from its mineral. Must: Be able to calculate the relative formula mass for any compound. Should: Be able to calculate the mass of a metal that can be extracted form its mineral. Should: Be able to write balanced symbol equations. Keywords: Atomic mass, relative formula mass, percentage composition, reactants and products. Starter: What do these numbers mean

counting atoms NaOH H2O CaCO3 H2SO4 Ca(OH)2 CONNECT What is the total number of atoms in the following formulae? NaOH H2O CaCO3 H2SO4 Ca(OH)2

rules recap! A small number just talks about the element it is behind. E.g. O2 = 2 x O Small numbers outside the bracket mean multiply everything inside the bracket by that number E.g. (CO3)3 = 3 x C, 9 x O

Relative atomic masses

Relative atomic masses Atoms of different elements have different masses The relative atomic mass (Ar) is the mass of the elements compared to the mass of a hydrogen atom Relative atomic masses

Relative Formula Mass The relative formula mass (Mr) is the relative mass of a molecule compared to the mass of a hydrogen atom Relative formula mass

Calculating relative formula mass. Top Tips. 1. The relative atomic mass is usually at the top – it is always the highest number. 2. When calculating the relative formula mass, a) Write out all the different atoms (Capital letter). b) Write down the number of each atom. c) Multiply the number of each type of atom by its atomic mass. d) Add all the masses together.

An example. Calculate the RFM – for calcium carbonate. Formula CaCO3 Type, number and atomic mass of each atom; Ca x 1 x 40 = 40 C x 1 x 12 = 12 O x 3 x 16 = 48 Total = 100 – Relative formula mass of CaCO3.

Relative formula mass Relative formula mass

Relative formula mass, Mr 18/07/2019 The relative formula mass of a compound is blatantly 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 2MgO 18/07/2019 CaCO3 40 + 12 + 3x16 100 HNO3 1 + 14 + 3x16 2MgO 2 x (24 + 16) 80 3H2O 3 x ((2x1) + 16) 4NH3 2KMnO4 3C2H5OH 4Ca(OH)2

142.5 ADD chemical CONUNDRUMS! PROGRESS CHECK Mr of CO2 Mr of NaCl Mr of MgO 142.5

58 Mr of CuSO4 SUBTRACT Mr of Al2O3 chemical CONUNDRUMS! PROGRESS CHECK Mr of CuSO4 SUBTRACT Mr of Al2O3 58

chemical CONUNDRUMS! PROGRESS CHECK Mr of H2O ADD Mr of ammonia 35

Calculating percentage mass 18/07/2019 If you can work out Mr then this bit is easy… Percentage mass (%) = Mass of element Ar Relative formula mass Mr x100% Calculate the percentage mass of magnesium in magnesium oxide, MgO: Ar for magnesium = 24 Ar for oxygen = 16 Mr for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Calculate the percentage mass of the following: Hydrogen in hydrochloric acid, HCl Potassium in potassium chloride, KCl Calcium in calcium chloride, CaCl2 Oxygen in water, H2O

Calculating the mass of a metal that can be extracted from its mineral. How much Fe could you get from 100kg of Fe2O3? There are 2 steps; 1. Calculate the percentage mass (composition) for the element in this case Fe. 2. Multiply the percentage mass (from step 1) by the mass of the mineral

An example. Percentage composition of Fe in Fe2O3 Mass of Fe in Fe2O3 = 2 x 56 =112 Mass of Fe2O3 = 2 x 56 + 3 x 16 = 160 Percentage Fe in Fe2O3 = 112 x 100 = 70% 160 Mass of Fe in 100 kg of Fe2O3 = 70 x 100 = 70 kg Click here for more examples of percentage composition problems.

How much gold is in your mobile phone? using Mr DEMONSTRATE How much gold is in your mobile phone? Use the procedure to calculate the percentage composition of gold in a mobile phone

More problems. 1. What mass of Al could be made from 1 tonne of Al2O3? 2. What mass of Na could be made from 2 tonnes of NaCl? 3. The main ore of chromium is FeCr2O4. What is the percentage mass of Cr in FeCr2O4? 4. 1000 tonnes of copper ore are dug out of the ground. Only 1% of this is the pure mineral, CuFeS2. What mass of the Cu could be made from 1000 tonnes of the ore?

Conservation of Mass Show that mass is conserved in this reaction. ZnCO3 ZnO + CO2 You need to use the ideas of Mr

Simple chemical reactions Mg O H Cl Magnesium oxide + Hydrochloric acid Magnesium chloride + Water MgO + 2HCl MgCl2 + H2O Mg O Cu S Magnesium + Copper sulphate Copper + Magnesium sulphate Mg + CuSO4 Cu + MgSO4

Balancing reaction equations This sequence of slides includes two approaches to balancing reaction equations. Individual slides may be copied elsewhere or used as templates. 1 Using symbols and no text 2 Using text prompts to explain each step of the process click here click here To return to this page, click the button to the top right of any slide.

2 Mg + O2  MgO 2  Symbols Mg + O2

2 Al + I2  AlI3 3 1½ 2  Symbols Al + I2

Write a balanced equation sodium hydroxide is ionic: what are the formulae of its ions? sodium hydroxide is ionic: what are the formulae of its ions? Na+ and OH- sodium + water  sodium hydroxide + hydrogen It’s balanced! Check the equation is balanced: count the number of reactant and product elements Add numbers in front of the ringed formula so that the total number of reactant symbols = total number of product symbols Write the formula of each substance as it is ringed above Reactants: 2Na 4H 2O Products: 2Na 4H 2O 2 Na + H2O 2  NaOH 2 + H2

References First example, with symbols only: Frank Harriss of Malvern College Second example, with text prompts: Steve Lewis

Some examples 2 2 3 2 3 Mg + O2 Zn + HCl Fe + Cl2 NaOH + HCl CH4 + O2 2 3 2 3 Mg + O2 Zn + HCl Fe + Cl2 NaOH + HCl CH4 + O2 Ca + H2O NaOH + H2SO4 CH3OH + O2 MgO ZnCl2 + H2 FeCl3 NaCl + H2O CO2 + H2O Ca(OH)2 + H2 Na2SO4 + H2O 2 2 4

Balancing equations Balancing equations