Moles Noadswood Science, 2016

Thursday, February 08, 2018 Moles To know how to calculate molarity

Precise Learning Chemical amounts are measured in moles. The symbol for the unit mole is mol (higher tier only) The mass of one mole of a substance in grams is numerically equal to its relative formula mass (higher tier only) One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance (higher tier only) The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole (higher tier only) Students should understand that the measurement of amounts in moles can apply to atoms, molecules, ions, electrons, formulae and equations, for example that in one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO2) (understand and use the symbols: =, <, <<, >>, >, ∝, ~) (higher tier only) Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa (higher tier only)

Recap Mass Number Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it. Protons and neutrons have a relative mass unit of 1 Electrons have a very small mass compared to protons and neutrons with a mass unit of 1/2000th Most of the mass of an atom is found in its nucleus: - The mass number of an atom is the total number of protons and neutrons it contains The atomic number (also called the proton number) is the number of protons it contains

Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) Chemical Equations In a balanced equation the big numbers in front of the chemical formulas tell you the relative number of moles of each reactant that take part in the reaction, and the relative number of moles of each product that are formed The little numbers within the chemical formulas tell you how many atoms of each element there are in the smallest unit of substance. for example Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) In the above reaction 1 mole of magnesium and 2 moles of hydrochloric acid react to form 1 mole of magnesium chloride and 1 mole of hydrogen gas Higher Tier

Moles The mole is a name given to a Avogadro's number: 6.023 x 1023 or 602,300,000,000,000,000,000,000 When you have precisely this number of atoms or molecules (of any element or compound) they weigh exactly the same number of grams as the relative atomic mass of the element or compound 1 mole of atoms of molecules of any substance will have a mass in grams equal to the relative formula mass (Ar or Mr) for that substance Higher Tier

Moles 1 mole of atoms of molecules of any substance will have a mass in grams equal to the relative formula mass (Ar or Mr) for that substance For example: - Carbon has an Ar of 12 – 1 mole of carbon weighs 12g Iron has an Ar of 56 – 1 mole of carbon weighs 56g Carbon dioxide has a Mr of 44 – 1 mole of carbon dioxide weighs 44g This means that 12g of carbon, 56g of iron, 44g of carbon dioxide etc… all contain the same number of atoms – 1 mole! Higher Tier

Ratio The ratio of moles of reactants and products in a reaction always stays the same – this can be used to work out how many moles of a reactant or product is involved in a reaction if you are given information about one of the other substances in the reaction How many moles of water are formed if 2 moles of methane combust completely in oxygen? CH4 + 2O2  CO2 + 2H2O From the balanced equation 1 mole of methane reacts to form 2 moles of water so the ratio is 1:2 (so 2 moles of methane will react to form 4 moles of water) Higher Tier

Ratio How many moles of oxygen will react if 3 moles of magnesium react completely to form magnesium oxide? 2Mg + O2  2MgO From the balanced equation 2 moles of magnesium react with 1 mole of oxygen, so the molar ratio is 2:1. So 3 moles of magnesium will react with 1.5 moles of oxygen Higher Tier

Number of moles = Mass (g) of element or compound Moles Within A Mass The number of moles within a given mass is worked out using the following equation: - Number of moles = Mass (g) of element or compound Mr of element or compound For example, to calculate the number of moles in 42g of carbon… Number of moles = 42g 12 = 3.5 moles Higher Tier

Number of moles = volume (litres) x molarity solution Molar Solutions A 1 molar solution contains 1 mole per litre Number of moles = volume (litres) x molarity solution For example – how many moles are in 185cm3 of a 2M solution? 0.185 x 2 = 0.37 moles Higher Tier

Balancing If you know the masses of the reactants and products that took part in a reaction you can work out the balanced symbol equation for the reaction using the following steps Divide the mass of each substance by its relative formula mass to find the number of moles Divide the number of moles of each substance by the smallest number of moles in the reaction If any of the numbers aren’t whole then multiply all the numbers by the same amount so that they all become whole Write the balanced symbol equation for the reaction by putting these numbers in from of the chemical formulas Higher Tier

Balancing 8.1g of zinc oxide (ZnO) reacts completely with 0.60g of carbon to form 2.2g of carbon dioxide and 6.5g of zinc Write a balanced symbol equation for this reaction Higher Tier

Balancing 2.7g of an element, X, reacts completely with 2.4g of oxygen to form 5.1g of an oxide X oxide. Write a balanced symbol equation for this reaction Higher Tier

Practice Questions - Application What is the relative number of moles of water in each of the following reactions Ca + 2H2O → Ca(OH2) + H2 HCl + NaOH → NaCl + H2O The balanced equation for the reaction between magnesium and oxygen is 2Mg + O2 → 2MgO. How many moles of magnesium would react to form 3 moles of magnesium oxide? The balanced equation for the reaction between chlorine and sodium bromide is Cl2 + 2NaBr → Br2 + 2NaCl. How many moles of bromine (Br2) would form 0.4 moles of sodium bromide?

Answers What is the relative number of moles of water in each of the following reactions 2 1 3 moles (ratio 1:1) 0.2 mol (ratio 2:1)

Practice Questions - Application 4.6g of sodium reacted with 1.6g of oxygen to form 6.2g of sodium oxide (Na2O). Use the reacting masses to write a balanced symbol equation for this reaction (Ar: O = 16 and Na = 23) 2.34g of potassium reacted with 2.19g of hydrochloric acid (HCl) to form 4.47g of potassium chloride (KCl) and 0.06g of hydrogen. Use the reacting masses to write a balanced symbol equation for this reaction (Ar: H =1; Cl = 35.5; and K = 39) 1.20g of a hydrocarbon, Z, combusts completely in 4.48g of oxygen to form 3.52g of carbon dioxide (CO2) and 2.16g of water (H2O). Write a balanced symbol equation for this reaction (Ar: Z = 30 and Mr O2 = 32; H2O = 18; CO2 = 44)

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