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Chemistry Knowledge Organiser Unit 3 - Chemical Quantities and Calculations Knowledge Organiser Equations must be BALANCED. This means that big numbers are put IN FRONT of the formulae to balance them. The following equation is not balanced: Mg + O2 MgO What to do next? Make a table like this: Reactants Products Mg = 1 Mg = 1 O =2 O = 1 You can see that the oxygen is not balanced – you need another O atom in the products so just multiply it by 2: Mg + O2 2MgO BUT there are now 2 MgO on the products side which means that there are now 2 Mg atoms. Just go back to the reactants side and multiply the Mg by 2: 2Mg + O2 2MgO Done! Moles are a measure of how much chemical you have. One mole of a substance is its atomic or molecular mass in grams: Formula mass (MR) = the relative atomic masses of all the atoms in a formula e.g. formula mass for H2O = 1 + 1 + 16 = 18. ‘Excess’ means you have more than you need of a chemical. ‘Limiting Reagent’ means you have a limited amount of a chemical and it will run out first. Moles When the mass of products is MORE than the reactants, the mass has been gained from the atmosphere around e.g. heating a metal in oxygen to make a metal oxide. Chemistry Knowledge Organiser Unit 3 - Chemical Quantities and Calculations Knowledge Organiser Moles in Equations In equations, the large numbers in front of the formulae show how many molecules are reacting, but also can be used as the number of moles aswell. Moles can be used to predict masses from equations. Looking at the example on the left, if only 6g of carbon was burned then you only need half the amount of oxygen and half the carbon dioxide would be produced. How many grams of CO2 would be produced? Concentration of solutions – keywords: Solute Solvent Solution Concentration v must be in litres not cm3. (1litre = 1000 cm3) Example calculation: what is the concentration of a solution containing 2 moles of solute in a volume of 100cm3? Answer: use equation c=n/v n= 2, v= 100/1000 = 0.1, c=2/0.1 = 20 moles/dm3. Good atom economy reduces waste and is what chemical factories try to achieve.