IGCSE›Oxford AQA IGCSE Chemistry›Mind mapsReacting MassesOxford AQA IGCSE Chemistry: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsConservationNo atoms lost or made in a reactionAtoms are rearranged onlyTotal mass of reactants = total mass of productsMethod Write the balanced equation Find Mr of known and unknown substances Known mass → moles: n=mass÷Mrn = \text{mass} \div M_rn=mass÷Mr Apply the mole ratio Moles → mass: mass=n×Mr\text{mass} = n \times M_rmass=n×Mr Mole ratioBig numbers in the equation give the ratio2 Mg+OX2→2 MgO\ce{2Mg + O2 -> 2MgO}2Mg+OX22MgO2 mol Mg gives 2 mol MgOReacting massesmasses from equationsnMrgWorked example6 g Mg burns: mass of MgO?Moles Mg = 6 ÷ 24 = 0.25Ratio Mg:MgO is 1:1, so 0.25 mol MgOMass = 0.25 × 40 = 10 gYieldActual yield can be lower than calculatedReaction may not go to completionProduct lost in filtering or transferringUnexpected side reactionsExam tipsNever use the mass ratio from the equationAlways convert to moles first, then back to mass