In a reaction when masses of all the reactants are given it is necessary to find which reactant is in excess and will not be completely used up in the reaction, and which one is the limiting reagent. The limiting reagent will be completely used up in the reaction and can therefore be used to determine the amount of product formed.
5.00g of iron and 5.00g of sulphur are heated together to form iron(II)sulphide. Which reagent is present in excess and what mass of product is formed?
Fe (s) + S (s) FeS (s) 1 mole Fe + 1 mole S forms 1 mole FeS 56g Fe + 32 g S form 88g FeS 5.00g Fe = 5 mol = 0.0893mol 56 and 5.00g S = 5 mol = 0.156mol 32 Fe is the limiting reagent and 0.0893 moles of FeS are formed = 0.0893 x 88g = 7.86g
Calculate the percentage yield when 180g of chromium are obtained from a reaction between 100g aluminium and 400g of chromium(III) oxide
Method: 1) calculate the no of moles of Al used 2) Calculate the no of moles of Cr 2 O 3 used 3) Decide which is the limiting reagent 4) Calculate the max no of moles of chromium obtainable 5) Calculate the percentage yield