posted by Sara .
Calculate the number of moles of excess reactant that will be left-over when .350 g if HCN react with .500 g of 02:
Balance the equation before you do anything.
mols HCN = grams/molar mass
mols O2 = grams/molar mass.
Determine which is the limiting reagent. Do that by converting mols HCN to mols CO2 using the coefficients in the BALANCED equation. Do the same for mols O2 to mols CO2. It is likely that the two values will not agree which means one of them is wrong. The correct value in limiting reagent problems is ALWAYS the smaller value and the reagent responsible for this lower value is the limiting reagent.
Finally, convert mols of the limiting reagent to mols of the excess reagent, convert that to grams excess reagent (the amount used), subtract from the initial amount to find the amount left over.