Finding mass of reactant

Question

When given a reaction equation and the mass of the PRECIPITATE, how would one go about finding out the mass of one of the reactants?

Sorry if this is unclear - I will add additional details if need be.

Thanks!

Answer 1

ALL these problems involve going by way of moles.

You start off by writing down the balanced equation for the reaction.  

Knowing the mass and nature of the precipitate, find number of moles of the precipitate.

The balanced equation will tell you the mole ratio, so you can work out the number of moles of the reactant of interest that were used up in the reaction.

Finally, multiply by molar mass (g/mole) of that reactant, and there you are.

Hope this helps.