Consider the following unbalanced reaction.

Question

P4(s) + F2(g) PF3(g)
What mass of F2 is needed to produce 212 g of PF3 if the reaction has a 84.2% yield?

Answer 1

Here is a worked example of a stoichiometry problem. http://www.jiskha.com/science/chemistry/stoichiometry.html
Since you want 212 grams AFTER an 84.2% yeild, you want to start with 212/0.842 = grams PF3.
Post your work if you get stuck.