Question

How many grams of fluorine are needed to produce 120. g of phosphorus trifluoride if the reaction has a 78.1% yield from the following reaction: P4+6F2 -> 4PF3

Answer 1

Here is a worked example of a stoichiometry problem. Just follow the steps. http://www.jiskha.com/science/chemistry/stoichiometry.html
You need to start with 120/0.781 = ?g to end up with 120 g at 78.1% yield.