Question

What is the theoretical yield of PI3 from the reaction of 27.0 g of P and 68.0 g I2? 2P(s)+3I2(s) 2PI3(s)

Answer 1

You have a limiting reagent problem How do I know? Because BOTH reactants are given. So basically you work two simple stoichiometry problems and use the one which produces the smaller amount of product as the limiting reagent. Here is the procedure for the first one.
Write and balance the equation.
Here is a worked example of a simple stoichiometry problem.
http://www.jiskha.com/science/chemistry/stoichiometry.html
Post your work if you get stuck.

Answer 2

73.5 g