How many moles of pcl5 can be produced from 60.0 g of cl2 (and excess p4) ?

The idea here is to convert 60.0.0 g of Cl2 to moles of Cl2, to moles PCl5 using dimensional analysis and equation coefficients. We can do that as follows:

1 mol Cl2 = 70.906 g Cl2

10 mol Cl2 = 4 mol PCl5

So what you do is the following:

[ (60.0 g Cl2) / 1][ (1 mol Cl2) / (70.906 g Cl2) ][ (4 mol PCl5) / (10 mol Cl2) ] = 0.3384762 mol PCl5 or 0.338 mol PCl5

That meanns that 0.338 mol of PCl5 can be produced from 60.0 g of Cl2 and excess P4.