Question

If the flask has a volume of 0.25 l , what is the minimum mass of ph3bcl3(s) that must be added to the flask to achieve equilibrium?

Answer 1

In order to attain equilibrium, you need to have a minimum of a minute amount of PH3BCl3(s) left in the flask. Alternatively, it will be all spend before you attain equilibrium concentration. Let PH3BCl3(s) be called C. Molecular weight of C is 151. 
4.32x10^-2molC/1L x 0.25 L x 151gC/1molC = 1.63 g C is the answer.