Alternative Approach
An alternative approach to arriving at the same answer involves using conservation
of mass. Since the reaction vessel initially contained 50.00 g of reactants (10.00
g of Sn and 40.00 g of I2), then at the conclusion of the reaction
50.00 g of material must remain. Assuming that the limiting reagent (I2)
was completely consumed and 49.35 g of SnI4 was produced, then the
reaction vessel also contains 50.00 g - 49.35 g = 0.65 g of unreacted tin.