need help on two answers

Question

The thermite reaction (shown below) is very important in welding projects and other important processes:

Fe₂O₃(s) + 2 Al(s) =====> 2 Fe(l) + Al₂O₃(s)

When the reaction is initiated with 21.5 g of Fe₂O₃ and an excess of Al, 12.3 g of Fe are collected.

Determine

a) the theoretical yield:  g

b) the percent yield of this process: 

Answer 1

Fe₂O₃(s) + 2 Al(s) ------> 2 Fe(l) + Al₂O₃(s)

The theoretical yield comes from the stoichiometry of our chemical reaction

(21.5g Fe₂O₃)(1 mol Fe₂O₃/159.69g Fe₂O₃)(2 mol Fe/1 mol Fe₂O₃)(55.845g Fe/1 mol Fe)

mass = 15.0g Fe

This is the amount of iron we should yield from this chemical reaction starting with 21.5g Fe₂O₃ and an excess of aluminum

However, because we attained fewer than 15.0g Fe, we want to calculate the percent yield

% Yield = actual yield/theoretical yield x 100%

<=> 12.3g/15.0g x 100%

% Yield = 81.8%

Cheers!