In your case, the reaction produced
28.7g *(1 mole H2/2.016g) = 14.236 moles H2
This implies that the reaction consumed
14.236 moles H2 * (2 moles Al/ 3 moles H2) = 9.491 moles Al
Finally, to convert the number of moles of each reactant to moles, use the molar masses of the two compounds.
9.491 moles Al * ( 26.982 g/1 mole Al) = 256 g
The answers are rounded to three sig figs, the number of sig figs you have for the mass of hydrogen gas.
