A. Since you are starting with equal moles of the two reactants, you can see from the balanced equation that you need 3 moles of H2SO4 for every 2 moles of Al(OH)3. So, H2SO4 is the limiting reactant.
B. 0.420 mol H2SO4 X (1 mol Al2(SO4)3 / 3 mol H2SO4) = 0.140 mol Al2(SO4)3
C. Moles of Al(OH)3 used:
0.420 mol H2SO4 X (2 mol Al(OH)3/3 mol H2SO4)
= 0.280 mol Al(OH)3 used Moles remaining
= 0.420 - 0.280
= 0.140 mol Al(OH)3 remaining.
