$n_{Fe}=14/56=0,25mol$
$n_{H_2SO_4}=49/98=0,5mol$
$PTHH :$
$Fe + H_2SO_4\to FeSO_4+H_2↑$
$\text{Theo pt : 1 mol 1 mol}$
$\text{Theo đbài : 0,25 mol 0,5 mol}$
Tỷ lệ : $\dfrac{0,25}{1}<\dfrac{0,5}{1}$
$\text{⇒Sau pư H2SO4 dư
$\text{Theo pt :}$
$n_{H_2SO_4\ pư}=n_{Fe}=0,25mol$
$⇒n_{H_2SO_4\ dư}=0,5-0,25=0,25mol$
$⇒m_{H_2SO_4\ dư}=0,25.98=24,5g$
$\text{b.Theo pt :}$
$n_{H_2}=n_{Fe}=0,25mol$
$⇒V_{H_2}=0,25.22,4=5,6l$
$\text{c.Giả sử H2 hao hụt 25%}$
$⇒V_{H_2\ tt}=5,6.75\%=4,2l$
$d. n_{H_2\ lt}=\dfrac{5,6}{22,4}=0,25mol$
$⇒n_{H_2\ tt}=\dfrac{0,25}{90\%}=\dfrac{5}{18}mol$
$\text{Theo pt :}$
$n_{Fe}=n_{H_2}=\dfrac{5}{18}mol$
$⇒m_{Fe}=56.\dfrac{5}{18}=15,56g$