3.
Gọi x, y là số mol Fe, Mg.
$\Rightarrow 56x+24y=9,92$ $(1)$
$Fe+4HNO_3\to Fe(NO_3)_3+NO+2H_2O$
$3Mg+8HNO_3\to 3Mg(NO_3)_2+2NO+4H_2O$
$n_{NO}=\dfrac{4,928}{22,4}=0,22(mol)$
$\Rightarrow x+\dfrac{2}{3}y=0,22$ $(2)$
$(1)(2)\Rightarrow x=0,1; y=0,18$
$\%m_{Fe}=\dfrac{56.0,1.100}{9,92}=56,45\%$
$\%m_{Mg}=43,55\%$
4.
Gọi x, y là số mol Fe, Mg.
$\Rightarrow 56x+24y=14$ $(1)$
$Fe+6HNO_3\to Fe(NO_3)_3+3NO_2+3H_2O$
$Mg+4HNO_3\to Mg(NO_3)_2+2NO_2+2H_2O$
$n_{NO_2}=\dfrac{17,92}{22,4}=0,8(mol)$
$\Rightarrow 3x+2y=0,8$ $(2)$
$(1)(2)\Rightarrow x=0,22; y=0,07$
$\%m_{Fe}=\dfrac{56.0,22.100}{14}=88\%$
$\%m_{Mg}=12\%$
