Câu 21: B
$NaX+AgNO_3\to AgX+NaNO_3$
$2AgX\to 2Ag+X_2$
$\Rightarrow n_{AgX}= n_{Ag}=\frac{1,08}{108}=0,01 mol= n_{NaX}$ (BTNT)
$\Rightarrow M_{NaX}=\frac{1,03}{0,01}=103=X+23$
$\Leftrightarrow X=80(Br)$
Câu 22: A
$Cl_2+2KI\to 2KCl+I_2$
$n_{Cl_2}=\frac{1}{2}n_{KI}=\frac{1}{2}.\frac{8,3}{166}=0,025 mol$
$2KMnO_4\to 5Cl_2$
$\Rightarrow n_{KMnO_4}=0,01 mol$
$\%KMnO_4=\frac{0,01.158.100}{2,5}=63,2\%$
Câu 24: B
$Ba(OH)_2+2HCl\to BaCl_2+2H_2O$
$n_{Ba(OH)_2}= 0,002 mol$
$n_{HCl}=0,009 mol < 2n_{Ba(OH)_2}$
$\Rightarrow$ Dư $HCl$.
$n_{BaCl_2}=n_{Ba(OH)_2}=0,002 mol$
$\Rightarrow C_M=\frac{0,002}{0,05+0,15}=0,01M$
Câu 26: A
$n_{NaCl}= x$
$\Rightarrow \frac{58,5x.100}{58,5x+210}=16$
$\Leftrightarrow x=0,684$
$\Rightarrow m_{NaCl}=0,684.58,5=40g$
Câu 27: D
Kim loại R hoá trị n.
$\Delta m= m_R - m_{H_2}$
$\Rightarrow m_{H_2}=0,6-0,55=0,05g$
$\Rightarrow n_{H_2}=0,025 mol$
Bảo toàn e: $n.n_R=0,025.2=0,05$
$\Leftrightarrow n_R=\frac{0,05}{n}$
$\Rightarrow M_R=\frac{0,6n}{0,05}=12n$
$n=2\Rightarrow M_R=24(Mg)$
