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: $n.n_R=0,025.2=0,05$

$\Leftrightarrow n_R=\frac{0,05}{n}$

$\Rightarrow M_R=\frac{0,6n}{0,05}=12n$

$n=2\Rightarrow M_R=24(Mg)$