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Đáp án:

Câu 7:

$225ml  = 225 (g)$

$m_{dd}=m_{ct} + m_{dd}=225 + 25= 250 (g)$

⇒ $\text{C%KCl=$\dfrac{m_{ct}}{m_{dd}}$ × 100%=$\dfrac{25}{250}$ × 100%= 10%}$

⇒ Chọn $A$

Câu 8:

$m_{H_{2}SO_{4}}=n × M=1 × 98= 98 (g)$

⇒ $m_{dd}=m_{ct} + m_{dd}=98 + 18= 116 (g)$

⇒ $\text{C%$H_{2}SO_{4}=\dfrac{m_{ct}}{m_{dd}}$ × 100%=$\dfrac{98}{116}$ × 100%= 84,48%}$

⇒ Chọn $D$

Câu 9: $B$

Câu 10: 

$Na_{2}O + H_{2}O→2NaOH$

Ta có: $n_{Na_{2}O}=\dfrac{m}{M}=\dfrac{124}{62}= 2 (mol)$

$m_{\text{dd sau pứ}}=124 + 876= 1000 (g)$

Theo PT: $n_{NaOH}=2n_{Na_{2}O}=2 × 2= 4 (mol)$

⇒ $m_{NaOH}=n × M=4 × 40= 160 (g)$

⇒ $\text{C%NaOH=$\dfrac{m_{ct}}{m_{\text{dd sau pứ}}}$ × 100%=$\dfrac{160}{1000}$ × 100%= 16%}$

⇒ Chọn $A$

Câu 11:

$n_{NaCl}=\dfrac{m}{M}=\dfrac{117}{58,5}= 2 (mol)$

⇒ $C_{M_{NaCl}}=\dfrac{n}{V}=\dfrac{2}{1,25}= 1,6 (l)$

⇒ Chọn $B$