Đáp án:
S = 25g
Giải thích các bước giải:
\(\text C\%_{\text{dung dịch NaOH}}=\dfrac{m_{\text{ct}}}{m_{\text{dd}}}=20\%\\\to \dfrac{m_{NaOH}}{m_{H_2O}+m_{NaOH}}=20\%\to m_{NaOH}=0,2m_{H_2O}+0,2m_{NaOH}\\\to 0,8m_{NaOH}=0,2m_{H_2O}\\\to m_{H_2O}=4m_{NaOH}\)
`-> S_{text{NaOH}}=\frac{m_{NaOH}}{m_{H_2O}}\times 100=\frac{m_{NaOH}}{4\times \text{m}_{\text{NaOH}}}\times 100=25\ \text {g`