\(X_{NaOH} =\frac{n_{NaOH}}{n_{NaOH}+n{H_2O}}\)

= \(\frac{\frac{W_{NaOH}}{M_{NaOH}}}{\frac{W_{NaOH}}{M_{NaOH}}+\frac{W_{H_2O}}{M_{H_2O}}}\)

= \(\frac{\frac{4}{40}}{\frac{4}{40}+\frac{36}{18}}\)

= \(\frac{\frac{1}{10}}{\frac{1}{10}+2}\) = \(\frac{\frac{1}{10}}{\frac{21}{10}}\) = \(\frac{1}{10}\) x \(\frac{10}{21}\) = \(\frac{1}{21}\)

= 0.047

\(\because\) x_{NaOH} + x_{H2O} = 1

\(\therefore\) x_{H2O} = 1 - 0.047 = 0.953

Mass of solution = 36 + 4 = 40g

Volume of solution = Mass x Specific gravity

= 40 x 1 = 40 ml

M = \(\frac{W_B}{M_B}\) x \(\frac{1000}{40}\) = 2.5 M