Molality and mole fraction

Exercise 2

    

The mass of a solution obtained by dissolving $4.00\;g$ of $NaOH$ in water is $504\; g$ What is the molality of $NaOH$ ?

$\mu_{NaOH}$ $=$ $\frac{n_{NaOH}\cdot 1000}{m_{H_2O}}$ $n_{NaOH}$ $=$ $\frac{m_{NaOH}}{M_{NaOH}}$ $=$ $0.100\;mol$ $\mu_{NaOH}$ $=$ $\frac{n_{NaOH}\cdot 1000}{m_S-m_{NaOH}}$ $=$ $0.2$