Question

The density of NaOH solution is 1.2 g cm^–3 . The molality of this solution is ...... m.  
(Round off to the Nearest Integer) 
[Use : Atomic masses : Na : 23.0 u O : 16.0 u H : 1.0 u  Density of H₂O : 1.0 g cm^–3]

Answer 1

Volume of the solution = x 
Mass of solute = 0.2 x gram
Mass of water = x gram 
Atomic mass of NaOH= 40
Molarity = \(\frac{\text{moles of solute}}{\text{liters of solution}}\)
 Substituting the value =\(\frac{0.2x \times1000}{40x}\)
\(= \frac{200}{4}\) 
= 5m
So, the correct answer is 5m.

Answer 2

Let's assume :
volume of solution is x ml
mass of solution = 1.2x
& mass of water (solvent) = x gram
Molality = \(\frac{W_{solute}\times1000}{GMM_{solute}\times W_{solvent}}\)
\(=\frac{0.2x\times1000}{40\times x}\)
\(=\frac{200}{40}=5\ m\)
So, the correct answer is 5m.