Answer :
Answer : The mass of NaCl added should be, 5.85 grams.
Explanation : Given,
Mass of [tex]H_2O[/tex] = 250 g
Molality of solution = 0.40 mol/kg
Molar mass of [tex]NaCl[/tex] = 58.5 g/mole
Molality : It is defined as the number of moles of solute present in one kilogram of solvent.
Formula used :
[tex]\text{Molality}=\frac{\text{Mass of NaCl}\times 1000}{\text{Molar mass of NaCl}\times \text{Mass of water (in g)}}[/tex]
Now put all the given values in this formula, we get:
[tex]0.40mol/kg=\frac{\text{Mass of NaCl}\times 1000}{58.5g/mole\times 250g}[/tex]
[tex]\text{Mass of NaCl}=5.85g[/tex]
Therefore, the mass of NaCl added should be, 5.85 grams.