Question

How many grams of NaCl should be added to 250 grams of waterto make a 0.40 m solutions?

Answer 1

Answer : The mass of NaCl added should be, 5.85 grams.

Explanation : Given,

Mass of $H_2O$ = 250 g

Molality of solution = 0.40 mol/kg

Molar mass of $NaCl$ = 58.5 g/mole

Molality : It is defined as the number of moles of solute present in one kilogram of solvent.

Formula used :

$\text{Molality}=\frac{\text{Mass of NaCl}\times 1000}{\text{Molar mass of NaCl}\times \text{Mass of water (in g)}}$

Now put all the given values in this formula, we get:

$0.40mol/kg=\frac{\text{Mass of NaCl}\times 1000}{58.5g/mole\times 250g}$

$\text{Mass of NaCl}=5.85g$

Therefore, the mass of NaCl added should be, 5.85 grams.