Answer:
Because for dilute and aqueous solutions the mass of solvent will be a very close value to the volume of solution.
Explanation:
Molar concentration is defined as:
[tex][M]=\frac{molessolute}{volumesolution}[/tex]
And molal concentration is defined as:
[tex][m]=\frac{molessolute}{kgsolvent}[/tex]
And:
Msolution=Msolute+Msolvent
For dilute solutions, we have small amounts of solute, then we have:
Msolution=Msolute+Msolvent, and as the mass of solute is very small: Msolution≅Msolvent
If the solution is also aqueous (water as solvent), and considering that the density of water is around 1 gm/cm3 or 1 kg/m3:
Msolvent≅Msolution≅Vsolution
Therefore, if we look to the molar and molal equations, we have the same numerator in both (moles of solute) and nearby numbers for the denominator, giving to the molar and molal concentration close values.
