Question

Calculate the mass of urea required in making 5 kg and point to find molar aqueous solution

Answer 1

Molality is defined as number of moles of solute present in 1 kg of solvent.

In the question,we are required to prepare a 5 molal solution of urea.

Molar mass of urea=60 g

From the equation,

Mass of solution=5 kg=5000 g

(Note that mass of solution and solvent are different quantities.)

Mass of solute and solvent both are unknown and we are required to find the mass of solute as per the question.

Let the mass of solute be xg.

Mass of solvent =(5000-x)g=(5000-x)/1000 g

No of moles of solute=x/60 moles

Molality = number of moles of solute/mass of solvent in kg

=5=1000(x/60)/(5000-x)

=0.3=x/(5000-x)

=1500-0.3x=x

=x=1500/1.3=1153.84 g

CHECKING:

Mass of solute= 1500 g

Moles of Solute =1154/60=19.23

Mass of solvent=5000-1154=3046 g=3.846 kg

Molality=moles of solute/mass of solvent=19.23/3.846=5