Question

Find required amunt of urea for prepration of 1l solution containing 0.5mol of urea

Answer 1

MOLE FRACTION OF SOLUTE =

MOLES OF SOLUTE/(MOLES OF SOLUTE+ MOLES OF SOLVENT)

==> 0.1= moles of solute/moles of solute + moles of solvent

Let the moles of solute + moles of solvent be 1

==> 0.1=moles of solute(urea) = 0.6 gm (molar mass of urea=60 gm/mol)

==> 0.9=moles of solvent(water) = 16.2 gm(molar mass of water=18 gm/mol)

Total mass of solution= 0.6gm +16.2gm= 16.8 gm

Volume= Mass/ Density=16.8/d mL= 16.8/1000d L

MOLARITY= MOLES OF UREA/LITERS OF SOLUTION

Molarity =0.1 ÷ 16.8/1000d

Molarity = 1/10 x 1000d/ 16.8

Molarity = 100d/16.8 = 125d/21 M

Now if you have the density, you can plug the value in this equation to get the answer. (Warning: This would only work when the density (d) is in gm/ml.)

As you have now provided the density, thus as d=0.21 gm/mL, we can put this value in the above equation to get-

Molarity= 125 x 0.21/21=1.25 M

Thus the Molarity of Urea in the solution is 1.25 M.