MOLALITY AND MOLE FRACTION

COLLIGATIVE PROPERTIES OF

SOLUTION

Why???

Although involving colligative

properties of the solution, colligative

properties do not depend on the

interaction between solvent and

solute molecules, but in

dependence on the amount of

solute dissolved in a solution.

MOLALITY

Molality is the number of moles of

solute in 1 kg (1000 grams) of solvent .

Example

Determine the molarity of solution that

produce 3 grams urea in 200 grams

water????

Answer :

Mole of urea= 3 g/60 g mole -1

Mass of solvent= 200 g =0.2 kg

m= n/p

=0.05/0.2

=o.25 mol kg-1

example

A total of 1.8 g of glucose, C6H12O6

dissolved in 100 grams of water (Ar C = 12, H

= 1, O = 16). Determine the molality of the

glucose solution!

Mole fraction

Mole fraction (X) is ratio of the number of moles

of solute or solvent to the number of moles of

solution.

example

In 100 grams of 20% urea solution

contained 20 grams of urea and 80

grams of water. Determine the moles of

urea fraction

Answer:

Mole of water= 80 g/18 g mole-1

= 4.44 mole

Mole of urea = 20 g/60 g mole-1

= 0.33

Xurea = XB= 0.33/4.44+0.33= 0.069

example

A total of 27.6 grams of ethanol

C2H5OH dissolved in 54 grams of

water (Ar C = 12, H = 1, O = 16).

Calculate the mole fraction mole

fraction of ethanol and water!

answer

Mr.C2H5OH = 46 gr/mol

Mr.H2O = 18 gr/mol

n.C2H5OH = 27,6 gr/(46 gr/mol)

n.H2O = 54 gr/(18 gr/mol)

χ etanol = 0,6 mol/(0,6 mol + 3 mol)=0,167

χ Air = 3 mol/(0,6 mol + 3 mol) = 0,833

exercise

1. Determine the amount (grams) of

NaOH must be dissolved in 1 liter of

water (water = 1.00 g / mL) to obtain

0.25 m NaOH.

2. In 100 grams of 20% urea solution

contained 20 grams of urea and 80

grams of water. Determine the moles of

urea fraction.

Cont.

3. Determine the mole fraction of

solute (Mr=60) in urea solution 10

%

4. Determine the percentage of urea

in urea solution 1 m

5.Determine the mole fraction of 2

molal of urea solution

Remember !!!!!!!!!!!!

Collect your assignment next week

:D