The molal boiling point elevation constant for water is 0.512 degree C./m. What

Question

The molal boiling point elevation constant for water is 0.512 degree C./m. What is the boiling point of a solution containing 673 g of water and 78.0 g of CaCl_2? 4. A solution containing 32.0 g of an unknown non-volatile molecular solute and 500 g of water freezes at -4.25 degree C. K_f for water is 1.86 degree C/m. What is the molar mass of the unknown solute? In step A4 of the experiment, it says, "The temperature will drop initially and then remain nearly constant as the cyclohexane begins to freeze." Why should the temperature stay the same while the solvent is freezing?

Explanation / Answer

1.

CaCl2 ionizes in water and

CaCl2--------àCa+2 + 2Cl-

I= 3, number of ions

deltaT= i*kb*m

m= molality of solute/ kg of solvent

Molar mass of CaCl2= 111, moles of solute= 78/111=0.7027

Molality, m= 0.7027/0.673 = 1.044 moles/kg

Boiling point elevation = 0.512*3*1.044 =1.6

Boiling point of water = 100+1.6= 101.6 deg.c

2.

Let M= Molar mass of unknown sample , moles = Mass/M= 32/M

Freezing point depression = 4.25= kf*m

Hence m= 4,25/1.86 =2.28 moles/kg solvent

1000 gm solvent water contains 2.28 moles of non-volatile component

500 gm of water contains 2.28*500/1000 =1.14 gmoles of solvent

32/M= 1.14

M= 32/1.14 =28 gm/mole

3 Cyclohexane freezes at constant temperature and until complete freezing is achieved, the temperature drops and once freezin point is reached. the temperature remains constant.

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