8. Rank the following substances in terms of boiling point from the lowest (1) t

Question

8. Rank the following substances in terms of boiling point from the lowest (1) to the highest (6), and indicate the significant intermolecular force(s) in each. HOCH2CH2CH2OH CaF2 CH3CH2CH2CH2CHa CH3CH3 HSCH2CH2CH2OH CH 3-O-C3H7 9. You have several different solutions of salts and want to impress your professor on being able to predict the order of boiling points from lowest, 1, to highest, 5. Each of the solutions is exactly 0.010 m in the compound indicated: Rank them from the lowest boiling point (1) to the highest boiling point (5) glucose AI(NOs)a KI CaCl2 Ga2(SO4) 10. For a solution of 2.500 g of an unknown compound in 100.0 g benzene (mp 5.455°C,K -5.065 C/m) upon cooling, the first crystal is observed at 4.880 °C. What is the molar mass of the compound?

Explanation / Answer

Q8)

From lowest to highest boiling

CH3CH3 < C5 H12 < Ch3O-C3H7 < HSCH2CH2CH2OH < HOCH2CH2CH2OH < CaF2

CH3CH3 only weak vander Waals forces, lower molecular weight

C5H12 weak vander Waals forces, higher molecular weight

CH3OC3H7 - slightly polar , vander Waals forces

HSCH2CH2CH2OH - polar, hydrogen bonds

HOCH2CH2CH2OH - more polar, more hydrogen bonds

CaF2 ionic forces

Q9)We know by dissolving solute the boiling point increases due to the colligative property called 'elevation in boiling point' which depends only on the number of particles of soute present in solution.

Thus the order or increasing boiling point is the order of increasing number of particles.[all have same molarity of solute]

thus the order of boiling point is

glucose (only one particle) < KI (two ions) < CaCL2( 3 ions) < Al(NO3)3 ( 4 ions) < Ga2(SO4)3 ( 5 ions)

Q10)

mass of solute = 2.500g

mass of solvent = 100 .0g

molar mass of solute = ?

depression in freezing point = 5.455-4.88

=0.575

Kf of solvent = 5.065 C/m

molality of solute = (mass/molarmass) / mass of solvent in kg

= (2.5/ molarmass) /0.1kg

Now depression in freezing point = kf x molality [ colligative property]

0.575 = 5.065C/m [2.5/molar mass] /0.1m

Thus molar mass of solute =220.2 g/mol

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