Two elements A and B form compounds having formula AB 2 and AB 4 . When dissolved in 20 g of benzene, 1 . 0 g of AB 2 lower the freezing point by 2 . 3 ° C , whereas 1 . 0 g of AB 4 lowers the freezing point by 1 . 3 ° C . The molal depression constant for benzene in 1000 g is 5 . 1 ° C . Calculate the atomic masses of A and B.

Freezing point depression refers to the lowering of the freezing point of solvents upon the addition of solutes. It is a colligative property of solutions that is generally proportional to the molality of the added solute. By using the depression in freezing point formula, M 2 = 1000 × k f × w 2 w 1 × ∆ T f M AB 2 = 1000 × 5 . 1 × 1 20 × 2 . 3 = 110 . 87 gmol - 1 M AB 4 = 1000 × 5 . 1 × 1 20 × 1 . 3 = 196 . 15 g mol - 1 Let atomic masses of A and B are p and q molar mass of AB 2 = p + 2 q = 110 . 87 g mol - 1 1 molar mass of AB 4 = p + 4 q = 196 . 15 gmol - 1 2 By subtracting equation (1) and (2) 2 q = 85 . 28 , q = 42 . 64 Put value of q in equation (1) p = 110 . 87 - 85 . 28 p = 25 . 59 Atomic mass of A = 25 . 58 g mol - 1 Atomic mass of B = 42 . 64 gmol - 1

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The van't Hoff factor for a dilute aqueous solution of glucose is less than one.

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IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>TRUE OR FALSE TYPE QUESTIONS>Q 34

The molecular mass of acetic acid (dissolved in benzene) as determined by cryoscopic method is almost double of the normal value.

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ICSE Chemistry>Chapter 2 - Solutions>TRUE OR FALSE TYPE QUESTIONS>Q 35

The osmotic pressure of

0.1 M KCl

solution will always be higher than that of

0.1 M

urea solution provided the temperature of the two solutions is the same.

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12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 1

Two elements A and B form compounds having formula

{AB}_{2}

and

{AB}_{4}

. When dissolved in

20 g

of benzene,

1.0 g

{AB}_{2}

lower the freezing point by

2.3 ° C

, whereas

1.0 g

{AB}_{4}

lowers the freezing point by

1.3 ° C

. The molal depression constant for benzene in

1000 g

5.1 ° C

. Calculate the atomic masses of A and B.

HARD

12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 2

An aqueous solution of a non-volatile and non-electrolytic substance boils at

100.5 ° C

. Calculate the osmotic pressure of this solution at

27 ° C

K_{b}

for water per

1000 g = 0.50

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12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 3

27 ° C

36 g

of glucose per litre has an osmotic pressure of

4.92

atmosphere. If the osmotic pressure of another solution of glucose is

1.5

atmosphere at the same temperature, what would be its concentration?

HARD

12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 4

A very small amount of a non-volatile solute (that does not dissociate) is dissolved in

56.8 {cm}^{3}

of benzene (density

= 0.889 g {cm}^{- 3}

). At room temperature, vapour pressure of this solution is

98.88 mm Hg

while that of benzene is

100 mm Hg

. Find the molality of this solution. If the freezing point of this solution is

0.73

degree lower than that of benzene, what is the value of molal freezing point depression constant of benzene?

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12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 5

A solution prepared by dissolving

0.300 g

of an unknown compound in

30 g

{CCl}_{4}

has a boiling point that is

0.392 ° C

higher than that of pure

{CCl}_{4}

. What is the molecular weight of solute? (Molal boiling point elevation constant of

{CCl}_{4}

5.02 ° C / m

)

MEDIUM

12th ICSE

IMPORTANT

ICSE Chemistry>Chapter 2 - Solutions>NUMERICAL PROBLEMS>Q 6

0.85 %

aqueous solution of

{NaNO}_{3}

is apparently

90 %

dissociated at

27 ° C

. Calculate its osmotic pressure.

(R = 0.0821 L atm K^{- 1} {mol}^{- 1})

The van't Hoff factor for a dilute aqueous solution of glucose is less than one.

Important Questions on Solutions

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