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Relative Lowering of Vapour Pressure

5 g of Na2S...

Question

$5 g$ of ${N a}_{2} {S O}_{4}$ was dissolved in $x$ $g$ of $H_{2} O$ . The change in freezing point was found to be ${3.82}^{o} C$ . If ${N a}_{2} {S O}_{4}$ is $81.5 %$ ionised, the value of $x$ .
( $K_{f}$ for water $= {1.886}^{o} C$ $k g$ ${m o l}^{- 1}$ ) is approximately
(Molar mass of $S = 32 g$ ${m o l}^{- 1}$ and that of $N a = 23 g$ ${m o l}^{- 1}$ )

A

45 g

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B

65 g

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C

15 g

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D

25 g

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Solution

The correct option is A $45 g$
$G i v e n Δ T_{f} = 3.82 \circ C = 277 K K_{f} f o r w a t e r = 1.886 \circ C = 275 K Δ T_{f} = k f \times w_{2} \times 1000 / m_{2} \times w_{1} w_{2} & m_{2} a r e w e i g h t o f t h e s o l u t e a n d m o l a r m a s s o f s o l u t e r e s p e c t i v e l y w_{1} i s t h e w e i g h t o f t h e s o l v e n t M o l a r m a s s o f N a_{2} S O_{4} = 2 \times 23 + 32 + 16 \times 4 = 142 g w_{1} = 275 \times 5 \times 1000 / 277 \times 142 w_{1} = 35$

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Similar questions

5 g

N a_{2} S O_{4}

was dissolved in

x g

H_{2} O

. The change in freezing point was found to be

{3.82}^{o} C

N a_{2} S O_{4}

81.5 %

ionised, the value of

x

(k_{f}

= {1.86}^{o} C k g m o l^{- 1})

is approximately:
(molar mass of

S = 32 g m o l^{- 1}

N a = 23 g m o l^{- 1}

)

Q. The freezing point depression constant for water is

- 1.86 C

5.00

{N a}_{2} {S O}_{4}

is dissolved in

45.0

H_{2} O

. The freezing point is changed by

- 3.82 C

Calculate the van't Hoff factor?

Q. 32g of hydrated magnesium sulphate

M g S O_{4} . x H_{2} O

is dissolved in 84g of water. The solution freezes at

- {4.836}^{0} C

K_{f} =

m o l^{- 1}

M g S O_{4}

is a strong electrolyte, then the value of

x

1.5 g

of an unknown substance was dissolved in

7.5 g

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