Question 18

Calculate the mass of a non-volatile solute (molar mass 40 g mol^-1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.

Answer

Let the vapour pressure of pure octane be p₁⁰.

Then, the vapour pressure of the octane after dissolving the non-volatile solute is 80/100 p₁⁰ = 0.8 p₁⁰.

Molar mass of solute, M₂ = 40 g mol ^{- 1}

Mass of octane, w₁ = 114 g

Molar mass of octane, (C₈H₁₈), M₁ = 8 × 12 + 18 × 1 = 114 g mol^{- 1}

Applying the relation,

(p₁⁰ - p₁) / p₁⁰= (w₂x M₁) / (M₂x w₁ )

⇒ (p₁⁰ - 0.8 p₁⁰) / p₁⁰= (w₂x 114) / (40 x 114 )

⇒ 0.2 p₁⁰ / p₁⁰= w₂/ 40

⇒ 0.2 = w₂/ 40

⇒ w₂ = 8 g

Hence, the required mass of the solute is 8 g.

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Pankaj 2019-05-24 11:24:16

Give the complete & correct solution, immediately

Tanya 2019-05-04 12:11:11

This solution is quite easy but this is not correct. The correct answer for this question is 10.

Tanya 2019-05-04 12:09:55

This is quite easy but correct answer is 10

Bhagya 2019-04-25 10:54:30

For the answer to be 10 , it should be equated with X of solute i.e. ((w1/M1)/(w1/M1)+(w2/M2))

Gautam 2019-04-22 22:21:37

Correct is 10

Neha 2019-03-28 23:44:30

But answer is 10

Neha 2019-03-28 23:43:03

Give the correct solution

Renuka 2019-03-10 23:58:19

Answer is right

Malavika Manish 2019-03-09 15:07:11

The answer is wrong ....the correct answer should be done like given below if vapour pressure of pure liquid is = Po 80 % of pure liquid Ps= 80ÃPo/100 = 0.8Po Ps =Po Ã Xsolute mass of solute = x gram mass of solvent = 114g Molar mass of solute= 40 g/mol Molar mass of solvent (octane C8H18) = 114g/mol Number of moles of solute = x/40 = 0.025x Number of moles of solvent = 114/114= 1 moles Mole fraction of solvent = 1/(1+0.025x) 0.8Po=PoÃ1/(1+0.025x) Cross multiply we get (1+0.025x))0.8Po= Po Divide by 0.8 Po we get 1+0.025x = 1.25 Subtract 1 both side we get 0.025x = 0.25 Now divide by 0.025 we get x = 10g

nikhil gupta 2019-01-17 15:37:35

this answer in wrong instead the formula used will be p-ps/ps

NCERT Chapter

Calculate the mass of a non-volatile solute (molar mass 40 g mol^-1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.

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Calculate the mass of a non-volatile solute (molar mass 40 g mol-1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.

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Calculate the mass of a non-volatile solute (molar mass 40 g mol^-1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%.