KCET · Chemistry · Solutions

The relative lowering of vapour pressure produced by dissolving \(18\) g of urea (Molar mass \(= 60\text{ g mol}^{-1}\)) in \(100\) g of water is

A \(0.025\)
B \(0.512\)
C \(0.051\)
D \(0.250\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(0.051\)

Step-by-step Solution

Detailed explanation

Moles of urea, \(n_2 = \dfrac{18}{60} = 0.3 \text{ mol}\)

Moles of water, \(n_1 = \dfrac{100}{18} = 5.55 \text{ mol}\)

According to Raoult's law, the relative lowering of vapour pressure is equal to the mole fraction of the solute.

\(\dfrac{\Delta P}{P^{\circ}} = \chi_2 = \dfrac{n_2}{n_1 + n_2}\)

\(\chi_2 = \dfrac{0.3}{5.55 + 0.3} = \dfrac{0.3}{5.85} = 0.0512\)

Rounding to three decimal places, we get \(0.051\).

Answer: \(0.051\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

Same subject

Match List-I with List-II:

List-I (Element-Atomic number)		List-II (Position in periodic table)
(a)	Ra – 88	(i)	\(4^{th}\) period, \(13^{th}\) group
(b)	Ga – 31	(ii)	\(6^{th}\) period, \(6^{th}\) group
(c)	W – 74	(iii)	\(5^{th}\) period, \(10^{th}\) group
(d)	Pd – 46	(iv)	\(7^{th}\) period, \(2^{nd}\) group

Choose the correct answer from the options given below.

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