   Chapter 12, Problem 12.108QP General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343

Solutions

Chapter
Section General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
ISBN: 9781305580343
Textbook Problem

Analysis of a compound gave 39.50% C, 2.21% H, and 58.30% Cl. When 0.855 g of this solid was dissolved in 7.50 g of naphthalene, the solution had a freezing point of 78.0°C. The pure solvent freezes at 80.0°C; its molal freezing point constant is 6.8°C/m. a What is the molecular formula of the compound? b What is its molecular weight to the nearest 0.1 g?

(a)

Interpretation Introduction

Interpretation:

A compound contains 39.50% Carbon , 2.21% Hydrogen and 58.30% Chlorine. 0.855 g of this substance is dissolved in 7.50 g of naphthalene and the solution has freezing point of 78°C. Molecular formula of the compound has to be determined.

Concept Introduction:

Depression in freezing point is the phenomenon of lowering of freezing point of a substance, which is considered as solvent, by adding a non-volatile solute.   It is expresses as –

ΔTf = Kf × m

Where,

ΔTf = depression in freezing pointKf  = cryoscopic constantm    = molality of the solute

Number of moles of a substance is related to molar mass of the substance as,

no.of moles massmolar mass

Explanation

Given that freezing point of the solution is 78°C. freezing point of pure naphthalene is 80°C . Kf for naphthalene is 6.8°C/m. rewriting the depression in freezing point equation, determine the molality of the solution.

m = ΔTfKf = [80°C78°C)]6.8°C/m = 0.294 m

Number of moles of the compound considering 1 kg of solvent,

moles = 0.294 mol1 kg of solvent×0.00750 kg = 2.21×103 mol

Molar mass of the compound is calculated as,

molar mass =  0.855 g2.21×103 mol = 387 g/mol

Number of moles of elements in 100 g of the compound are calculated as follows -

moles of C = 39

(b)

Interpretation Introduction

Interpretation:

A compound contains 39.50% Carbon , 2.21% Hydrogen and 58.30% Chlorine. 0.855 g of this substance is dissolved in 7.50 g of naphthalene and the solution has freezing point of 78°C. Molar mass of the compound nearest to the tenth of a gram has to be determined.

Concept Introduction:

Depression in freezing point is the phenomenon of lowering of freezing point of a substance, which is considered as solvent, by adding a non-volatile solute.   It is expresses as –

ΔTf = Kf × m

Where,

ΔTf = depression in freezing pointKf  = cryoscopic constantm    = molality of the solute

Number of moles of a substance is related to molar mass of the substance as,

no.of moles massmolar mass

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