   Chapter 13, Problem 44PS

Chapter
Section
Textbook Problem

Anthracene, a hydrocarbon obtained from coal, has an empirical formula of C7H5. To find its molecular formula, you dissolve 0.500 g in 30.0 g of benzene. The boiling point of pure benzene is 80.10 °d whereas the solution has a boiling point of 80.34 °C. What is the molecular formula of anthracene?

Interpretation Introduction

Interpretation: The molecular formula of anthracene has to be determined.

Concept introduction:

Colligative properties: Properties of solutions which having influence on the concentration of the solute in it. Colligative properties are,

• Decrease in the vapor pressure
• Increase in the boiling point
• Decline in the freezing point
• Osmotic pressure

Change in boiling point is calculated by using the equation,

ΔTbp=Kbpmsolute

where,

Kbp is the molal boiling point elevation constant.

The number of moles of any substance can be determined using the equation

Numberofmole=GivenmassofthesubstanceMolarmass

Explanation

The molecular formula of anthracene is determined.

Given,

ΔT=(80.340C80.100C)=0.240C

Mass of orange crystalline compound is 0.500g

Mass of benzene is 30g=0.03kg

Molal boiling point elevation constant of benzene is 2.530C/m

Change in boiling point is calculated by using the equation,

ΔTbp=Kbpmsolute

Hence,

The concentration is calculated by,

Concentration,msolute=ΔTbpKbp=(0.240C)(2.530C/m)=0.09486m

The amount of compound dissolved is,

(0.09486m1kg)(0.03kg)=2

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