Chapter 13, Problem 82GQ

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# A compound is known to be a potassium halide, KX. If 4.00 g of the salt is dissolved in exactly 100 g of water, the solution freezes at −1.28 °C. Identity the halide ion in this formula.

Interpretation Introduction

Interpretation: The halide ion in KX 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 freezing point is calculated by using the equation,

ΔTfp=Kfpmsolute

where,

Kfp is the molal freezing point depression constant.

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

Numberofmole=GivenmassofthesubstanceMolarmass

Explanation

The molar mass of potassium halide is determined.

Given,

â€‚Â Î”Tâ€‰=â€‰âˆ’1.280C

â€‚Â Mass of potassium halide is 4â€‰g

â€‚Â Mass of water is 100â€‰gâ€‰=â€‰0.1â€‰kg

â€‚Â Molal freezing point depression constant of water is â€‰âˆ’1.860C/m

â€‚Â Molar mass of potassium is 39.10â€‰g/mol

Change in freezing point is calculated by using the equation,

â€‚Â Î”Tbpâ€‰=â€‰Kbpâ€‰msolute

Hence,

The concentration is calculated by,

â€‚Â Concentration,msolute=â€‰Î”TbpKbp=â€‰(â€‰âˆ’1.280C)(âˆ’1.860C/m)=â€‰0.6882â€‰m

The amount of potassium halide is,

â€‚Â (â€‰0.6882â€‰m1â€‰kg)â€‰(0

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