Chapter 4.4, Problem 4.4CYU

### 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 0523-g sample of the unknown compound CxHy was burned in air to give 1.612 g of CO2 and 0.7425 g of H2O. A separate experiment gave a molar mass for CxHy of 114 g/mol Determine the empirical and molecular formulas for the hydrocarbon.

Interpretation Introduction

Interpretation:

The empirical and molecular formula for the unknown hydrocarbon CxHy should be determined.

Concept introduction:

• Empirical formula of a compound represents the smallest whole number relative ratio of elements in that compound.
• Equation for finding Molecular formula from the empirical formula,

MolarmassEmpiricalformula mass × Empirical formula

• The molar mass of an element or compound is the mass in grams of 1 mole of that substance, and it is expressed in the unit of grams per mol (g/mol).
Explanation

From the given masses the amount of CO2â€‰andâ€‰H2O can be calculated.

Thus, amount of CO2â€‰andâ€‰H2O isolated from the combustion are,

â€‚Â 1.612â€‰gâ€‰CO2â€‰Ã—â€‰1â€‰molâ€‰CO244.010â€‰gâ€‰CO2â€‰=â€‰0.03662â€‰â€‰molâ€‰CO20.7425â€‰gâ€‰H2Oâ€‰Ã—1â€‰molâ€‰H2O18.015â€‰gâ€‰H2Oâ€‰=â€‰0.04121â€‰molâ€‰H2O

For every mole of CO2 isolated, 1 mol of C must have been present in the unknown compound.

So,

Â Â Â Â 0.03662â€‰molâ€‰CO2â€‰Ã—1â€‰molâ€‰Câ€‰inâ€‰unknown1â€‰molâ€‰CO2â€‰=â€‰0.03662â€‰molâ€‰C

For every mole of H2O isolated, 2 mol of H must have been present in the unknown compound.

Therefore,

Â Â Â Â 0.04121â€‰molâ€‰H2Oâ€‰Ã—2â€‰molâ€‰Hâ€‰inâ€‰unknown1â€‰molâ€‰H2Oâ€‰=â€‰0

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