Chapter 4, Problem 35PS

### 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

# An unknown compound has the formula CxHyOz. You burn 0.0956 g of the compound and isolate 0.1356 g of CO2 and 0.0833 g of H2O. What is the empirical formula of the compound? If the molar mass is 62.1 g/mol, what is the molecular formula?

Interpretation Introduction

Interpretation:

The empirical and molecular formula of Unknown compound 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

When 0.0956â€‰g of unknown compound is burned in oxygen, 0.1356g of CO2 and â€‰0.0833â€‰gâ€‰H2O can be isolated.

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

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

â€‚Â 0.1356gâ€‰CO2â€‰Ã—â€‰1â€‰molâ€‰CO244.010â€‰gâ€‰CO2â€‰=â€‰0.00308â€‰â€‰molâ€‰CO20.0833â€‰gâ€‰H2Oâ€‰Ã—1â€‰molâ€‰H2O18.015â€‰gâ€‰H2Oâ€‰=â€‰0.00462â€‰molâ€‰H2O

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

So,

â€‚Â 0.00308â€‰molâ€‰CO2â€‰Ã—1â€‰molâ€‰Câ€‰inâ€‰unknown1â€‰molâ€‰CO2â€‰=â€‰0.00308â€‰molâ€‰C

The given unknown compound has the formula CxHyOz.

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

Therefore,

â€‚Â 0.00462molâ€‰H2Oâ€‰Ã—2â€‰molâ€‰Hâ€‰inâ€‰unknown1â€‰molâ€‰H2Oâ€‰=â€‰0

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