   Chapter 2, Problem 162IL

Chapter
Section
Textbook Problem

When analyzed, an unknown compound gave these experimental results: C, 54.0%; h, 6.00%; and O, 40.0%. Four different students used these values to calculate the empirical formulas shown here. Which answer is correct? Why did some students not get the correct answer? (a) C4H5O2 (b) C5H7O3 (c) C7H10O4 (d) C9H12O5

Interpretation Introduction

Interpretation: The correct empirical formula for the compound should be determined from the given empirical formulas.

Concept introduction:

• Empirical formula of a compound represents the smallest whole number relative ratio of elements in that compound.
• Equation for number moles from mass and molar mass,

Numberofmoles=MassingramsMolarmass

• Mass percent of elements of a compound is the ratio of mass of element to the mass of whole compound and multiplied with hundred.
Explanation

Reason for the correct answer:

Mass percent of an element means, 100g of compound contains that mass percent of an element in grams. Therefore, mass of  54g C and 6g of hydrogen and 40g of oxygen are present respectively in 100g of unknown compound.

Equation for number moles from mass and molar mass is,

Numberofmoles=MassingramsMolarmass

Therefore,

The number of moles of carbon is,

Numberofmoles=54g12.01g/mol=4.5mol

The number of moles of hydrogen is,

Numberofmoles=6g1g/mol=6mol

The number of moles of oxygen is,

Numberofmoles=40g16g/mol=2.5mol

So, the mole ratio between elements in unknown compound is,

C:H:O=4.5:6:2

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