   Chapter 2.8, Problem 2.2ACP

Chapter
Section
Textbook Problem

Salvarsan was long thought to be a single substance. Recently, however, a mass spectrometry study of the compound shows it to be a mixture of two molecules with the same empirical formula. Each has the composition 39.37% C, 3.304% H, 8.741% O, 7.652% N, and 40.932% As. One has a molar mass of 549 g/mol and the other has a molar mass of 915 g/mol. What are the molecular formulas of the compounds?

Interpretation Introduction

Interpretation: For the given percent compositions of two compounds, the molecular formula have to be determined using given molar masses of the compound as 549g/mol and 915g/mol.

Concept introduction:

• Molecular Formula: It is the simplest representation of composition of chemical substance where each and every type of atom is written with the total number of the atom present in the substance.
• Equation for finding Molecular formula from the empirical formula,

MolarmassEmpiricalformula mass × Empirical formula

• Moles: One mole is equivalent to the mass of the substance consists same number of units equal to the atoms present in 12g of 12C.
• From given mass of substance moles could be calculated by using the following formula,

Molesofsubstance GivenmassofsubstanceMolecularmass

• 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

Mass percent of C, H, O, N and As in compound is given as 39.37%, 3.304%, 8.741%, 7.652% and 40.932% .

Mass percent of an element means, 100g of compound contains that mass percent of an element in grams. Therefore, mass of C, H, N, O and As are 39.37g, 3.304g, 8.741g, 7.652g and 40.932g respectively in 100g of compound.

Equation for number moles from mass and molar mass is,

Numberofmoles=MassingramsMolarmass

Therefore, the number of moles of C is,

Numberofmoles=39.37g12.01g/mol=3.278mol

The number of moles of H is,

Numberofmoles=3.304g1.008g/mol=3.27mol

The number of moles of N is,

Numberofmoles=7.652g14.007g/mol=0.5465mol

The number of moles of O is,

Numberofmoles=8.741g16g/mol=0.5463mol

The number of moles of As is,

Numberofmoles=40.932g74.9g/mol=0

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