   Chapter 2.8, Problem 2.1ACP

Chapter
Section
Textbook Problem

Arsenic is found widely in the environment and is a major problem in the ground water supply in Bangladesh. Orpiment is one arsenic-containing mineral and enargite is another. The latter has 19.024% As, 48.407% Cu, and 32.569% S. What is the empirical formula of the mineral?

Interpretation Introduction

Interpretation: Considering the given percent compositions of compounds in mineral, the empirical formula of mineral should be determined.

Concept introduction:

• Empirical formula of a compound represents the smallest whole number relative ratio of elements in that compound.
• 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 As, Cu and S in compound is given as 19.024%, 48.407% and 32.569%.

Mass percent of an element means, 100g of compound contains that mass percent of an element in grams. Therefore, mass of As, Cu and S are 19.024g, 48.407g and 32.569g respectively in 100g of compound.

Equation for number moles from mass and molar mass is,

Numberofmoles=MassingramsMolarmass

Therefore,

The number of moles of As is,

Numberofmoles=19.024g74.9g/mol=0.254mol

The number of moles of Cu is,

Numberofmoles=48

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