   Chapter 4, Problem 116GQ

Chapter
Section
Textbook Problem

Calcium and magnesium carbonates occur together in the mineral dolomite. Suppose you heat a sample of the mineral to obtain the oxides, CaO and MgO, and then treat the oxide sample with hydrochloric acid. If 7.695 g of the oxide sample requires 125 mL of 2.55 M HCl,CaO(s) + 2 HCl(aq) → CaCl2(aq) + H2O(ℓ)MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(ℓ)what is the weight percent of each oxide (CaO and MgO) in the sample?

Interpretation Introduction

Interpretation:

The weight percent of CaOandMgO in the given sample has to be determined.

Concept introduction:

• The relation between the number of moles and mass of the substance is ,

Numberofmole=Mass in gramMolarmass

Massingramofthesubstance=Numberofmole×Molarmass

• 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).
• For chemical reaction balanced chemical reaction equation written in accordance with the Law of conservation of mass.
• Law of conservation of mass states that for a reaction total mass of the reactant and product must be equal.
• Stoichiometric factor is a relationship between reactant and product which is obtained from the balanced chemical equation for a particular reaction.
• Amountof substance=Concentrationofsubstance×volumeofthesubstance
• Weight percent of elements of a compound is the ratio of weight of element to the weight of whole compound and multiplied with hundred.
Explanation

Calcium and magnesium carbonates occurs together in the mineral dolomite.

Here dolomite sample is heated to obtain the oxides CaOandMgO and these oxides sample is then treated with hydrochloric acid.

Balanced equations for these reactions are,

CaO(s)+2HCl(aq) CaCl2(aq)+H2O(l)MgO(s)+2HCl(aq)MgCl2(aq)+H2O(l)

If 7.695g of the oxide sample requires 125mL of 2.55MHCl, then the amount of hydrochloric acid can be determined as follows,

Amountof HCl =ConcentrationHCl×volumeHCl =2.55molL×0.125L =0.319 mol

The stoichiometric ratio between the reactant metal oxides MgO,CaOandHCl is 1:2

Thus,

Amount of MgO is 0.319molHCl×1molMgO2molHCl=0.159molMgO

Amount of CaO is 0.319molHCl×1molCaO2molHCl=0

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