Concept explainers
On October 21, 1982, the Bureau of the Mint changed the composition of pennies (see Exercise 120). Instead of an alloy of 95% Cu and 5% Zn by mass, a core of 99.2% Zn and 0.8% Cu with a thin shell of copper was adopted. The overall composition of the new penny was 97.6% Zn and 2.4% Cu by mass. Does this account for the difference in mass among die pennies in Exercise 120? Assume the volume of the individual metals that make up each penny can be added together to give the overall volume of the penny, and assume each penny is the same size. (Density of Cu = 8.96 g/cm3; density of Zn = 7.14 g/cm3).

Interpretation:
The density of the old and new pennies has to be calculated and the reason for the change in mass with change in alloy used has to be explained.
Concept introduction:
The quantity density the amount of substance per unit volume of the substance. Density is a unique property of a substance.
Answer to Problem 113CP
The density of the old penny is
The density of the new penny is
From the density measurements of the old and new pennies, we have found that there is a change in density along with the change in the composition. Hence, we can conclude that the change in mass is due the difference in the alloys that is used for the preparation of pennies.
Explanation of Solution
To find the density of the old penny
Given,
Density of one mole of copper
Density of one mole of zinc
Let's assume there are
The total volume is the sum of the volume of copper and zinc.
To find the density of the new penny
Given,
Density of one mole of copper
Density of one mole of zinc
Let's assume there are
The total volume is the sum of the volume of copper and zinc.
From the density measurements of the old and new pennies, we have found that there is a change in density along with the change in the composition. Hence, we can conclude that the change in mass is due the difference in the alloys that is used for the preparation of pennies.
The density of the old and new pennies has been calculated and the reason for the change in mass with change in alloy used has been explained.
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Chapter 1 Solutions
Chemistry-Stud. Solution Guide
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