The karat scale used to describe gold alloys is based on mass percentages. (a) If an alloy is formed that is 50 mol% silver and 50 mol% gold, what is the karat number of the alloy? Use Figure 12.18 to estimate the color of this alloy.

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Determine the molar masses of silver (Ag) and gold (Au).

Calculate the mass percentages of Ag and Au in the alloy using the given molar percentages (50 mol% Ag and 50 mol% Au).

Use the mass percentages to locate the position on the triangular diagram provided.

Identify the color of the alloy based on its position on the diagram.

Determine the karat number of the alloy by comparing the mass percentage of gold to the karat scale (24 karat = 100% Au).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Karat Scale

The karat scale is a measure of the purity of gold alloys, where 24 karats represents pure gold. The karat number indicates the mass percentage of gold in the alloy; for example, 18-karat gold contains 75% gold and 25% other metals. Understanding this scale is essential for determining the quality and value of gold jewelry.

Mole Percent vs. Mass Percent

Mole percent refers to the ratio of the number of moles of a component to the total number of moles in a mixture, while mass percent is the mass of a component divided by the total mass of the mixture. In the context of the question, converting from mole percent to mass percent is necessary to accurately determine the karat number of the alloy.

Alloy Color and Composition

The color of a metal alloy is influenced by its composition, particularly the types and amounts of metals present. The triangular diagram provided illustrates how varying proportions of gold, silver, and copper affect the alloy's color. This relationship is crucial for predicting the visual appearance of the alloy based on its composition.

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(a) The density of diamond is 3.5 g/cm³, and that of graphite is 2.3 g/cm³. Based on the structure of buckminsterfullerene, what would you expect its density to be relative to these other forms of carbon?

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(b) X-ray diffraction studies of buckminsterfullerene show that it has a face-centered cubic lattice of C₆₀ molecules. The length of an edge of the unit cell is 14.2 Å. Calculate the density of buckminsterfullerene.

Textbook Question

The karat scale used to describe gold alloys is based on mass percentages. (b) If an alloy is formed that is 50 mol% copper and 50 mol% gold, what is the karat number of the alloy? What is the color of this alloy?

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Textbook Question

(a) What are the C—C—C bond angles in diamond?

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