Aim for FiveChemistry

Short free-response question

Gallium: mass spectrum and composition

  • Unit 1
  • 4 points
  • About 9 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

The mass spectrum of a pure sample of an element shows two peaks.

Table 1. Mass spectrum data

Isotope mass (amu)Relative abundance (%)
68.9360.11
70.9239.89

Source: Standard isotope data, rounded

Suggested time: 9 minutes

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Part (a)

1 point

Calculate the average atomic mass of the element, and identify the element.

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Part (b)

1 point

Calculate the number of atoms in a 2.50 g sample of this element.

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Part (c)

1 point

Calculate the mass percent of gallium in gallium oxide, Ga₂O₃.

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Part (d)

1 point

A 5.00 g sample of a mixture contains only Ga₂O₃ and SiO₂. Analysis shows that the sample contains 2.80 g of gallium. Calculate the mass percent of Ga₂O₃ in the mixture.

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