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PHY 104 PHY 104 Past Questions Objective Question
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Question

Naturally occurring rubidium consists of just two isotopes. One of the isotopes consists of atoms having a mass of 84.912 amu; the other of 86.901 amu. What is the percent natural abundance of the heavier isotope? Average atomic mass of Rb is 85.47 amu.
Options
A 15%
B 28%
Correct Answer
C 37%
D 72%
Correct Answer

Option B is the correct answer.

Detailed Explanation

Let x be the fraction of the heavier isotope (86.901 amu). Then (1-x) is the fraction of the lighter isotope (84.912 amu). Average mass = 84.912(1-x) + 86.901x = 85.47. 84.912 - 84.912x + 86.901x = 85.47 => 84.912 + 1.989x = 85.47 => 1.989x = 0.558 => x = 0.2805 ≈ 28%.

Hint

Use the weighted average formula.

Question Info

Course: PHY 104
Topic: PHY 104 Past Questions
Type: Objective
Difficulty: Not specified
Question 924 of 947
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