Isotopes and Atomic Mass: Quiz

Isotopes and Atomic Mass: Quiz

7 questions

1. Question 1EasyWhy are most atomic masses on the periodic table not whole numbers?

   AProtons and neutrons have fractional chargesBThey are weighted averages of the masses of an element's isotopesCElectrons add a large, varying massDThey are rounded incorrectly
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   Answer: They are weighted averages of the masses of an element's isotopes

   Most elements are mixtures of isotopes. The atomic mass is the average of their masses, weighted by how common each isotope is.

2. Question 2MediumCopper is 69.15% copper-63 (62.930 u) and 30.85% copper-65 (64.928 u). What is its average atomic mass?

   A63.55 uB63.93 uC64.00 uD64.31 u
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   Answer: 63.55 u

   (0.6915 × 62.930) + (0.3085 × 64.928) = 43.516 + 20.030 = 63.55 u. The answer 63.93 u is the simple (unweighted) average.

3. Question 3EasyAn element has two isotopes with masses 10.0 u and 11.0 u. Its average atomic mass is 10.8 u. Which isotope is more abundant?

   AThe 10.0 u isotopeBThe 11.0 u isotopeCThey are equally abundantDIt cannot be determined
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   Answer: The 11.0 u isotope

   The average (10.8 u) is closer to 11.0 than to 10.0, so the heavier isotope must be more common.

4. Question 4MediumBromine is 50.69% bromine-79 (78.918 u) and 49.31% bromine-81 (80.916 u). What is its average atomic mass?

   A79.90 uB79.92 uC80.00 uD7990 u
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   Answer: 79.90 u

   (0.5069 × 78.918) + (0.4931 × 80.916) = 79.90 u. Using percentages without dividing by 100 gives 7990, 100 times too big.

5. Question 5MediumLithium is 7.59% lithium-6 (6.015 u) and 92.41% lithium-7 (7.016 u). What is its average atomic mass?

   A6.52 uB6.94 uC7.02 uD6.09 u
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   Answer: 6.94 u

   (0.0759 × 6.015) + (0.9241 × 7.016) = 6.94 u, close to 7 because lithium-7 is far more common.

6. Question 6HardGallium has two isotopes: gallium-69 (68.926 u) and gallium-71 (70.925 u). Its average atomic mass is 69.723 u. What is the abundance of gallium-69?

   A39.9%B50.0%C60.1%D69.7%
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   Answer: 60.1%

   Let x = fraction of gallium-69: 68.926x + 70.925(1 − x) = 69.723, so x = 1.202 ÷ 1.999 = 0.601, or 60.1%. 39.9% is the abundance of gallium-71.

7. Question 7EasyWhich statement about chlorine-35 and chlorine-37 is true?

   AThey have different numbers of protonsBThey have different numbers of neutronsCThey have very different chemical propertiesDChlorine-37 has more electrons in a neutral atom
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   Answer: They have different numbers of neutrons

   Both have 17 protons and, as neutral atoms, 17 electrons, so they behave the same chemically. Chlorine-35 has 18 neutrons and chlorine-37 has 20.