Chemistry 9th Chap 3 : Topic 20 - Maryam Publications

Chemistry Chapter 3 : Topic 20

Stable-Isotope Atomic Mass

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An element X exists naturally as two stable isotopes. Isotope A has a mass of 10 amu (20% abundance) and isotope B has a mass of 11 amu (80% abundance). What is the relative atomic mass of element X?

A) 10.5 amu

B) 10.2 amu

C) 11.0 amu

D) 10.8 amu

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A naturally occurring element has two isotopes. The first isotope is lighter but more abundant, while the second isotope is heavier but less abundant. Which statement correctly explains why the relative atomic mass is closer to the lighter isotope?

A) Because heavier isotopes are always ignored

B) Because abundance has no effect on atomic mass

C) Because weighted average depends more on abundance than mass number

D) Because isotopic masses are not considered in calculations

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Chlorine exists as two isotopes:

A) 36.5 amu

B) 36.0 amu

C) 35.5 amu

D) 35.0 amu

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A student incorrectly calculates atomic mass by simply averaging isotopic masses without considering abundance. Which element property will most likely cause the largest error?

A) Isotopic abundance variation

B) Atomic number

C) Valency

D) Number of electrons

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An element has isotopes 12 amu (90%) and 14 amu (10%). A student claims the atomic mass is 13 amu. Why is this incorrect?

A) Because isotopic masses must be added directly

B) Because weighted average is not used in chemistry

C) Because heavier isotope has less influence due to low abundance

D) Because abundance must be used as simple addition

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Which mathematical method is used to calculate relative atomic mass from isotopic data?

A) Simple average of isotopic masses

B) Weighted average based on abundance

C) Multiplication of masses only

D) Difference between isotopic masses

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A fictional element Z has isotopes with the following data:

A) 21.0 amu

B) 20.8 amu

C) 21.2 amu

D) 22.0 amu

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Why is the relative atomic mass of most elements not a whole number?

A) Because electrons affect mass

B) Because of isotopic mixtures in nature

C) Because atoms are unstable

D) Because protons change mass randomly

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An element has isotopes with equal abundance. Which statement is correct?

A) The relative atomic mass is exactly the average of isotopic masses

B) Abundance does not affect calculation

C) The lighter isotope is ignored

D) The heavier isotope dominates the atomic mass

10 / 15

A student calculates atomic mass as 50 amu for an element, but correct isotopic data gives 52 amu. What is the most likely mistake?

A) Ignoring isotope abundance

B) Using electron mass incorrectly

C) Using wrong atomic number

D) Using correct formula

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Which situation will cause a larger shift in relative atomic mass?

A) Only one isotope present

B) Rare isotope with very high mass

C) Common isotope with slightly higher mass

D) Equal isotopic abundance

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An element has isotopes 30 amu (10%) and 31 amu (90%). What best describes its relative atomic mass?

A) Exactly 30.5 amu

B) Slightly less than 31 amu

C) Equal to 30 amu

D) Equal to 31 amu

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Why is abundance expressed in percentage when calculating relative atomic mass?

A) To ignore rare isotopes

B) To make calculations more complex

C) To change atomic number values

D) To convert directly into weighting factors

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Which statement best defines relative atomic mass?

A) Mass of the heaviest isotope only

B) Weighted average mass of all naturally occurring isotopes

C) Average mass of protons and neutrons

D) Sum of all isotopic masses

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If an element has one dominant isotope (99% abundance), its relative atomic mass will be:

A) Equal to the heavier isotope approximately

B) Unrelated to isotopic masses

C) Exactly half of isotopic masses

D) Equal to the lighter isotope

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