Question

Consider an element Z that has two naturally occurring isotopes with the following percent abundances: the isotope with a mass number of 19.0 is 55.0% abundance and the isotope with a mass number of 21.0 is 45.0 % abundant. What is the atomic mass for element Z
A
20.0 amu

B
20.10 amu

C
19.90 amu

D
40.0 amu

Answer 1

Answer:

The atomic mass for element Z = 19.90 amu (option C is correct)

Explanation:

Step 1: Data given

Element Z has 2 isotopes:

First isotope: Mass number of 19.0 has a natural abundance of 55.0 %

Second isotope: Mass number of 21.0 has a natural abundance of 45.0 %

Step 2: Calculate atomic mass for element Z

Atomic mass = isotopic mass 1 * natural abundance + isotopic mass 2 * natural abundance

Atomic mass = 19.0 * 0.55 + 21.0 *0.450

Atomic mass = 19.90 amu

The atomic mass for element Z = 19.90 amu (option C is correct)

Answer 2

The atomic mass of element Z is 19.90 amu.

HOW TO CALCULATE AVERAGE ATOMIC MASS:

• The average atomic mass of an isotopic element can be calculated by multiplying the natural abundance of the each isotope by its mass number and summing them.

• Isotope 1: Mass number of 19.0 has a natural abundance of 55.0% = 0.55

• 19 × 0.55 = 10.45

• Isotope 2: Mass number of 21.0 has a natural abundance of 45.0% = 0.45

• 21.0 × 0.45 = 9.45

• Average atomic mass = 10.45 + 9.45 = 19.90 amu.

• Atomic mass = 19.90 amu

• Therefore, the atomic mass of element Z is 19.90 amu.

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