Question

Calculate the atomic mass of X. The three X isotopes have atomic mass and relative abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%). What is this atom?
Co


Al


Ni


Si

Answer 1

Answer:  Si   (choice D)

This is the element Silicon.

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Explanation:

First convert each percentage to its decimal form.

For example, 92.2297% converts to 0.922297 after moving the decimal point two spots to the left.

After doing that, multiply those decimal values with their respective atomic mass unit (amu) values.

• 27.9769 * 0.922297 = 25.8030109393
• 28.9765 * 0.046832 = 1.357027448
• 29.9738 * 0.030872 = 0.9253511536

Then we add up the results

25.8030109393 + 1.357027448 + 0.9253511536 = 28.0853895409

That rounds to about 28.085

Then look at the periodic table to see the atomic mass of Cobalt (Co), Aluminum (Al), Nickel (Ni) and Silicon (Si). The mass values listed in the periodic table are weighted averages of all the isotopes. The units for the mass are still in amu.

• Cobalt = 58.933
• Aluminum = 26.982
• Nickel = 58.693
• Silicon = 28.085

We have a match with silicon, showing that choice D is the final answer.