Average Atomic Mass

Rubidium has two naturally occurring isotopes, ⁸⁵Rb (relative mass 84.9118 amu) and ⁸⁷Rb (relative mass 86.9092 amu). The abundance of ⁸⁷Rb is 27.8%, If rubidium has an average atomic mass of 85.47 amu, what is the % abundance of ⁸⁵Rb (in percent)?

• 27.9%

• 72.1%

• 74.63%

• 34.29%

Copper is made up of two isotopes, Cu-63 (62.9296 amu) and Cu-65 (64.9278 amu). Given copper's atomic weight of 63.546, what is the percent abundance of each isotope?

• Cu-63: 30.85% Cu-65: 69.15%

• Cu-63: 45.45% Cu-65: 54.55%

• Cu-63: 55.45% Cu-65: 44.55%

• Cu-63: 69.15% Cu-65: 30.85%

Tags

NGSS.HS-PS1-7

What does the 1.00794 stand for?

• Hydrogen

• atomic number

• atomic mass

• atomic explosion

Tags

NGSS.HS-PS1-1

Neon does not occur in the human body, but it is in the aire we breathe. Neon has two naturally occuring isotopes, neon-20 and neon-22. If neon 20 has a natural abundance of 90% and neon-22 has a natural abundance of 10%, what is the average atomic mass of neon?

• 15.8 amu

• 20.2 amu

• 21.8 amu

• 22.0 amu

Answer explanation

Since neon-20 has 90% natural abundance, which means it is more likely to be found in the nature = more of neon-20 vs. neon-22. Therefore the average atomic mass would be very close to 20 amu. Not less than 20 amu, or more than 22.

24.1% of all the isotopes of a an element have a mass of 75.23 amu, 48.7% have a mass of 74.61 amu, and 27.2% have a mass of 75.20 amu.
How many protons does this element have?

• 32

• 33

• 34

• no way to know

Chlorine has two naturally occurring isotopes, Cl-35 and Cl-37, The atomic mass of chlorine is 35.45. Which of these two isotopes of chlorine is more abundant?

• Cl-35

• Cl-37

• Cannot be determined from the information provided.

The average atomic mass on the periodic table is found from...

• calculating the weighted average mass of all the isotopes of that element

• Calculating different types of elements together

• calculations the average of all elements in that period together.

• making a guess about how much all the isotopes might wiegh together.

An element X has two isotopes: X-10 (10.012 amu) and X-11 (11.009 amu). If the average atomic mass of X is 10.81 amu, what is the percent abundance of X-11?

• A) 19%

• B) 81%

• C) 50%

• D) 25%

Tags

NGSS.HS-PS1-8

The element boron has two isotopes, B-10 and B-11. If the average atomic mass of boron is 10.81 amu, which isotope is more abundant?

• A) B-10

• B) B-11

• C) Both are equally abundant

• D) Cannot be determined

An element has three isotopes with masses 20 amu, 21 amu, and 22 amu. If their abundances are 90%, 0%, and 10% respectively, what is the average atomic mass of the element?

• A) 20.2 amu

• B) 21.0 amu

• C) 20.0 amu

• D) 22.0 amu