Average Atomic Mass Practice Problems

Element Q is an unknown element. ²⁰Q has abundance of 90.51%, ²¹Q has abundance of 0.21%, and ²²Q has abundance of 9.22%. Find the average atomic mass of Q and identify the unknown element.

• F with mass = 19,894 amu

• Ne with mass = 20.17 amu

• Sodium with mass = 22.90 amu

• Mg with mass = 24.12 amu

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%

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.

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.

Referring to the table, what is the relative mass of a proton?

• 10

• 1

• 0.1

• 0.01

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.

Element X has two isotopes: X-10 with a mass of 10.012 amu and X-11 with a mass of 11.009 amu. If the average atomic mass of X is 10.81 amu, what is the percent abundance of X-10?

• A) 19%

• B) 81%

• C) 50%

• D) 25%

An element has three isotopes with the following masses and abundances: Isotope A: 10 amu (20%), Isotope B: 11 amu (50%), Isotope C: 12 amu (30%). What is the average atomic mass of the element?

• A) 10.8 amu

• B) 11.1 amu

• C) 11.5 amu

• D) 11.3 amu

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