Average Atomic Mass Practice Problems

An element has three isotopes. Given the abundances and relative masses, calculate the average atomic mass and determine (from the periodic table) which element it is.

Abundances | Relative masses

0.005% | 234.04 amu

0.720% | 235.04 amu

99.275% | 238.05 amu

• Uranium (#92, Atomic Mass: 238.03 amu)

• Fluorine (#9, Atomic Mass: 19.00 amu)

• Mercury (#89, Atomic Mass: 200.59 amu)

• Polonium (#84 209 amu)

Tags

NGSS.HS-PS1-1

NGSS.HS-PS1-8

A meteor crashes to Earth, is collected and analyzed. To everyone’s surprise, a new element is discovered with an atomic number of 120. Calculate the weight-average atomic mass for this new element, Ubn (Unbinnulium). Show all work below.

• 315.250 amu

• 313.827 amu

• 313.333 amu

• 316.248 amu

Tags

NGSS.HS-PS1-8

An element with 4.35% have a mass of 49.9461 amu, 83.79% have amass of 51.9405 amu, 9.50% have a mass of 52.9407 amu, and 2.36% have a mass of 53.9389amu.

• 51.99 amu

• 52.19 amu

• 53.45 amu

• 17.33 amu

Four isotopes of lead include lead-204, lead-206, lead-207, and lead-208.  The average atomic mass of lead is 207.2.  Which isotope of lead is likely to be the most abundant.

• 204

• 206

• 207

• 208

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

• 27.9%

• 72.1%

• 74.63%

• 34.29%

What is the average atomic mass for thallium, Tl, if there are two isotopes with the following masses and abundances? Tl-203 has a mass of 203 amu with an abundance of 29.5 % Tl-205 has a mass of 205 amu with an abundance of 70.5 %

• 203.12 amu

• 204.31 amu

• 204.41 amu

• 204.0 amu

A sample of cesium is 75% Cs-133, 20% Cs-132, and 5% Cs-134. What is the average atomic mass?

• 132.97 amu

• 133.00 amu

• 133.16 amu

• 132.85 amu

An element has two isotopes with masses 10.012 amu and 11.009 amu. If the isotopic abundances are 19.91% and 80.09% respectively, what is the average atomic mass of the element?

• 10.81 amu

• 10.50 amu

• 11.00 amu

• 10.10 amu

A certain element has three isotopes with the following abundances and masses: 5% at 50 amu, 15% at 51 amu, and 80% at 52 amu. Calculate the average atomic mass.

• 51.75 amu

• 51.00 amu

• 52.00 amu

• 50.50 amu

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

• Boron-10

• Boron-11

• Both are equally abundant

• Cannot be determined