Average Atomic Mass Practice Problems

Determine the percent abundance for Fe-57, given the following above. The average atomic mass for Fe is 55.845 u.

• 2.13

• 0.27

• 2.70

• 0.21

Tags

NGSS.HS-PS1-7

Naturally occurring strontium consists of four isotopes, Sr-84, Sr-86, Sr-87 and Sr-88, whose isotopic masses are 83.9134, 85.9094, 86.9089 and 87.9056 amu respectively.

The most abundant isotope is Sr-88 which accounts for 82.58% of naturally occurring strontium and the least abundant isotope is Sr-84 which accounts for 0.56% of naturally occurring strontium.

Given that the observed atomic mass of strontium is 87.62 amu, calculate the percentages of Sr-87 in nature.

• 7.34

• 9.52

• 92.66

• 9.27

Oxygen is composed of three isotopes: One has a mass of 16.999 amu. This isotope makes up 0.037% of oxygen. Of the other two, one has a mass of 15.995 amu, and the other has a mass of 17.999 amu.

Calculate the abundance of the other two isotopes, using the average atomic mass of 15.9994 amu.

• 99.76 & 0.203

• 89.67 & 1.28

• 99.76 & 0.037

• 0.203 & 3.70

The atomic weight of lead is 207.19 amu. Assuming that it's made up of entirely the isotopes of Pb-206 (205.98 amu); Pb-207 (206.98 amu); and Pb-208 (207.98 amu) and that the percentages of the two lightest isotopes are equal, calculate the relative percentages of these isotopes in the natural element.

• Pb-206: 26.3%

  Pb-207: 26.3%

  Pb-208: 47.4%

• Pb-206: 36.3%

  Pb-207: 36.3%

  Pb-208: 27.4%

• Pb-206: 16.3%

  Pb-207: 16.3%

  Pb-208: 67.4%

• Pb-206: 10.3%

  Pb-207: 10.3%

  Pb-208: 79.4%

A sample of boron with a relative atomic mass of 10.8 gives a mass spectrum with two peaks, one at m/z = 10, and one at m/z = 11.

Calculate the ratio of the heights of two peaks.

• 4:1

• 2:3

• 2:2

• 4:2

Antimony, Sb, has two stable isotopes. Given that 43.8% of natural antimony is Sb-123 with the experimentally determined mass of 122.904 amu.

What is the other stable isotope?

• Sb-121

• Sb-122

• Sb-124

• Sb-125