Worksheet: Percentage Isotopic Abundance

In this worksheet, we will practice calculating percentage isotopic abundances from the relative atomic mass and isotopic masses.

Q1:

Chlorine has two stable isotopes, 3 5 C l and 3 7 C l , with atomic masses of 34.96885 u and 36.96590 u, respectively. The average atomic mass of naturally occurring chlorine is 35.453 u. Calculate to 4 significant figures the percentage abundance of 3 5 C l in naturally occurring chlorine.

Q2:

Copper has two stable isotopes, 6 3 C u and 6 5 C u , with atomic masses of 62.9296 u and 64.9278 u, respectively. The average atomic mass of naturally occurring copper is 63.546 u. Calculate to 4 significant figures the percentage abundance of 6 3 C u in naturally occurring copper.

Q3:

Boron has two stable isotopes, 1 0 B and 1 1 B , with atomic masses of 10.0129 u and 11.0931 u, respectively.

Calculate to 3 significant figures the abundance of 1 0 B for a sample of boron with an average atomic mass of 10.807 u.

Calculate to 3 significant figures the abundance of 1 0 B for a sample of boron with an average atomic mass of 10.819 u.

Q4:

Chlorine has two stable isotopes, 3 5 C l and 3 7 C l , with atomic masses of 34.9689 u and 36.9659 u, respectively. The relative abundance of 3 7 C l in an average sample of chlorine is 3 7 3 5 C l C l = 0 . 3 1 9 6 .

Calculate the average absolute abundance of 3 7 C l .

  • A 4 0 . 4 9 %
  • B 1 9 . 0 2 %
  • C 4 6 . 9 7 %
  • D 2 4 . 2 2 %
  • E 5 1 . 5 6 %

Calculate the average atomic mass of chlorine.

  • A 35.45 u
  • B 35.97 u
  • C 35.61 u
  • D 36.48 u
  • E 36.33 u

Q5:

Gallium has two stable isotopes, 6 9 G a and 7 1 G a , with atomic masses of 68.9256 u and 70.9247 u, respectively.

Calculate to 3 significant figures the abundance of 7 1 G a for a sample of gallium with an average atomic mass 69.723 u.

Calculate to 3 significant figures the abundance of 6 9 G a for a sample of gallium with an average atomic mass of 69.620 u.

Q6:

Magnesium has three stable isotopes, 2 4 M g , 2 5 M g and 2 6 M g , with atomic masses of 23.9850 u, 24.9858 u and 25.9826 u, respectively.

The natural abundances of 2 4 M g and 2 5 M g are 7 8 . 9 9 % and 1 0 . 0 0 % , respectively. Calculate the average atomic mass of magnesium.

  • A 24.20 u
  • B 24.10 u
  • C 24.88 u
  • D 24.31 u
  • E 24.98 u

The relative abundances of magnesium isotopes in a sample are 2 5 2 4 M g M g = 0 . 1 1 8 1 5 and 2 6 2 4 M g M g = 0 . 1 4 6 8 7 . Calculate to 4 significant figures the absolute abundance of 2 4 M g in the sample.

  • A 7 9 . 0 5 %
  • B 7 7 . 9 8 %
  • C 7 3 . 5 0 %
  • D 5 8 . 1 0 %
  • E 9 7 . 2 1 %

Q7:

Antimony has two stable isotopes, 1 2 1 S b and , with atomic masses of 120.9038 u and 122.9042 u, respectively. The relative abundance of in an average sample of antimony is 1 2 3 1 2 1 S b S b = 0 . 7 4 7 9 .

Calculate the average absolute abundance of 1 2 3 S b .

  • A20.13%
  • B14.42%
  • C33.71%
  • D42.79%
  • E25.21%

Calculate the average atomic mass of antimony.

  • A 121.76 u
  • B 121.90 u
  • C 121.86 u
  • D 121.41 u
  • E 122.05 u

Q8:

Silicon has three stable isotopes, 2 8 S i , 2 9 S i and 3 0 S i , with atomic masses of 27.9769 u, 28.9765 u and 29.9738 u, respectively.

The natural abundances of 2 8 S i and 2 9 S i are 9 2 . 2 3 % and 4 . 6 8 % , respectively. Calculate to 4 significant figures the average atomic mass of silicon.

  • A 28.11 u
  • B 28.02 u
  • C 28.05 u
  • D 28.09 u
  • E 28.03 u

The relative abundances of silicon isotopes in a sample are 2 9 2 8 S i S i = 0 . 0 8 0 6 9 and 3 0 2 9 S i S i = 0 . 4 9 5 8 3 . Calculate to 4 significant figures the absolute abundance of 2 8 S i in the sample.

  • A 8 9 . 2 3 %
  • B 8 7 . 9 3 %
  • C 6 3 . 4 3 %
  • D 6 3 . 1 0 %
  • E 8 8 . 9 7 %

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