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, 35Cl and 37Cl, 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 35Cl in naturally occurring chlorine.

Q2:

Copper has two stable isotopes, 63Cu and 65Cu, with atomic masses 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 63Cu in naturally occurring copper.

Q3:

Boron has two stable isotopes, 10B and 11B, with atomic masses 10.0129 u and 11.0931 u respectively.

Calculate, to 3 significant figures, the abundance of 10B for a sample of boron with an average atomic mass of 10.807 u.

Calculate, to 3 significant figures, the abundance of 10B for a sample of boron with an average atomic mass of 10.819 u.

Q4:

Chlorine has two stable isotopes, 35Cl and 37Cl, with atomic masses 34.9689 u and 36.9659 u respectively. The relative abundance of 37Cl in an average sample of chlorine is 3735ClCl=0.3196.

Calculate the average absolute abundance of 37Cl.

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

Calculate the average atomic mass of chlorine.

  • A35.61 u
  • B36.48 u
  • C36.33 u
  • D35.97 u
  • E35.45 u

Q5:

Gallium has two stable isotopes, 69Ga and 71Ga, with atomic masses 68.9256 u and 70.9247 u respectively.

Calculate, to 3 significant figures, the abundance of 71Ga for a sample of gallium with an average atomic mass 69.723 u.

Calculate, to 3 significant figures, the abundance of 69Ga for a sample of gallium with an average atomic mass of 69.620 u.

Q6:

Magnesium has three stable isotopes, 24Mg, 25Mg, and 26Mg, with atomic masses 23.9850 u, 24.9858 u, and 25.9826 u respectively.

The natural abundances of 24Mg and 25Mg are 78.99% and 10.00% respectively. Calculate the average atomic mass of magnesium.

  • A24.31 u
  • B24.98 u
  • C24.20 u
  • D24.10 u
  • E24.88 u

The relative abundances of magnesium isotopes in a sample are 2524MgMg=0.11815 and 2624MgMg=0.14687. Calculate, to 4 significant figures, the absolute abundance of 24Mg in the sample.

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

Q7:

Antimony has two stable isotopes, 121Sb and 123Sb, with atomic masses 120.9038 u and 122.9042 u respectively. The relative abundance 123Sb in an average sample of antimony is 123121SbSb=0.7479.

Calculate the average absolute abundance of 123Sb.

  • A42.79%
  • B14.42%
  • C33.71%
  • D20.13%
  • E25.21%

Calculate the average atomic mass of antimony.

  • A121.86 u
  • B122.05 u
  • C121.90 u
  • D121.41 u
  • E121.76 u

Q8:

Silicon has three stable isotopes, 28Si, 29Si, and 30Si, with atomic masses 27.9769 u, 28.9765 u, and 29.9738 u respectively.

The natural abundances of 28Si and 29Si are 92.23% and 4.68% respectively. Calculate, to 4 significant figures, the average atomic mass of silicon.

  • A28.05 u
  • B28.03 u
  • C28.09 u
  • D28.11 u
  • E28.02 u

The relative abundances of silicon isotopes in a sample are 2928SiSi=0.08069 and 3029SiSi=0.49583. Calculate, to 4 significant figures, the absolute abundance of 28Si in the sample.

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

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