# 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, and , 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 in naturally occurring chlorine.

Q2:

Copper has two stable isotopes, and , 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 in naturally occurring copper.

Q3:

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

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

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

Q4:

Chlorine has two stable isotopes, and , with atomic masses 34.9689 u and 36.9659 u respectively. The relative abundance of in an average sample of chlorine is .

Calculate the average absolute abundance of .

• A
• B
• C
• D
• E

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, and , with atomic masses 68.9256 u and 70.9247 u respectively.

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

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

Q6:

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

The natural abundances of and are and 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 and . Calculate, to 4 significant figures, the absolute abundance of in the sample.

• A
• B
• C
• D
• E

Q7:

Antimony has two stable isotopes, and , with atomic masses 120.9038 u and 122.9042 u respectively. The relative abundance in an average sample of antimony is .

Calculate the average absolute abundance of .

• 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, , , and , with atomic masses 27.9769 u, 28.9765 u, and 29.9738 u respectively.

The natural abundances of and are and 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 and . Calculate, to 4 significant figures, the absolute abundance of in the sample.

• A
• B
• C
• D
• E