# Worksheet: Percentage Isotopic Abundance

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:

Magnesium has three stable isotopes, , and , with atomic masses of 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.

• 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 and . Calculate to 4 significant figures the absolute abundance of in the sample.

• A
• B
• C
• D
• E

Q3:

Silicon has three stable isotopes, , and , with atomic masses of 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.

• 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 and . Calculate to 4 significant figures the absolute abundance of in the sample.

• A
• B
• C
• D
• E

Q4:

Chlorine has two stable isotopes, and , with atomic masses of 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.

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

Q5:

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

Calculate the average absolute abundance of .

• 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

Q6:

Gallium has two stable isotopes, and , with atomic masses of 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.

Q7:

Boron has two stable isotopes, and , with atomic masses of 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.

Q8:

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