# 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 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.

**Q3: **

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.

**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: **

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.

**Q6: **

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

**Q7: **

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

**Q8: **

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