# Worksheet: Percentage Isotopic Abundance

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

**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