Selenium atom has 6 main isotopes:

⁷⁴Se (0.860%), ⁷⁶Se (9.23%), ⁷⁷Se (7.60%), ⁷⁸Se (23.7%), ⁸⁰Se(49.8%) and ⁸²Se (8.82%).

What is the average atomic mass of selenium?

On another planet, the selenium atom has only 2 main isotopes: ⁷⁷Se and ⁸⁰Se.

Its atomic mass is the same as that on Earth.

Determine the relative percentage of these two isotopes.

Question

Selenium atom has 6 main isotopes:

⁷⁴Se (0.860%), ⁷⁶Se (9.23%), ⁷⁷Se (7.60%), ⁷⁸Se (23.7%), ⁸⁰Se(49.8%) and ⁸²Se (8.82%).

What is the average atomic mass of selenium?

On another planet, the selenium atom has only 2 main isotopes: ⁷⁷Se and ⁸⁰Se.

Its atomic mass is the same as that on Earth.

Determine the relative percentage of these two isotopes.

| General Chemistry 1

Accepted Answer

Average atomic mass of selenium on Earth:

74 x 0.00860 + 76 x 0.0923 + 77 x 0.0760 + 78 x 0.237 + 80 x 0.498 + 82 x 0.0882 = 79.1 amu

Relative percentage of ⁷⁷Se and ⁸⁰Se on the other planet:

% of ⁷⁷Se = X%

% of ⁸⁰Se = Y% = (100-X)%

79.1 amu = $\frac{77 X}{100}$ + $\frac{80 Y}{100}$ = $\frac{77 X}{100}$ + $\frac{80 (100 - X)}{100}$ ⇒ X = 30

% of ⁷⁷Se = 30.0%

% of ⁸⁰Se = 70.0%

Midterm 1 - Question 5 | General Chemistry 1