Question

Which compound has the highest percent com­position by mass of strontium?

(1) SrC12 (2) SrI2 (3) SrO (4) SrS

Answer 1

Answer: option (3) SrO, with 84.56%

Step-by-step explanation:

You have to calculate the mass of strontium and the mass of the compound, for each of the four compounds, and compute the percent:

percent of strontium = (mass of strontium / mass of the chemical formula) × 100 %.

These are the atomic masses of the elements that you have to use:

Atomic mass of Sr: 87.62 g/mol

Atomic mass of Cl: 35.453 g/mol

Atomic mass of I: 126.904 g/mol

Atomic mass of O: 15.999 g/mol

Atomic mass of S: 32.065 g/mol

(1) SrCl₂:

i) mass of Sr: 87.62 g/mol

ii) mass of Cl₂: 2×35.453 g/mol = 70.906 g/mol

iii) mass of SrCl₂: 87.62 g/mol + 70.906 g/mol = 158.526 g/mol

iv) Sr % = [87.62 / 158.526 ] × 100 = 55.27%

(2) SrI₂

Following the same procedure:

i) Sr % = [87.62 / (87.62 + 2× 126.904) ] × 100 = 25.66%

(3) SrO

i) Sr % = [87.62 / (87.62 + 15.999) ] × 100 = 84.56%

(4) SrS

i) Sr % = [87.62 / (87.62 + 32.065) ] × 100 = 73.21%

Answer 2

The answer is (3). The number of Sr is the same so if the compound has the smallest gram formula mass, it has the highest percent composition by mass of strontium. So the answer is (3).