Question

A 2.000g sample of a compound containing chromium, sulfur, and oxygen contains 0.787g chromium. Each formula unit contains one chromium atom and one sulfur atom. What is its formula mass? What is its formula?

Answer 1

Formula = CrSO3, Formula mass = 132.06 g/mol

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Mass of compound = 2.000 g

Mass of Chromium = 0.787 g

Molar mass of Chromium = 51.996 g/mol

No. of moles of Chromium in 0.787 g = 0.787 g / 51.996 g/mol = 0.015135 mol

Since number of Chromium atom and number of sulfur atoms are equal in the formula unit,

No. of moles of Sulfur = 0.015135 mol

Molar mass of sulfur = 32.065 g/mol

Mass of sulfur = 0.015135 mol x 32.065 g/mol = 0.4853 g

Total mass of Chromium and sulfur = 0.787 g + 0.485 g = 1.272 g

Mass of oxygen = Total Mass of Sample - Mass of Chromium and sulfur = 2.000 g - 1.272 g = 0.728 g

Molar mass of oxygen = 16.00 g/mol

No. of moles of Oxygen atom in the formula = 0.728 g / 16.00 g/mol = 0.0455 mol

Number of oxygen atoms with respect to chromium in the formula = No. of moles of Oxygen / No. of moles of Chromium = 0.0455 mol / 0.015135 mol = 3.006277 = 3

Number of oxygen atom in the formula = 3

Final formula,

No. of Cr = 1

No. of S = 1

No. of O = 3

Formula = CrSO3

Formula mass = 132.06 g/mol

Name = chromium (II) sulfite