What is the mass of chromium in 250.0 g of potassium dichromate, K2Cr2O7? (Molar masses: K = 39.1 g/mol, Cr = 52.0 g/mol, O = 16.0 g/mol)
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Correct answer
B. 88.4 g
Principle or equation
Mass fraction of an element in a compound is (total molar mass of that element in the formula) / (molar mass of compound). Mass of element = mass of sample × mass fraction.
Why this answer is correct
Molar mass of K2Cr2O7 = 2(39.1) + 2(52.0) + 7(16.0) = 78.2 + 104.0 + 112.0 = 294.2 g/mol. Mass fraction of Cr = 104.0 / 294.2 = 0.3535. Mass of Cr = 250.0 g × 0.3535 = 88.4 g.
Example
For Fe2O3 (molar mass 159.7 g/mol), mass fraction of Fe = 111.7/159.7 = 0.6994. In 100 g Fe2O3, mass of Fe = 69.94 g.
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