Percent Composition, Molecular and Empirical Formula

Front

What is percent composition?

Back

Percent composition is the percentage by mass of each element in a compound.

Front

How do you calculate percent composition?

Back

Percent composition = (mass of element in 1 mole of compound / molar mass of compound) x 100%.

Front

What is the empirical formula?

Back

The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.

Front

What is the molecular formula?

Back

The molecular formula shows the actual number of atoms of each element in a molecule of a compound.

Front

How do you determine the empirical formula from percent composition?

Back

Convert percent to grams, find moles of each element, divide by the smallest number of moles, and write the ratio.

Front

What is the molar mass of MgO?

Back

Molar mass of MgO = 24.31 g/mol (Mg) + 16.00 g/mol (O) = 40.31 g/mol.

Front

Calculate the percent by mass of magnesium in MgO.

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Percent by mass of Mg = (mass of Mg / molar mass of MgO) x 100% = (24.31 / 40.31) x 100% = 60%.

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What is the gram-formula mass of CaF2?

Back

Gram-formula mass of CaF2 = 40.08 g/mol (Ca) + 2 x 19.00 g/mol (F) = 78.08 g/mol.

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Calculate the percent by mass of calcium in CaF2.

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Percent by mass of Ca = (mass of Ca / gram-formula mass of CaF2) x 100% = (40.08 / 78.08) x 100% = 51%.

Front

What is the empirical formula for a compound with 72.2% Mg and 27.8% N?

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The empirical formula is Mg3N2.