Percent Composition, Empirical & Molecular Formulas

Front

What is percent composition?

Back

Percent composition is the percentage by mass of each element in a compound, calculated by dividing the mass of each element by the total mass of the compound and multiplying by 100.

Front

How do you calculate the percent by mass of an element in a compound?

Back

Percent by mass = (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

How do you determine the empirical formula from percent composition?

Back

Convert the percent composition to grams, then to moles, and find the simplest ratio of moles of each element.

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 find the molecular formula from the empirical formula?

Back

Multiply the subscripts in the empirical formula by a whole number (n) to match the molecular mass.

Front

What is the relationship between empirical and molecular formulas?

Back

The molecular formula is a multiple of the empirical formula, representing the actual number of atoms in a molecule.

Front

Calculate the percent by mass of fluorine in CaF2 (Ca = 40.08 g/mol, F = 19.00 g/mol).

Back

Percent by mass of F = (2 * 19.00 / (40.08 + 2 * 19.00)) * 100 = 49%.

Front

What is the empirical formula of C3H6?

Back

The empirical formula is CH2.

Front

Calculate the percent by mass of sodium in NaCl (Na = 22.99 g/mol, Cl = 35.45 g/mol).

Back

Percent by mass of Na = (22.99 / (22.99 + 35.45)) * 100 = 39%.