Percent Composition of Compounds

Percent by Mass Chemical formulas show the ratio of atoms in a compound. Sometimes, it is helpful to know the composition of a compound based on the masses of different elements represented.

Calculating Percent Composition Mass fraction = mass of element in 1 mol of compound mass of 1 mol of compound How do you convert a fraction to a percentage? Multiply by 100

Example Problem 1 Ethanol is an alcohol obtained by fermenting sugar in grapes, corn and other fruits and grains. It is often added to gasoline as an octane enhancer to form a fuel called gasahol. Ethanol’s formula is C 2 H 5 OH. This means each molecule of ethanol contains 2 C atoms, 6 H atoms, and 1 O atom. What is the mass percent of each element present in ethanol?

Example Problem 2 Carvone is a substance that occurs in two forms, both of which have the same molecular formula (C 10 H 14 O) and molar mass. One type of carvone gives caraway seeds their characteristic smell. The other is responsible for the smell of spearmint oil. Compute the mass percent of each element in carvone.

Example Problem 3 A compound is analyzed and determine to contain 1.75 g of carbon and g of bromine. What is the percent composition of this compound?

Determining Empirical Formula

Difference between Empirical and Molecular Formulas Empirical formula: expresses the smallest whole-number ratio of atoms in a compound  Ionic compounds are always written in empirical formula Molecular formula: the actual formula of a compound giving the types of atoms and numbers of each type of atom.  Covalent compounds (molecules) are always written in molecular formula

Difference between Empirical and Molecular Formulas Glucose has a molecular formula of C 6 H 12 O 6. We must write its formula this way to show that a glucose molecule is actually made up of 6 C atoms, 12 H atoms, and 6 O atoms. What is the empirical formula of glucose? CH 2 O

Empirical Formula Finding the empirical formula of a compound is determining the ratio of atoms in a compound. Determining empirical formula is important when you know the elements present in a compound but you do not know what the compound is.

Steps for Determining the Empirical Formula of a Compound 1. Obtain the mass of each element present (in grams). If percent composition is given, assume a 100-g sample of the compound. 2. Determine the number of moles of each type of atom present. 3. Divide the number of moles of each element by the smallest number of moles to convert the smallest number to 1.

4. If the numbers you get in step #3 are whole numbers they are the subscripts in the empirical formula. If one or more of these numbers are not whole numbers, go on to step Multiply the numbers from step #3 by the smallest whole number that will convert all of them to whole numbers. This set of numbers will be the subscripts for the empirical formula.

Example Problem 1 An oxide of aluminum is formed by the reaction of g of aluminum with g of oxygen. Calculate the empirical formula for this compound.

Example Problem 2 When a g sample of vanadium metal is heated in air, it reacts with oxygen to reach a final mass of g. Calculate the empirical formula of this vanadium oxide.

Example Problem 3 A sample of lead arsenate, an insecticide used against the potato beetle, contains g of lead, g of hydrogen, g of arsenic, and g of oxygen. Calculate the empirical formula of lead arsenate.

Example Problem 4 Cisplatin, the common name for a platinum compound that is used to treat cancerous tumors, has the composition (mass percent) 65.02% platinum, 9.34% nitrogen, 2.02% hydrogen and 23.63% chlorine. Calculate the empirical formula for cisplatin.

Determining Molecular Formula

Molecular Formula If we know the percent composition of a compound in terms of the masses of the elements present we can calculate the empirical formula but not the molecular formula. In order to calculate the molecular formula we must know the molar mass.

Molecular Formula 1. If the empirical formula is not given, follow the steps calculate the empirical formula. 2. Find the empirical formula mass. 3. Compare the empirical formula mass to the molar mass. molecular formula = (empirical formula) n n = molar mass/empirical formula mass

Example Problem 1 A white powder is analyzed and found to have an empirical formula of P 2 O 5. The compound has a molar mass of g/mol. What is the compound’s molecular formula?

Example Problem 2 A compound used as an additive for gasoline to help prevent engine knock shows the following percentage composition: 24.27% C, 4.07% H, and 71.65% Cl. The molar mass is known to be g. Determine the empirical formula and the molecular formula for this compound.

Example Problem 3 A certain hydrocarbon (a compound consisting of carbon and hydrogen) is 82.66% carbon by mass and has a molar mass of g/mol. Determine the molecular formula of this hydrocarbon.