Percent Composition

What is the percent composition ofnitrogen in 1 mol of potassium

nitrate?• This means:• How much of the mass of potassium

nitrate is due to just the mass of nitrogen?

• In other words…• How does the mass of just nitrogen

• KNO3

• Compare to the mass of the whole compound?

• KNO3

• The answer will be a percent…• Answer: 13.85% of the mass of

potassium nitrate (KNO3) comes from the nitrogen!!!

Percent Composition Problem Summary

• To find percentage composition, you must:

• 1. Find the mass of the entire compound• 2. Find the mass of just the element you

are concerned with• 3. Divide the mass of the element by the

mass of the compound• 4. Multiply by 100 to convert to a percent

Example

• 1. What is the percent composition of hydrogen in water?

• 2. What is the percent composition of oxygen in water?

Finding Percent Composition from Experimental Data

Example

• A 0.545 gram sample of an unknown compound is analyzed. It is found that it contains:

• 0.137 g of C• 0.0080 g of H• 0.400 g of O.• What is the percentage composition of

each element?

Why do we want to know the percent composition?

• It is possible to use the percentage composition to determine the chemical formula of an unknown.

• This formula would be called the empirical formula and refers to the simplest (lowest) whole number ratio of atoms

• An empirical formula is NOT necessarily the true formula of the unknown compound

Empirical Formulae

How to determine empirical formula from percent composition:

• 1. Assume you have a 100.0 g sample Ex. 80.0% carbon becomes 80.0g

carbon• 2. Convert the mass of each element to

moles.• 3. Divide each amount of moles by the

smallest amount of moles (round to nearest whole number)

Example

Empirical Formulas are helpful, but…

• Empirical formulas are just the simplest ratio of atoms in a compound

• They do not tell the actual number of atoms found in the compound in nature

Ex. HO is the empirical formula for hydrogen peroxide, but we know the “real” formula is H2O2

So how do we get the “real” formula of a compound?

• If the “actual” molar mass is twice the “empirical formula” molar mass, then the real formula (the molecular formula) is obtained by multiplying every subscript in the empirical formula by 2.

• If the “actual” molar mass is triple the “empirical formula” molar mass, then the real formula (the molecular formula) is obtained by multiplying every subscript in the empirical formula by 3.

Example

• The empirical formula for a substance is CH2O.

• Experiments show it’s actual molar mass is 180 g/mol;

• what is it’s molecular formula?

Source

• http://www.rockwood.k12.mo.us/rsummit/Gray/genchem/U4%20The_Mole/NOTES%204%20Percent%20Comp%20EmpMolec%20Formulas.pdf