Unit – The MoleFormula Mass – The total mass of the

formula for a compound.

- To calculate formula mass, multiply the number of atoms of each element by the

atomic mass (rounded to two decimal places) and then sum them all up.

Percent Composition

• Is useful to know what amounts (or percentages) of a compound are contributed by each element present in that compound.

% comp. = mass of element (part)

------------------------------------ X 100

mass of compound (whole)

The Mole

- The mole is a unit of quantity (how much) of something is present. It has 3 definitions.

1) 1 mole = atomic mass (element = periodic table) or formula mass (compound = calculated) in grams.

2) 1 mole = 6.02 X 1023 atoms or molecules.

(6.02 X 1023 = Avogadro’s number)

1) 1 mole = 22.4 liters iff the material is a gas and is at STP (standard temperature and pressure).

Helpful Hints in Solving Mole Problems

***** If “GRAMS” appear in the problem and you have a “COMPOUND”, you must calculate the “FORMULA MASS”!

***** If moles do “NOT” appear in the problem you “NEED” a path.

Empirical Formula

• Empirical formula = simplest formula or smallest ratio of atoms.

(ex) C6H6

EF = CH

Steps to Determine the Empirical Formula

1) Convert all info to moles.

2) Divide answers to step # 1 by the smallest.

3) If the answers to step # 2 are close to whole numbers, round them off and use them as the subscripts for your formula.

4) If the answers to step #2 are not close to whole numbers, multiply by the smallest whole number that gives you a whole number. Then, use those as the subscripts to your formula.

True (Molecular) Formulas

- True molecular formulas are some ratio of an empirical formula.

- EF = empirical formula

- FM = formula mass (mass of the EF)

- TMF = true molecular formula

- MM = molecular mass (mass of the TMF)

Steps to Determining True Molecular Formulas

1) Find the EF.

2) Calculate the FM

3) Divide the MM/FM = #

4) Multiply the subscripts of the EF by the number obtained in step #3 = TMF.

Stoichiometry

• Problems using balanced equations.

• There are 3 types of Stoichiometry problems:

1) Mass – Mass

2) Mass – Volume

3) Volume - Volume

Steps Used to Solve Stoichiometry Problems

1) Write the correct balanced equation.

2) Convert all given info to moles given.

3) Convert all moles given to moles sought.

4) Convert all moles sought to info sought.

In other words,

info given -- moles given -- moles sought -- info sought