E: Moles and Molar Masses

• The Mole: The most important concept you’ve never heard about

A mole of objects contains the samenumber of objects as there are in 12.0 gof Carbon-12

6.022x1023 somethings/mole = Avogadro’s Number

• 1 mole of any element always has 6.0221x1023 atoms in it

• 1 mole of elephants always has 6.0221x1023 elephants in it.

• In equations, you will frequently see the number of moles represented by the variable “n”

• 1 kmol = 1000 mol

• 1 mol = 1x10-6 mol• # of objects = n(NA) where NA is Avogadros’ number

Molar Mass• The molar mass is the mass of one mole of

material– Also referred to as molecular weight or formula weight

for compounds

• The molar mass of an element is the mass of one mole of its atoms

• The molar mass of a compound is the mass of a mole of its molecules.

• Molar mass (M) is usually given in g/mol

Mass of Sample = nM

• Remember: An element is a collection of isotopes present in different amounts

• The molar mass of an element takes this into account and is really an average of the different isotope masses scaled by their percentage abundance.

• We can use this to determine the percent composition of the elements of to calculate the average molar mass

M = matom averageNA

• To calculate the average molar masses of molecular and ionic compounds, we use the molar masses of the elements present

The Molar Mass of a Compound is the

sum of the molar masses of the elements

that make up the molecule or the formula

unit of the ionic compound

Determination of Chemical Formulas

• For hundreds of years, people knew that drinking willow bark tea would belp cure a headache

• When scientists identified a compound form Willow bark that was biologically active, they had to determine its structure before they could synthesize it.

• Determining the chemical formula is the first step in the structure determination process…

Chemical Formulas

• We will deal with 2 types of chemical formulas:

1. Empirical formula: Shows the relative numbers of each element present in the compound

2. Molecular formula: The precise number of atoms of each element present in the compound

Mass Percentage Composition

• The mass percentage composition of a compound tells us the percentage (by mass) that a given element comprises of the compound.

• We frequently determine the mass % composition using combustion analysis

Mass % Composition Summary:

Mass % composition is found by calculating the fraction of the total mass contributed by each element present in a compound and expressing the fraction as a percentage.

Determining Empirical Formula from Mass % Data

• To convert the mass % composition obtained from a combustion analysis into an empirical formula, we must convert the mass % of each type of atom into the relative number of atoms.– To do this, assume that we have 100g of

sample– The mass % will then be in grams