E: Moles and Molar Masses The Mole: The most important concept you’ve never heard about A mole of objects contains the same number of objects as there are in 12.0 g of Carbon x10 23 somethings/mole = Avogadro’s Number 

1 mole of any element always has x10 23 atoms in it 1 mole of elephants always has x10 23 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(N A ) where N A 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 = m atom average N A 

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 