Mole Concept

Counting Units  A pair refers to how many shoes?  A dozen refers to how many doughnuts or eggs?  How many pencils are in a gross?  How many pieces of paper are in a ream? In chemistry, a counting unit called a mole is used to keep track of the number of atoms, molecules, or formula units that are in a sample of a substance.

Mole  Unit used to measure the amount of a pure substance (abbreviated mol)  1 mol = x10 23 representative particles  representative particles are the smallest piece of any pure substance  Element – atoms  Covalent compounds – molecules  Ionic compounds - formulas units  x10 23 is known as Avogadro’s number  x10 23 can be used as a conversion factor to determine the number of particles in a substance

Molar Mass  mass (in grams) of one mole of any substance  units are grams per mole (g/mol)  molar mass of any element = the AAM of the element from the periodic table

Examples: the molar mass for  lithium (Li) =  manganese (Mn) =  silver (Ag) =

 molar mass of a molecule or formula unit – number of atoms of each element multiplied by the mass of each element Examples: H2OH2OH2OH2O C 6 H 12 O 6

Using Molar Mass in Conversions Example: Determine the mass of mol of chromium. Example: Determine the mass of mol of chromium.

Example: How many moles are in 3.25 g of water?

Example: How many molecules of glucose, C 6 H 12 O 6 are found in a 5.37 g sample?

Example: Determine the percent composition of the elements in copper (II) nitrate.

Empirical Formulas Empirical formula – smallest whole number mole ratio of elements in a compound Solving Empirical Formula Problems 1.Grams to moles – use the molar mass to convert the grams of each element to moles 2.Divide by small – divide the number of moles of each element by the smallest number of moles

example… What is the formula of a compound that is comprised of 56.4g K, 8.7g C and 34.9g O?

Example 2- Determine the empirical formula for methyl acetate, which is known to have the following percent composition: 48.64% carbon, 8.16% hydrogen, and 43.20% oxygen.

Empirical and Molecular Formulas Molecular formula – the actual number of atoms in a molecular compound Example: glucose  molecular formula: C 6 H 12 O 6  empirical formula: C 1 H 2 O 1  ratio of the elements is the same in both formulas, just reduced in the empirical formula

To solve molecular formula problems…  Calculate the empirical formula  divide empirical molar mass into the given molecular molar mass, then distribute whole number answer through empirical formula

example problem… Find the molecular formula for a compound that is known to contain 26.7% phosphorus, 12.1% nitrogen, and 61.2% chlorine. Molecular mass of the compound is known to be 695 g/mol.

Percent Composition  is the percent by mass of any element in a compound  uses molar mass for elements and compounds  is found by using the following formula: % of element = mass of element x 100 mass of compound mass of compound