Mole Calculations I.Molar Mass II. Percent Composition III. Converting Mass to Moles IV. Converting Moles to Mass

What is a Mole? A mole is the SI (Metric ) unit for the amount of substance. 1 mole= 6.02 X particles This huge number is known as Avogadro’s Number

Back to the Periodic Table Atomic Number =16 (the number of protons) Average Atomic Mass = amu amu = atomic mass units So, 1 atom of sulfur has a mass of amu Here is a really neat fact, 1 mole of S atoms has a mass of grams This works for all of the elements, not just sulfur!

I. Calculating Molar Mass To calculate Molar Mass, you will need a calculator and a Periodic Table! The units of molar mass are grams/mole, pronounced “grams per mole.”

The Simplest Molecule, H 2 To calculate the molar mass of a molecule, we add up the Atomic Masses of the atoms that make it up. Look at the formula, H 2. The subscript means there are two atoms of H Multiply 2 x the atomic mass of H (from the periodic table) 2 x = g/mol

Molar Mass of KMnO 4 ElementNumber of atoms Atomic Mass Mass of element K1 X Mn1 X O4 X Molar Mass(Sum) g/mol

II. Percent Composition Percent Composition tells us which elements are in a compound, and in what proportions. If our class has 18 girls and a total of 31 people, the percent girls is: % girls = 18 X 100 = 58 % 31 Therefore, the percent of boys = 42%

Calculating Percent Composition % element = mass of the element X 100 molar mass of compound Round the percent compositions to the 100 th place, i.e. go to two decimal places Check to be sure that your percentages total up to 100 %

Percent Composition of KMnO 4 ElementNo. of atoms Atomic Mass Mass of Element Percent of Element K X 100 = % Mn x 100 = % O x 100 = % Molar Mass = Total = 100%

III. Converting Mass to Moles We need to convert mass of a substance to the number of moles so that we can understand chemical reactions. If you use dimensional analysis, it will be easy! Start with grams, then divide by the Molar Mass of the compound

Mass to Moles Convert 25 g of NaCl to moles: –First we need to calculate the Molar Mass: ( 1 X ) + ( 1 X ) = g/mol 25 g NaCl 1 mole NaCl = 0.43 mol g

Mass to Moles Convert 125 g of H 2 SO 4 to moles: –First we need to calculate the Molar Mass: ( 2 X 1.008) + ( 1 X ) + (4 x ) = g/mol 125 g H 2 SO 4 1 mole H 2 SO 4 = 1.27 mols g

IV. Moles to Mass To convert from moles to grams, we multiply the moles by the Molar Mass of the substance. Convert 2.50 mols of NaCl to grams: 2.50 mols g = 146 g NaCl 1 mol NaCl (We already calculated the molar mass of NaCl)