3.10 – NOTES Measuring Matter - Moles

V. Measuring Matter A. Counting particles It is convenient and easier to count certain items in groups: e.g. eggs in a dozen, paper in reams (500 sheets), etc. Matter is composed of items so small that we must mass out groups of atoms or molecules, and those groups are called moles.

B. What is a mole? Definition: A mole (mol) is defined as the number of particles (carbon atoms) in exactly 12g of C-12. The number has been found to be 6.022 x 1023; this is known as Avogadro’s number.

C. Converting moles to particles and particles to moles Conversion factor is 1 mole = 6.022 x 1023 particles

Examples a. How many atoms are in a 5.67 mole sample of Cu? 5.67 mol x 6.02 x 1023atoms of Cu = 3.41 x 1024atoms 1 mole **maintain sig figs

Examples b. How many molecules are in 0.0034 mole of water? 0.0034 mol x 6.02 x 1023m-c of H2O = 2.0 x 1021 m-c 1 mole **maintain sig figs

Examples c. How many moles are in a 2.78 x 1024 atom sample of Ar? 2.78 x 1024 atoms Ar x 1 mole = 4.62 moles 6.02 x 1023 atoms

d. 9.12 x 1021 molecules of N2O = _____________ mole 9.12 x 1021 m-c of N2O x 1 mole = 0.0151 moles 6.02 x 1023 m-c

II. Mass and the Mole A. The mass of a mole of elements and compounds Molar mass is __the sum of the elemental masses in a compound_. The molar mass of a substance contains 6.022 x 1023 particles of the substance. Particles would be atoms for most elements; molecules for compounds. Molar mass is a conversion factor!

1. What is the molar mass of oxygen gas? 2 x 16.0 = 32.0 g/mol

2. What is the molar mass of calcium phosphate, Ca3(PO4)2? Ca – 3 x 40.1 = 120. P – 2 x 31.0 = 62.0 O – 8 x 16.0 = 128 310. g/ mol

B. Using molar mass Examples: 1. How many moles are in a 45 gram sample of sulfur dioxide, SO2? 45 g SO2 x 1 mol = 0.70 mol 64.0 g

2. 0.0718 mole of nitric acid, HNO3, would have a mass of ________________ g. 0.0718 mol HNO3 x 63.0 g = 4.52 g 1 mol

C. Conversions from mass to particles and particles to mass These problems have 2 steps since you must go through moles first.

Examples: 1. How many atoms are in a 3.4 gram sample of gold? 3.4 g Au x 1 mol x 6.02 x 1023atoms = 1.0 x 1022atoms 197 g 1 mol *MM and Avogadro’s number should still have a minimum of 3 sig figs even though the given only has 2.

2. 4.51 x 1023 molecules of fluorine gas would have a mass of _________________ g. 4.51 x 1023 mc F2 x _____1 mol____ x 38.0 g = 28.5 g 6.02 x 1023mc 1 mol

3. 2.543 x 1023 molecules of carbon dioxide will have ______ grams of oxygen. 2.543 x 1023 mc CO2 x 2 atoms O x ____1 mol_____ x 16.00 g = 13.03 g 1 mc CO2 6.022 x 1023atoms 1 mol **Use 4 sig figs for mass and Avogadro’s number since 4 sig figs were originally given.