Counting Atoms and Moles Chapter 3.4 us us.

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Presentation transcript:

Counting Atoms and Moles Chapter 3.4 us us

Atomic Mass Atomic Mass: the mass of an atom expresses in atomic mass units –Atomic mass units (AMU) This number is based on an average do to the isotopes found in nature.

Average Atomic Mass Calculating the Average Atomic Mass Pb-204 (1.4%); Pb-206 (24.1); Pb-207 (22.1%); Pb-208 (52.4%) –Lead ALWAYS has an atomic number of 82 even though the number of neutrons can vary based on isotopes. Average Atomic Mass

Calculating Average Atomic Mass Calculating the Average Atomic Mass Pb-204 (1.4%); Pb-206 (24.1); Pb- 207 (22.1%); Pb-208 (52.4%) 204 (.014) = (.241) = (.221) = (.524) =

Average Atomic Mass Here is some information about two isotopes of boron: What is the average atomic mass of boron?

Atomic Mass What to do: –Change percent to decimal (move decimal point 2 places to the left) –Multiply by mass –Add up answers to get atomic mass Boron-10: (0.1978)(10) = 1.98 amu Boron-11: (0.8022)(11) = amu amu

Atomic Mass Copper-63: (0.6917)(62.94) = amu Copper-65: (0.3083)(64.93) = amu amu %Copper %Copper-63 Atomic mass (amu) AbundanceIsotope

THE MOLE Mole: defined as the number of atoms in exactly 12 grams of carbon-12. –Symbol or unit (mol) Avogadro’s Number: 6.02 x eggs = 1 dozen eggs How do you ask for eggs…a person says you need a dozen…it’s understood as 12. In 1 mole of sulfur, there are 6.02 x sulfur atoms

The Mole Avogadro’s # can be used to convert: –# of atoms to moles –moles to # of atoms Use conversion factors & dimensional analysis!!

Conversion Factors §A simple way to make conversions is to use a conversion factor §A conversion factor is a ratio of equivalent measurements

Dimensional Analysis How to set up problem: *Start with what’s given (ex. 2.5 days) *Choose conversion factor so that what you know is in the denominator & the unknown is in the numerator *Keep choosing conversion factors until the unit you want to get to is in the numerator

Problems How many moles are in 2.84 x atoms of iron? 2.84 x10 22 atoms x 1 mol Fe = mol Fe 6.02 x atoms

Problems How many moles are in 2.84 x atoms of iron? 2.84 x10 22 atoms x 1 mol Fe = mol Fe 6.02 x atoms

Practice Problems How many atoms are in 3.5 mol of Na? How many moles are in 4.78 x atoms of Ag? How many atoms are in 0.78 mol of Mg? How many moles are in 6.85 x atoms of Hg? How many atoms are in 22 mol of Al? How many moles are in 1.23 x atoms of He?

Molar Mass Molar Mass: is an elements mass in grams of one mole of the element. –Molar mass has the unit grams per mol (g/mol) Example: –Atomic Mass of Copper (Cu) = 63.55amu –Molar Mass of Copper (Cu) = 63.55g/mole

Molar Mass Q: What is the molar mass of H 2 O? HINT: USE THE PERIODIC TABLE A: The molar mass of Hydrogen (H)=1.00g/mol The molar mass of Oxygen (O)=15.99g/mol 2 (1.00g/mol) + 1(15.99g/mol) = 18g/mol

Calculating Mole Conversions Questions you need to consider before answering any questions involving moles or atoms. –What do I know? –Where do I need to get to? –How do I get there?

Mole Conversions How many moles are in 24 g of carbon?

Mole Conversions How many grams are in 1.8 moles of nickel?

Problems 1.How many grams are in 2.3 mol of Na 2 O? 2.How many moles are in 68.3 g of S? 3.How many grams are in 0.26 mol of Fe 2 O 3 ? 4.How many moles are in 222 g of Au? 5.How many atoms are in 16.2 g of Ca? (hint: #7 will have 2 steps, think about Avogadro)