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The Mole Molar Masses

The Mole Molar Masses. The Mole. The mole is a counting unit in chemistry. Abbreviated mol . Defined as the number equal to the number of carbon atoms in exactly 12 grams of pure 12 C . Has the value 6.022 x 10 23 .

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The Mole Molar Masses

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  1. The Mole Molar Masses

  2. The Mole • The mole is a counting unit in chemistry. • Abbreviated mol. • Defined as the number equal to the number of carbon atoms in exactly 12 grams of pure 12C . • Has the value 6.022 x 1023. • Called Avogradro’s number to honor his contributions to chemistry. • One mole of anything consists of 6.022 x 1023units of that substance. • Just like a dozen eggs is 12 eggs; a mole of eggs is 6.022 x 1023 eggs.

  3. How big is a mole? • 1 mole of seconds represents a span of time 4 million times as long as the Earth as already existed. • 1 mole of marbles is enough to cover the entire Earth to a depth of 50 miles.

  4. Using the Mole in Calculations • Remember that the mole is the number of atoms in exactly 12 grams of 12C therefore 12 grams of 12C contains exactly 6.022 x 1023atoms. • Plus it tells us that a 12.01 gram sample of natural carbon contains 6.022 x 1023atoms (natural carbon is a mixture of the isotopes of carbon). • Notice that the value 12.01 grams of natural carbon is the same as the atomic mass value (12.01 amu). • It also tells us that 26.98 grams of aluminum contains exactly 6.022 x 1023atoms of aluminum.

  5. If a mole of natural carbon and aluminum have the same number of atoms why do they have different masses? • Consider a dozen doughnuts and a dozen tires. • Would they have the same mass? • We can also define the mole such that a sample of a natural element with a mass equal to the element’s atomic mass expressed in grams contains 1 mole of atoms.

  6. Example: Determining Moles of Atoms • How many moles of copper are in an old copper penny that has a mass of 3.14 g? How many atoms are in this 3.14 g penny?(New pennies are made of zinc with only a thin surface coating of copper.) • Solution: • The mass of 1 mole (6.022 x 1023 atoms) of copper is 63.55 g. • The sample we are considering has a mass of 3.14 g. • Since the mass is less than 63.55 g, this sample contains less than 1 mole of copper atoms. • 1 mol Cu = 63.55 g Cu (gives two conversion factors)

  7. Which conversion factor is appropriate? We are converting from g to mol. • How many atoms are in this 3.14 g penny? • If we have 1 mol Cu then we have 6.022 x 1023atoms Cu. • 1 mol Cu = 6.022 x 1023atoms Cu

  8. Example: Calculating Moles and Mass • Uranium is a silvery-white metallic element in the actinide series of the periodic table. Calculate both the number of moles in a sample of uranium containing 2.530 x 1015 atoms and the mass of the sample. • Solution: • We know 1 mol U = 6.022 x 1023atoms U.

  9. How many grams of U are contained in this sample? • We know if we have 1 mol U we have 238.03 g U. • 1 mol U = 238.03 g U

  10. Molar Mass • Molar mass of a substance is the mass in grams of one mole of the compound. • Molar mass has units of g/mol. • Remember a chemical compound is a collection of bonded atoms. • If we want to know the mass in grams of one mole of methane, CH4 (the mass of 6.02 x 1023atoms of CH4) we need to take into account that 1 mole of CH4 contains 1 mole of carbon atoms and 4 moles of hydrogen atoms.

  11. To determine the mass of 1 mole of CH4 we sum the atomicmasses of carbon and hydrogen present: • Mass of 1 mol C = 12.02 g • Mass of 4 mol H = 4 x 1.008 g • Mass of 1 mol CH416.04 g • Since 16.04 g represents the mass of 1 mole of methane molecules it is called the molar mass of methane.

  12. Example: Calculating Molar Mass of a Compound • What is the molar mass of calcium phosphate, Ca3(PO4)2, expressed to the second decimal place? • Solution: Use atomic masses of 40.08 for Ca, 30.97 for P, and 16.00 for O. • Ca3(PO4)2 = 3 Ca + 2 P + 8 O • Molar mass of Ca3(PO4)2 = • (3 x 40.08) + (2 x 30.97) + (8 x 16.00) = • 120.24 + 61.94 + 128.00 = • 310.18 g/mol or • 1 mol Ca3(PO4)2 = 310.18 g Ca3(PO4)2

  13. Example: Calculating Grams from Moles for Compounds • A student has worked out the details for an experiment that will use 0.115 mol of Ca3(PO4)2, calcium phosphate, as a starting material. How many grams of Ca3(PO4)2 should be weighed out? • Solution: For this problem we need an equality that relates the number of moles to the number of grams. We know that 1 mol Ca3(PO4)2 = 310.18 g Ca3(PO )2. • Notice units of moles cancel and we are left with grams.

  14. Example: Calculating formula units (molecules if covalent) from grams. • How many formula units of Na2CO3 does 45.8 g of Na2CO3 contain? • Solution: This problem requires 2 equalities. • 1 mol Na2CO3 = ? g Na2CO3 • 1 mol Na2CO3 = 6.022 x 1023 formula units • To determine the number of grams in 1 mol of Na2CO3 sum the atomic masses of the atoms in the compound. • (2 x 22.99 for Na) + (1 x 12.01 for C) + (3 x 16.00 for O) = • 105.99 g/mol

  15. How many sodium atoms were present in the 45.8 g of Na2CO3? • Solution: • In the 45.8 g of Na2CO3 we calculated that we had 2.60 x 1023formula units. Since each formula unit of Na2CO3 contains 2 Na atoms we can calculate as follows.

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