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Mass and the Mole: Understanding Atomic Mass and Molar Mass

This chemistry lesson explains the concept of atomic mass and molar mass, as well as how to calculate the molecular mass of a compound. It also discusses Avogadro's constant and the use of relative atomic and molecular masses. The assignment at the end reinforces the concepts learned.

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Mass and the Mole: Understanding Atomic Mass and Molar Mass

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  1. DP Chemistry 11Part 3Mass and the Mole

  2. The Periodic Table shows the atomic masses of the elements. Atomic masses are measured in amu’s (atomic mass units) because they are too small to measure directly. The atomic mass of an element, expressed in grams, is equal to the mass of one mole of that element.Therefore:If mercury (Hg) has an atomic mass of 200.6 amu, 1 mole ofmercury atoms has a mass of 200.6 gIf carbon (C) has an atomic mass of 12.0 amu, 1 mole of carbon atoms has a mass of 12.0 g If sulfur (S) has an atomic mass of 32.1 amu, 1 mole of sulfur atoms has a mass of 32.1 g

  3. This is also true of molecules. One mole of a compound = the molecular mass of the compoundTo determine the molecular mass of a compound add up the individual atomic masses: 1 mole of water (H2O) = 2 hydrogens + 1 oxygen = 2 (1) + 16 = 18 g 1 mole of glucose (C6H12O6 ) = 6 carbon + 12 hydrogen + 6 oxygen = 6 (12) + 12(1) + 6 (18) = 192 g

  4. The mass, in grams, of one mole of any pure substance is called its molar mass. The units are given as g/mol.Example: As given on the Periodic Table (refer to www.ptable.com) , an atom of iron has an atomic mass of 55.845 amu. Therefore, the molar mass of iron is 55.845 g/moland 1 mole of iron (6.02 x 1023 atoms of iron) has a mass of 55.845 g

  5. Why are atomic masses exact integers? Recall, from grade 10 Chemistry, that most elements have isotopes.Definition: Isotopes are the atoms of the same element with different numbers of neutrons.There are 3 isotopes of hydrogen:Most hydrogen atoms have a mass of 1 amu. When all masses are added up, the average mass is 1.008 amu. This is called the relative mass.

  6. Although atomic mass is measured in amu, relative atomic mass has no units.The symbol used to show relative atomic mass is Ar

  7. Relative molecular masses (Mr )are calculated using the relative atomic masses (Ar) of the elements making up a substance.

  8. To review:Avogadro’s Constant = 6.02 x 1023 = 1 molemolar mass = the mass in grams of one mole of any pure substance, derived from the Periodic Table - measured as g mol-1 (read as g/mole)Relative atomic mass = Ar = average atomic mass of a substance - no unitsRelative molecular mass = Mr = sum of the Ar of all atoms making up a molecule - no units

  9. Assignment #3due date: Monday October 5, 2009 1. Review: Avogadro’s number: How much time, in years, would it take to count the number of particles in one mole of any substance? (assume that it takes 1 second to count 1 particle). • How many particles make up 0.05 moles of Sulfur? • How many moles of carbon are found in 200 grams of carbon? (atomic mass of carbon = 12.0 amu) • Complete page 10, questions 2,3,4

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