6.3 The Gram Formula Mass

1. 6.3 The Gram Formula Mass • You are always working with large numbers of atoms even when you are working with very small amounts of a substance. • Because of this, it is easier to work with grams of atoms gram atomic mass (gam) – the atomic mass of an element expressed in grams • Example: One gram atomic mass of carbon is 12.0 g

2. 6.3 The Gram Formula Mass • You determine the gram atomic mass (gam) of a substance by finding the atomic mass of a substance. • What is the gam for: H, Fe, Hg, and S • 1 gam H = 1.0 g H • 1 gam Fe = 55.8 g Fe • 1 gam Hg = 200.6 g Hg • 1 gam S = 32.1 g S

3. 6.3 The Gram Formula Mass • The gram atomic masses of any two elements must contain the same number of atoms • A mole is defined as the amount of a substance that contains as many representative particles as the number of atoms in 12.0 g of carbon-12 (6.02 x 1023 representative units) • This means that the gram atomic mass is the mass of one mole of atoms of any element. • Example: 1 mol C = 1 gam C = 12.0 g C

4. 6.3 The Gram Formula Mass 12. What is the mass of 1 mole of each of the following monatomic elements? a. sodium A: 23.0 g b. selenium A: 79.0 g c. lead A: 207.2 g

5. 6.3 The Gram Formula Mass • To determine the mass of a mole of a compound you must know the formula of a compound • The formula tells you the number of atoms of each element in a representative particle of that compound

6. 6.3 The Gram Formula Mass • Example: sulfur trioxide, SO3, contains 1 atom of S and 3 atoms of O • You can use the atomic masses of S and O to determine the molar mass of SO3 • 1 atom of S = 32.1 amu • 3 atoms of O = 3 x 16.0 = 48.0 amu • molecular mass SO3 = 32.1 amu + 48.0 amu = 80.1 amu • You can substitute the unit grams for amu’s to obtain the gram molecular mass of SO3

7. 6.3 The Gram Formula Mass gram molecular mass (gmm) – (for any molecular compound) is the mass of one mole of that compound Example: 1 mol SO3 = 6.02 x 1023 molecules SO3 = 80.1 g SO3 * Gram molecular masses may be calculated directly from gram atomic masses

8. 6.3 The Gram Formula Mass Example: What is the gram molecular mass of dihydrogen dioxide, H2O2? U: gmm of dihydrogen dioxide K: Molecular Formula - H2O2 = 2 H atoms, 2 O atoms 1 mol H = 1.0 g H 1 mol O = 16.0 g O S: 2 mol H x (1.0 g H/1 mol H) = 2.0 g H 2 mol O x (16.0 g O/1 mol O) = 32.0 g O Gram Molecular Mass H2O2 = 2.0 g + 32.0 g = 34.0 g

9. 6.3 The Gram Formula Mass • Since the representative unit ionic compounds is the formula unit and not a molecule, you cannot represent these compounds with the gram molecular mass unit. • However, you can use the gram formula mass (gfm), which is calculated in the same way.

10. 6.3 The Gram Formula Mass Example: Calculate the gram formula unit for CaI2. U: gfm of calcium iodide K: Formula Unit – CaI2 = 1 Ca atom, 2 I atoms S: gfm = 40.1 g Ca + (2 x 126.9 g I) = 293.9 g CaI2

11. 6.4 The Molar Mass of a Substance • The terms gram atomic mass (gam), gram molecular mass (gmm), and gram formula mass (gfm) represent the mass of a mole of a particular substance. • It is appropriate to use the term gram formula mass, or just molar mass, to refer to the mass of a mole of any element or compound.