6.3 The Gram Formula Mass

6.3 The Gram Formula 6.3 The Gram Formula MassMass

You are always working with large You are always working with large numbers of atoms even when you are numbers of atoms even when you are working with very small amounts of a working with very small amounts of a substance.substance. Because of this, it is easier to work with Because of this, it is easier to work with

grams of atomsgrams of atoms

gram atomic mass (gam)gram atomic mass (gam) – the atomic – the atomic mass of an element expressed in gramsmass of an element expressed in grams Example: One Example: One gram atomic massgram atomic mass of of

carboncarbon is is 12.0 g12.0 g


You determine the gram atomic You determine the gram atomic mass (gam) of a substance by finding mass (gam) of a substance by finding the atomic mass of a substance.the atomic mass of a substance.

What is the gam for: H, Fe, Hg, and What is the gam for: H, Fe, Hg, and SS 1 gam H = 1.0 g H1 gam H = 1.0 g H 1 gam Fe = 55.8 g Fe1 gam Fe = 55.8 g Fe 1 gam Hg = 200.6 g Hg1 gam Hg = 200.6 g Hg 1 gam S = 32.1 g S1 gam S = 32.1 g S


The gram atomic masses of any two The gram atomic masses of any two elements must contain the same elements must contain the same number of atomsnumber of atoms

A mole is defined as the amount of a A mole is defined as the amount of a substance that contains as many substance that contains as many representative particles as the number representative particles as the number of atoms in 12.0 g of carbon-12 (6.02 x of atoms in 12.0 g of carbon-12 (6.02 x 10102323 representative units) representative units) This means that the gram atomic mass This means that the gram atomic mass

is the mass of one mole of atoms of any is the mass of one mole of atoms of any element.element.

Example: 1 mol C = 1 gam C = 12.0 g CExample: 1 mol C = 1 gam C = 12.0 g C


12. What is the mass of 1 mole of each 12. What is the mass of 1 mole of each of the following monatomic of the following monatomic elements?elements?

a. sodiuma. sodium

A: 23.0 gA: 23.0 g

b. seleniumb. selenium

A: 79.0 gA: 79.0 g

c. leadc. lead

A: 207.2 gA: 207.2 g


To determine the mass of a mole of a To determine the mass of a mole of a compound you must know the compound you must know the formula of a compoundformula of a compound The formula tells you the number of The formula tells you the number of

atoms of each element in a atoms of each element in a representative particle of that representative particle of that compoundcompound


Example: sulfur trioxide, SOExample: sulfur trioxide, SO33, contains 1 , contains 1 atom of S and 3 atoms of Oatom of S and 3 atoms of O You can use the atomic masses of S and O to You can use the atomic masses of S and O to

determine the molar mass of SOdetermine the molar mass of SO33

1 atom of S = 32.1 amu1 atom of S = 32.1 amu 3 atoms of O = 3 x 16.0 = 48.0 amu3 atoms of O = 3 x 16.0 = 48.0 amu molecular mass SOmolecular mass SO33 = 32.1 amu + 48.0 amu = = 32.1 amu + 48.0 amu =

80.1 amu80.1 amu You can substitute the unit You can substitute the unit gramsgrams for for amu’samu’s to obtain the to obtain the gram molecular gram molecular massmass of SO of SO33


gram molecular mass (gmm)gram molecular mass (gmm) – (for – (for any molecular compound) is the mass any molecular compound) is the mass of one mole of that compoundof one mole of that compound

Example:Example:1 mol SO1 mol SO33 = 6.02 x 10 = 6.02 x 102323 molecules SO molecules SO33 = =

80.1 g SO80.1 g SO33

* Gram molecular masses may be calculated * Gram molecular masses may be calculated directly from gram atomic massesdirectly from gram atomic masses


Example: What is the gram molecular Example: What is the gram molecular mass of dihydrogen dioxide, Hmass of dihydrogen dioxide, H22OO22??

U: gmm of dihydrogen dioxideU: gmm of dihydrogen dioxide

K: Molecular Formula - HK: Molecular Formula - H22OO2 2 = 2 H atoms, 2 O = 2 H atoms, 2 O atomsatoms

1 mol H = 1.0 g H1 mol H = 1.0 g H 1 mol O = 16.0 g O1 mol O = 16.0 g O

S: 2 mol H x (1.0 g H/1 mol H) = 2.0 g HS: 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 O2 mol O x (16.0 g O/1 mol O) = 32.0 g O

Gram Molecular Mass HGram Molecular Mass H22OO22 = 2.0 g + 32.0 g = = 2.0 g + 32.0 g = 34.0 g34.0 g


Since the representative unit ionic Since the representative unit ionic compounds is the formula unit and compounds is the formula unit and not a molecule, you cannot represent not a molecule, you cannot represent these compounds with the gram these compounds with the gram molecular mass unit.molecular mass unit. However, you can use the However, you can use the gram gram

formula mass formula mass ((gfmgfm)), which is , which is calculated in the same way.calculated in the same way.


Example: Calculate the gram formula Example: Calculate the gram formula unit for CaIunit for CaI22..

U: gfm of calcium iodideU: gfm of calcium iodide

K: Formula Unit – CaIK: Formula Unit – CaI22 = 1 Ca atom, 2 = 1 Ca atom, 2 I atomsI atoms

S:S:

gfm = 40.1 g Ca + (2 x 126.9 g I) = gfm = 40.1 g Ca + (2 x 126.9 g I) = 293.9 g CaI293.9 g CaI22

6.4 The Molar Mass of a 6.4 The Molar Mass of a SubstanceSubstance

The terms gram atomic mass (gam), The terms gram atomic mass (gam), gram molecular mass (gmm), and gram molecular mass (gmm), and gram formula mass (gfm) represent gram formula mass (gfm) represent the mass of a mole of a particular the mass of a mole of a particular substance.substance.

It is appropriate to use the term It is appropriate to use the term gram formula mass, or just gram formula mass, or just molar molar massmass, to refer to the mass of a mole , to refer to the mass of a mole of any element or compound.of any element or compound.

6.3 The Gram Formula Mass

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