Section 7.3 – Using Chemical Formulas

 They allow chemists to calculate a number of characteristic values for a given compound.

Formula Mass

 Like individual atoms, the formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all the atoms represented in its formula. Reported in amu (atomic mass units).

Example – Water (H

2

O)

   1 H atom = 1.0079 amu 1 O atom = 15.9994 amu 1 water molecule = (2 x 1.0079) + (1 x 15.9994) = 18.0152 amu/molecule

Remember The Mole?

 The mass of 6.022 x 10 23 particles.

Molar Mass

Calculated by summing the masses of the elements present in a mole (6.022 x 10 23 particles) of the molecules, formula units, or ions of a compound. Reported in grams.

Example – Water (H 2 O)  1 mole H atoms = 1.0079 grams  1 mole O atoms = 15.9994 grams  1 mole water molecules = (2 x 1.0079) + (1 x 15.9994) = 18.0152 g/mole H 2 O

Notice

 The molar mass and the formula mass are numerically equal; one is reported in amu, the other in grams.

EXAMPLE

 Find the formula mass for KClO 3  Find the formula mass for Ca(NO 3 ) 2   Find the molar mass for Al 2 S Find the molar mass for Ba(OH) 2

Molar Mass As A Conversion Factor   See p. 224 To convert moles to grams: # mol x g/mol = g  To convert mass to moles: grams x 1/molar mass = moles

EXAMPLES

   What is the mass in grams of 2.50 mol of oxygen gas (O 2 )?

How many moles of compound are there in 6.60g (NH 4 ) 2 SO 4 How many molecules are there in 25.0g of H 2 SO 4 ?

Percentage Composition  Often useful to know this, it is the percentage by mass of each element in a compound. There are two ways to determine it:

By Mass  Mass of element in a sample divided by the mass of the sample multiplied by 100. The percentage will be the same regardless of the sample size.

Or By Moles (Easier)

 Mass of the element in 1 mol divided by the molar mass of the compound multiplied by 100.

EXAMPLES

Find the percentage compositions of: – PbCl 2 Ba(NO 3 ) 2 – Magnesium Hydroxide is 54.87% oxygen by mass. How many grams of oxygen are in 175 g of the compound. How many moles of oxygen is this?

Info. You Will Need For Later



Hydrates

are salts that crystallize from a water solution and trap water (known as water of hydration) in the crystal structure.

Na 2 CO 3 •10H 2 O  This means that 10 moles of water are trapped within each mole of the sodium carbonate salt.

Assignment – Due Friday   7.3 Worksheet 7.3 Textbook Problems