Section 7.3 – Using Chemical Formulas   They allow chemists to calculate a number of characteristic values for a given compound.   Formula mass   Molar mass   Percentage composition

Formula Mass   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)   mass H atom = amu   mass O atom = amu   Mass of 1 water molecule = (2 x ) + (1 x ) = amu/molecule

Your turn #1   What is the formula mass of sulfuric acid?   H 2 SO 4   2xH + 1xS + 4xO = 2( ) + 1(32.066) + 4( ) = amu

Your turn #2   What is the formula mass of calcium nitrate?   Ca(NO 3 ) 2   1xCa + 2xN + 6xO = 1(40.078) + 2( ) + 6( ) = amu

Remember The Mole?   The molar mass of a substance is the mass in grams of one mole (i.e., the mass in grams of x particles).

Molar Mass Calculated by summing the masses of the elements present in one mole of the substance (6.022 x particles) A mole of molecules, formula units, or ions of a compound. Reported in grams.

How many moles of atoms are there? How many moles of atoms of each element are there in aluminum sulfide? Al 2 S 3 One mole of aluminum sulfide contains 2 moles of Al, and 3 moles of S

How many moles of atoms are there? How many moles of atoms of each element are there in barium hydroxide? Ba(OH) 2 1 mol Ba 2 mol O 2 mol H

Molar Mass – Water (H 2 O)   1 mole H atoms = grams   1 mole O atoms = grams   1 mole water molecules = (2 x ) + (1 x ) = g/mole H 2 O

Molar Mass – Example   What is the mass of one mole of sodium nitrate?   NaNO 3   1 mol Na( g/mol) + 1 mol N( g/mol) + 3mol O( g/mol) = g   Molar mass is g/mol

Notice   The molar mass and the formula mass are numerically equal;   formula mass is reported in amu   Molar mass is reported in grams/mol

Molar Mass As A Conversion Factor   See p. 224   Can use molar mass to convert   grams  moles  molecules   Molecules  moles  grams

Examples   What is the mass in grams of 3.04 mol of ammonia vapor, NH 3 ?   What is the mass of mol of calcium nitrate?

Examples   How many moles of compound are there in 6.60g of ammonium sulfate?   How many molecules are there is 25.0g of sugar (C 12 H 22 O 11 )?

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

Determine % composition 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.

Percent composition by moles (Easier)

Example

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   Chapter 7 Element Handbook Problems   7.3 Textbook Problems