**Ch. 6 - Chemical Quantities**

4 sig figs

6.1 - Atomic Mass and Formula Mass

In most samples of elements we treat every atom as if it has the same atomic mass.

N= 14.01amu

Zn= 65.41amu

Sn= 118.7amu

14.01amu

1 N atom

OR

1 N atom

14.01amu

atomic mass conversion factor

Mass of a Sample of Atoms

atomic mass can be used to convert a known quantity of atoms to their mass (amu)

10 N atoms X

14.01amu

1 atom

=

140.1 amu

Using Atomic Mass to Count Atoms

6420amu X

1 S atom

32.07amu

=

2.00 X 10 S atoms

2

1500amu of N = ? atoms of N

Formula Mass

the sum of all the atomic masses of all the atoms in a formula

Formula Mass of Carbon Dioxide = ?

1 C atom X

12.01amu

1 C atom

=

12.01amu

2 O atoms X

16.00amu

1 O atom

=

32.00amu

Formula Mass of CO =

2

44.01amu

Formula Mass of water = ?

6.2 - The Mole

Just like eggs are counted by the dozen and paper is sold by the ream; atoms, molecules, and ions are counted in moles(mols).

12 eggs = 1 dozen

6.022 x 10 atoms = 1 mole

23

Subscripts State Moles of Elements

Subscripts in chemical formulas tell us how many atoms are in the compound.

If we have 1 mole of carbon dioxide how many moles of each element would we have? How many atoms?

1 mol of C and 2 mol of O

1 x 6.022 x 10 = atoms of C

23

2 x 6.022 x 10 = atoms of O

23

2 mol O

1 mol CO

2

1 mol C

1 mol CO

2

6.3 - Molar Mass

quantity in grams that equals the atomic mass of that element

6.4 - Calculations Using Molar Mass

molar mass is used to change moles into grams or grams into moles

equalities can be written as:

24.31 g Mg

1 mol Mg

1 mol Mg

24.31 g Mg

18.02 g H O

1 mol H O

2

2

2

2

&

1 mol H O

18.02 g H O

elements

compounds

? g Ag in a .750 mol sample

? mols of NaCl in a 737 g sample

pg. 176-177

g/mol

6.022 x 10

23

A 10.00 -lb bag to table sugar contains 4536 g sucrose, C H O . How many molecules of sugar are present?

or Atoms

element or compound

pg. 178

12

22

11

6.5 - Percent Composition and Empirical Formulas

Percent Composition - the mass percent of each element present in a compound

Molecular Formula - actual or true formulas of compounds

Empirical Formula - formula that represents the lowest whole-number ratio of the atoms in a compound

Empirical Formulas

determined by converting the # of grams of each element to moles and finding the lowest whole # ratio to use as subscripts

Converting Decimals to Whole numbers

Sometimes the result of dividing by the smallest number of moles gives a decimal instead of a whole number.

less than 0.1 or greater than 0.9, round to the nearest whole number

between 0.1 & 0.9, multiply by small integer until a whole number is obtained

Try the sample on pg. 184

6.6 Molecular Formulas

Molecular formula = small integer x empirical formula

Molar mass = small integer x empirical formula mass

small integer =

molar mass of compound (g/mol)

empirical formula mass (g/mol)

molar mass of ribose

empirical formula mass of ribose

=

150.0 g/mol

30.0 g/mol

= 5

5 x empirical formula (CH O) = molecular formula (C H O )

2

5

10

5

Calculating Molecular

Formula

Calculate mass of empirical formula

Divide molar mass by the empirical formula mass to obtain a small integer

Multiply the empirical formula by the small integer to obtain molecular formula

Try sample problem on pg. 187