Beginning Chapter 6

Moles

An analogy before we begin, the concept of a mole is identical to the concept of a dozen.   If you can answer these questions, you can easily understand what a mole is.

How many eggs are in 3 dozen?

 3 dozen 12 eggs = 36 eggs 1 dozen

How many dozen eggs is 50?

 50 eggs 1 dozen =4.2 dozen 12 eggs

Remember the mass number.

 1 atom of carbon has a mass of 12 amu 1atom of nitrogen has a mass of 14 amu

Moles and molar mass - In carrying out chemical reactions we generally can't weight out one molecule so we need a quantity we can deal with.  More convenient to sell eggs by the dozen rather than one by one.

Mole = number of atoms in 12 g of carbon = 6.02 x 1023 = Avagadro's number

How big is Avagadro's number?  Imagine winning the power ball lottery for Avagadro's number \$\$\$\$.    If you spend 1 billion dollars of your winnings each second of your life, you would still have over 99.999% of it left when you die.

 1 mole B 6.02x1023 atoms of B 1 mole CO2 6.02x1023 molecules of CO2 6.02x1023 atoms of C 2 x 6.02x1023 atoms of O

Molar mass of an element = mass of 6 x 1023 atoms of that element (just like the mass of a dozen eggs, vs. a dozen apples). Units g  / mole

Molar mass = atomic mass from the periodic table in units of grams.

 Molar mass of C 12.0 g Molar mass of N 14.0 g Molar mass of H2 (2*1.008g) = 2.02 g

The formula mass (mass in amu of a formula unit of a compound), molecular mass (mass in amu of a molecule) and the mass of a mole of molecules (in grams) are all numerically equivalent.

In other words if we ignore their different units,

molar mass = formula mass  = molecular mass = mass of a mole of molecules

Examples:

1. What is the atomic mass and molar mass of Boron?

atomic mass = 10.81 amu / atom B (does one atom really weigh 10.81 amu?)

molar mass = 10.81 grams / mole B

2. What is the molecular and molar mass of CO2

molecular mass = 2(16) + 12 = 44 amu

molar mass = 2(16) + 12 = 44 g

3. If you have 0.5 moles of CO2, how many grams is this?

(# of moles)(molecular mass) = # grams

All we need to do is figure out whether we need to multiply by 44 g / mole or 1 mole / 44 g (just like 12 eggs / 1 dozen; 1 dozen / 12 eggs).  Just look at the units to figure it out.

 0.5 moles 44.0 g = 22.0 grams 1 mole

4. If you have 5.8 grams of NaCl, how many moles is that?

molar mass of NaCl = 23.0 + 35.5 = 58.5 g / mole

 5.8 g 1 mole 0.1 mole 58.5 g

Moles and chemical reactions -

Chemical equation - reactants -> products

Ex:  1C(s) + 1O2(g) -> 1CO2(g)

C(s) + O2(g) -> CO2(g)

(s) = solid
(l) = liquid
(g) = gas
(aq) - aqueous solution (i.e. dissolved in water)

reactants: C and O2, products: CO2

Ex: 4 Al + 3 O2 -> 2Al2O3

reactants:  Al and O2, products: Al2O3

Coefficients -  number before each substance indicates the number of units

4 moles Al + 3 moles O2 -> 2 moles Al2O3
4 atoms Al + 3 atoms O2 -> 2 atoms Al2O3

Chemical reaction equations are balanced

# of atoms of each element in the reactants = # of atoms of each element in the products

Show Fig 6.3

How does this relate to the conservation of mass and atomic theory?

Is the following balanced?

 2H2(g) + O2(g) -> 2 H2O(l) 2 moles H2 + 1 mole O2 -> 2 mole H2O 2 molecules H2 + 1 molecule O2 -> 2 molecules H2O 4 atoms of H + 2 atoms O -> 4 atoms of H 2 atoms of O

Yes, it is balanced!

Is the following balanced?

 CaCl2(aq) + AgNO3(aq) -> AgCl(s) + Ca(NO3)2(aq) 1 atom Ca 2 atoms Cl + 1 atom Ag 1 atom N 3 atoms O -> 1 atom Ag 1 atom Cl + 1 atom Ca 2 atom N 6 atom O

NO!, number of Cl do not balance, # of O do not balance, # of N do not balance.  A balanced version of the reaction is as follows.

 CaCl2(aq) + 2AgNO3(aq) -> 2AgCl(s) + Ca(NO3)2(aq) 1 atom Ca 2 atoms Cl + 2 atom Ag 2 atom N 6 atoms O -> 2 atom Ag 2 atom Cl + 1 atom Ca 2 atom N 6 atom O