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?

How many dozen eggs is 50? 

Remember the mass number.

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. 

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.

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.

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

Moles and chemical reactions -

Chemical equation - reactants -> products

Ex:  1C_(s) + 1O_2(g) -> 1CO_2(g)

C_(s) + O_2(g) -> CO_2(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?

Yes, it is balanced!

Is the following balanced?

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.