3.4: The Mole

MOL

The mol can be define by the following:

1. Amount of a substance whose weight/mass in grams is numerically equal to its atomic weight in amu.

2. Amount of a substance that contains the same number of units as the atoms in exactly 12 grams of C-12.

     (C-12 = 12 amu --> 12 grams of Carbon = 1 mol of Carbon)

Example:

*All have different weights, but all are 1:1 ratio in the equation.

AVOGADRO'S NUMBER

1 dozen = 12

1 barrel = 42

1 mole = 6.02 x 10^23 atoms/molecules/formula unit

Example:

One mole of H2O = Molar mass of a compound. Therefore:

H2O = (2 x 1.01) + (1 x 16.00) = 18 g/mole = 6.02 x 10^23 molecules of H2O in one mole

CONVERSIONS OF CHEMICAL UNITS

Things to keep in mind:

*Always start with what is given in the problem.

*Always make sure your units cancel out. If they do your procedure is most likely to be correct.

*Same procedures are followed for other type of chemical conversions.

1. Grams to mole:

Example:

How many moles are in 89.0 grams of NH4Cl?

2. Mole to Grams:

Example:
What is the mass of 0.150 moles of Na2SO4?

3. Mole to mole:

 

Example:
Using the following equation, how many mols of N2 are needed to produce 3.45 mol of NH3?

N2(g) + 3H2(g) --------> 2NH3

Step 1: Make sure your equation is balanced, otherwise balance it.

Step 2: Before answering any questions, take note of what's given vs. was being asked:

Given: 3.45 mol of NH3

Asked: Moles of N2

Step 3: Start your unit analysis by writing the given, and then the ratio found in the balanced equation as follows:

   

 

4. Grams to grams:

 

 
Example:
Using the following equation, how many grams of NH3 will be produced by 60 grams of H2?

N2(g) + 3H2(g) --------> 2NH3

Step 1: Make sure your equation is balanced, otherwise balance it.

Step 2: Before answering any questions, take note of what's given vs. was being asked:

Given: 60 grams of H2

Asked: Grams of NH3

Step 3: Start your unit analysis by writing the given, and then use the periodic table to find the grams of the given as follows:

 

Step 4: Cancel the moles of the given by doing a ratio of asked over ratio of given obtained from the balanced equation as follows:


 

Step 5: Cancel the moles of the asked by using the periodic table of that specific compound/element: