SCH 3U/4C - Simple Calculations Using the Mole

Simple Calculations Using the Mole

In everyday life, it is important to understand the quantitative measures of substances. For instance, when making decisions about quantities in cooking recipes and in determining dosages in medical prescriptions, it is very important to measure the proper quantities.

• chemists normally measure out substances in grams or kilograms 
• sometimes we need to know the number of moles of a substance used for a given mass
• we need a method to convert between mass and moles, so we use the following equation:

n = m/M
where n = number of moles (mol), m is the mass (g) and M is the molar mass (g/mol)

👇👇👇👇👇👇👇👇👇👇

 

***When carrying out calculations, you MUST use the proper format (I have colour-coded each bit of proper format in the first example).  Place the known and unknown data along the left margin. In the middle of the page, provide the mathematical equation.  Beneath that, substitute in the data, with units, and then solve the problem.  Include a final statement, in which the answer has been rounded to the correct number of significant digits. Failure to follow proper format will result in lost marks (see the penalties toward the end of this post).***



Changing Grams to Moles
ex. A reaction requires the use of 10.3 g of sodium carbonate.  What amount* of sodium carbonate is used in the reaction?

Na₂CO₃

m = 10.3 g                            n = m/M

n = ?                                        = 10.3 g/105.99 g/mol

M = 105.99 g/mol**                 =  0.097178 mol          

                                   ∴ the number of moles is 0.0972 mol.

*When a chemist says, "what amount?" she is asking "how many moles?"
**You do not need to show the calculations for the molar mass.  Just cut to the chase.

Changing Moles to Grams

ex. If a container is holding 12.68 mol of water, what mass of water is in the container?

H₂O

n = 12.68 mol                       m = nM

m = ?                                       = 12.68 mol(18.02 g/mol)

M = 18.02 g/mol                      = 228.4936 g          

                                     ∴ the mass is 228.5 g.

Calculating the Number of Particles from Mass

Sometimes, we will run across questions that require us to use the equation from yesterday's lesson, along with today's.  No problem!

ex. How many atoms of silver are in a 120.5 g pure silver bracelet?

Ag

m = 120.5 g                       n = m/M                                       

M = 107.9 g/mol                   = 120.5 g/107.9 g/mol                  

N = ?                                    = 1.117 mol                                   

                                         
                                          N = nN_A
                                              = 1.117 mol(6.02 x 10²³ atoms/mol)
                                              = 6.72 x 10²³ atoms

                                     ∴ there is 6.72 x 10²³ atoms of silver in the bracelet.

Still need some guidance, check out this video.  She explains well but be sure to use the format the way I have asked when you do your questions.  You will find my format makes life easier in the long run.


Homework:

Practice Problems, p. 237 # 41, 47, 49

Practice Problems, p. 239 # 55, 58

Practice Problems, p. 242 # 61, 62, 65

 

Student questions:

1. Can you do a couple questions in #61 on p. 242?

Sure I can.  Just try and stop me.

2. Can you show me p. 239 #55b? 

But, of course...

3.  How did you get the exponent of "2" on the final answer of p.237 #47?

Did you forget to convert the final answer (which is in "g") to milligrams ("mg")?

4.  What happens if I don't use proper format?

Failure to employ proper format will result in the following penalties on each question:

• Not listing out all data; the knowns/unknowns (should locate this at the left hand margin) → -1 mark
• Not providing the equation being used (should locate this in the centre of the page) → -1 mark
• Not substituting in data, with units (should locate directly beneath equation) → up to -1 mark per occasion
• Not providing an answer, with units (should locate directly beneath substitution) → -1 mark
• Not providing a final statement (should locate directly below answer) → -1 mark
• Incorrect rounding/sig digs for final answer → -0.5 mark

The reason I am so picky about format is three-fold: 

• It will serve you well the future when the questions get more difficult.
• Science, like language, has rules of communication.  It is important to be literate in science "language."
• If you skip steps and get the answer wrong, you make it difficult, if not impossible for me to trace the error and give you part marks.

 

5.  Can you do p. 237 #47 and p. 239 #55a,c and p. 242 #65?