Unit 2, Topic 3:

Mole Calculations

Lesson Instructions

Review the learning goals and success criteria for Topic 3: Mole Calculations
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals (continued from Topic 1 & 2)

I am learning to ...

understand how two atoms of the same element can have different masses
describe what a “mole” is as and how it relates to chemistry
use the concept of a “mole” is conversion calculations

Success Criteria (continued from Topic 1 & 2)

I am able to ...

use significant digits appropriately in calculations
describe the term isotope and how it is related to the atomic abundance
describe the concept of the mole as 6.02 × 10²³ particles (such as atoms or molecules) of a sample
use the mole to perform conversion calculations

Minds-On

Although you now have a familiar counting quantity, the mole, there is unfortunately no measuring cup or scale that will dispense one mole of water or salt or iron. One mole of each of these substances has a different mass. The internationally accepted unit of mass is grams, so you must relate mass (in grams) to moles. We call this the Molar Mass.

What is Molar Mass?

The average atomic mass of an element, is the mass of all the different isotopes of that element. Atomic mass units are indicated by the letter u and are listed under the atomic symbols on the periodic table of elements for this course. This value (listed on the periodic table) is equal to the mass of one mole of that particular element. This value is called the element’s molar mass, M, and it is defined as the mass of one mole of the element in question.

Figure 1. Molar mass is given the units grams/mole (g/mol). To use an example from earlier in this lesson, magnesium is listed with an atomic mass of approximately 24.30 u. Therefore, the molar mass of magnesium is 24.30 grams per mole, or simply 24.30 g/mol.

Action

Molar Mass and the Mole

Molar Mass (M) – the mass of 1 mole of a substance (g/mol).

How is Molar Mass Calculated?

First thing's first, you need your Periodic Table! I have attached another copy of the Periodic Table for easy access, below. Keep this with you at all times as we will be relying on it frequently throughout this unit.

Example 1a - Calculate the Molar Mass of copper, Cu.

Using your periodic table, find copper (Cu).
Locate the molar mass of copper. On your periodic table, this will be in the upper right hand corner of the box that contains the element.
Thus, the molar mass of copper is 63.55 g/mol.

Figure 2. Mass of copper.

Example 1b - Calculate the Molar Mass of phosphorus, P.

Using your periodic table, find phosphorus (P).
Locate the molar mass of phosphorus. On your periodic table, this will be in the upper right hand corner of the box that contains the element.
Thus, the molar mass of phosphorus is 30.97 g/mol.

Figure 3. Mass of phosphorus.

Example 1c - Calculate the Molar Mass of chlorine, Cl₂.

Remember: On its own, chlorine is found as Cl₂ because it is diatomic.

Using your periodic table, find chlorine (Cl).
Locate the molar mass of chlorine. On your periodic table, this will be in the upper right hand corner of the box that contains the element.
This time, we have Cl₂which means we need to MULTIPLY the mass of one chlorine molecule by 2.
Thus, the molar mass of chlorine, Cl_{2 ,}is 70.90 g/mol.

Figure 4. Calculating the molar mass of chlorine, Cl₂.

Example 1d - Calculate the Molar Mass of sulfuric acid, H₂SO₄.

Using your periodic table, find and record the mass of each element in the compound: hydrogen (H), sulfur (S), and Oxygen (O).
1. - Hydrogen: 1.01 g/mol
  - Sulfur: 32.07 g/mol
  - Oxygen: 16.00 g/mol
Make yourself an equation that represents the quantity of each element in the compound:
1. - Molar Mass of H₂SO₄= 2H + 1S + 4O
  - In H₂SO₄ there are 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms. This is represented by the equation above.
Finally, plug in the masses of each element into your equation to solve for the Molar Mass of H₂SO_{4 .}
1. - Molar Mass of H₂SO₄= 2H + 1S + 4O
  - Molar Mass of H₂SO₄= 2 (1.01 g/mol) + 1 (32.07 g/mol) + 4 (16.00 g/mol)
  - Molar Mass of H₂SO₄= 98.09 g/mol

Figure 5. Calculating the molar mass of sulfuric acid, H₂SO₄_.

Example 1e - Calculate the Molar Mass of iron (III) nitrate, Fe(NO₃)₃.

Using your periodic table, find and record the mass of each element in the compound: iron (Fe), nitrogen (N), and oxygen (O).
1. - Iron: 55.85 g/mol
  - Nitrogen: 14.01 g/mol
  - Oxygen: 16.00 g/mol

2. Make yourself an equation that represents the quantity of each element in the compound:

- - Molar Mass of Fe(NO₃)₃ = 1Fe + 3N + 6O
  - In Fe(NO₃)₃ there is 1 iron atom, 3 nitrogen atoms and 6 oxygen atoms.
  - Remember, when there is a subscript on the outside of the brackets, everything inside of the brackets gets multiplied by that number.

Figure 6. Remember to expand the number outside of the brackets to the entities inside of the brackets.

3. Finally, plug in the masses of each element into your equation to solve for the Molar Mass of Fe(NO₃)_3.

- - Molar Mass of Fe(NO₃)₃= 1Fe + 3N + 9O
  - Molar Mass of Fe(NO₃)₃= 1 (55.85 g/mol) + 3 (14.01 g/mol) + 9 (16.00 g/mol)
  - Molar Mass of Fe(NO₃)₃= 241.88 g/mol

The following problems are not written on your handout for this lesson. Please complete these problems on the back of the handout or on lined paper. It is important to practice these calculations.

Practice - Problem Set #1

Try the following calculations. Then, check your solutions.

1. What is the molar mass of sodium (Na) in g/mol?
2. What is the molar mass of diatomic oxygen (O₂) in g/mol?
3. What is the molar mass of water (H₂O) in g/mol?
4. What is the molar mass of calcium phosphate (Ca₃(PO₄)₂) in g/mol?

Show the Solution

Practice - Problem Set #2

Try the following calculations. Then, check your solutions.

1. Determine the molar mass of ammonia, NH₃in g/mol.
2. Determine the molar mass of glucose, C₆H₁₂O₆ in g/mol.
3. Determine the molar mass of potassium dichromate, K₂Cr₂O₇ in g/mol.
4. Determine the molar mass of iron (III) sulphate, Fe₂(SO₄)₃ in g/mol.

Show the Solution

The Molar Mass Formula

Think back to Topic 2 when we related moles (n), number of particles (N) and Avogadro's Number (N_A) using a mole triangle.

Now, we can relate moles (n), mass (m), and the molar mass (M) using a similar triangle.

Watch the following video to see how we derive the formulas for mass, number of moles, and Molar Mass, using the triangle above. I will provide you with this triangle on assessments. However, it is good practice to copy this triangle down as often as you can so that you can familiarize yourself on how to use it correctly.

Example 2 - Calculate the number of moles in 29.0 g of sodium chloride, NaCl.

Example 3 - Calculate the mass of 0.0143 mol of potassium hydroxide, KOH.

Example 4 - To treat lake water to make it safe to drink, a tablet of sodium chlorite can be added. 100 tablets contains 2.56 g of sodium chlorite. How many moles of sodium chlorite are in 100 tablets?

The Mole Hill

We've now seen two different triangles that can help us perform calculations involving moles. What if I told you BOTH triangles were related to one another? We can convert between all values in our triangle using something called, "The Mole Hill". This diagram will be provided on assessments. With that being said, you need to familiarize yourself with how to use it.

Example 5 - How many molecules are in 300.0 g of Iron (III) chloride?

Example 6 - A fertilizer contains 3.00x10²⁵ molecules of KCl. What mass of KCl does the fertilizer contain?

Example 7 - How many molecules of glucose, C₆H₁₂O₆, are in 375 g of glucose?

Homework - Practice Questions

Using your textbook, complete the following practice questions: Page 185 #3-10

Once you have completed the homework questions, you can check your answers below.

This lesson is now complete. Return to D2L - Brightspace and complete the assigned tasks to consolidate your learning.

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