Now we explore the next step in our conversion map. This step will help us explain the definition of a mole. A mole is something that is hard to understand at first. Quite simply, a mole is just a number of objects in this case atoms or molecules. Therefore, a mole is an amount of something. The best analogy to a mole is a dozen. A dozen is an amount of something, specifically 12 of them. The only difference between a dozen and a mole is that the number or amount of things is different. A mole is 6.022 * 10^{23} things, whereas a dozen is 12 things. So this is where we get the conversion between 1 mole and 6.022 * 10^{23} molecules.
6.022 * 10^{23} molecules |
1 mol |
or
1 mol |
6.022 * 10^{23} molecules |
This ratio is seen on the right arrow (the green text) in the conversion map below.
On a side note, I tend to abbreviate the units of molecules as ( molec ) for reasons of saving space and to help distinguish it from moles which is something different.
VIDEO Converting Between Grams and Molecules Demonstrated Example 1: If you have 5.4g of Cr then how many molecules of Cr would you have? You will need the periodic table for this question.
Step 1:
What information are we given?
Answer: 5.4 g Cr
Step 2:
What units does the question ask for?
Answer: ? molecules Cr
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 2 arrows when we go from Grams —> Moles —> molecules. 2 arrows = 2 conversions
Step 4:
How do we set up the problem?
Answer: First box is info given, next two boxes are the conversions, last box (fourth box) is what the question asked for
5.4 g Cr | Molecules Cr | ||
1 |
Step 5:
What is the first conversion?
Answer: molar mass (grams to mole ratio) of Chromium found on the periodic table
Step 6:
What is the molar mass of Cr?
Answer: about 52 g/ 1 mol
Step 7:
How do we set that up in the conversion?
Answer: units first, set up the units that need to cancel out (in red)
5.4 g Cr | mol | molec Cr | |
g |
Step 8:
What comes next?
Answer: fill in the numbers and cross out units
5.4 g Cr | 1 mol | molec Cr | |
52 g |
Step 9:
Simplify by removing all crossed out units.
5.4 Cr | 1 mol | molec Cr | |
52 |
Step 10:
What is the next conversion?
Answer: Avogadro’s number (moles to molecules ratio) found on the conversion map. 6.022 * 10^{23} molecules / 1 mol
Step 11:
How do we set that up in the conversion?
Answer: units first, set up the units that need to cancel out (in red)
5.4 Cr | 1 mol | molec = | molec Cr |
52 | mol |
Step 12:
What comes next?
Answer: fill in the numbers and cross out units
5.4 Cr | 1 mol | 6.022 * 10^{23} molec = | molec Cr |
52 | 1 mol |
Step 13:
Simplify by removing all crossed out units.
5.4 Cr | 1 | 6.022 * 10^{23} molec = | molec Cr |
52 | 1 |
Step 14:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case molec and Cr.
5.4 Cr | 1 | 6.022 * 10^{23} molec = | molec Cr |
52 | 1 |
Step 15:
How do I do the calculations?
Answer: (5.4 * 6.022 * 10^{23}) / (52) = 6.25 * 10^{22}
5.4 Cr | 1 | 6.022 * 10^{23} molec = | 6.25 * 10^{22} molec Cr |
52 | 1 |
Step 16:
COMPLETE ANSWER: 6.3 10^{23} molecules of Cr
VIDEO Converting Between Grams and Molecules Demonstrated Example 2: When you have 7.4 * 10^{25} molecules of O_{2} how many grams is that? You will need the periodic table for this question.
Step 1:
What information are we given?
Answer: 7.4 * 10^{25} molecules O_{2}
Step 2:
What units does the question ask for?
Answer: ? g O_{2}
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 2 arrows when we go from Molecules —> Moles —> Grams. 2 arrows = 2 conversions
Step 4:
How do we set up the problem?
Answer: First box is info given, next two boxes are the conversions, last box (fourth box) is what the question asked for
7.4 * 10^{25} molec O_{2} | g O_{2} | ||
1 |
Step 5:
What is the first conversion?
Answer: Avogadro’s number (moles to molecules ratio) found on the conversion map. 6.022 * 10^{23} molecules / 1 mol
Step 6:
How do we set that up in the conversion?
Answer: units first, set up the units that need to cancel out (in red)
7.4 * 10^{25} molec O_{2} | mol | g O_{2} | |
molec |
Step 7:
What comes next?
Answer: fill in the numbers and cross out units
7.4 * 10^{25} molec O_{2} | 1 mol | g O_{2} | |
6.022 * 10^{23} molec |
Step 8:
Simplify by removing all crossed out units.
7.4 * 10^{25} O_{2} | 1 mol | g O_{2} | |
6.022 * 10^{23} |
Step 9:
What is the next conversion?
Answer: molar mass (grams to mole ratio) of O_{2} found on the periodic table
Step 10:
What is the molar mass of O_{2}?
Answer: about 32 g/ 1 mol
Step 11:
How do we set that up in the conversion?
Answer: units first, set up the units that need to cancel out (in red)
7.4 * 10^{25} O_{2} | 1 mol | g = | g O_{2} |
6.022 * 10^{23} | mol |
Step 12:
What comes next?
Answer: fill in the numbers and cross out units
7.4 * 10^{25} O_{2} | 1 mol | 32 g = | g O_{2} |
6.022 * 10^{23} | 1 mol |
Step 13:
Simplify by removing all crossed out units.
7.4 * 10^{25} O_{2} | 1 | 32 g = | g O_{2} |
6.022 * 10^{23} | 1 |
Step 14:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case g and O_{2}.
7.4 * 10^{25} O_{2} | 1 | 32 g = | g O_{2} |
6.022 * 10^{23} | 1 |
Step 15:
How do I do the calculations?
Answer: (7.4 * 10^{25} * 32) / (6.022 * 10^{23}) = 3932
7.4 * 10^{25} O_{2} | 1 | 32 g = | 3932 g O_{2} |
6.022 * 10^{23} | 1 |
Step 16:
COMPLETE ANSWER: 3900 g of O_{2}
PRACTICE PROBLEMS: Solve the conversions between grams and molecules. Make sure you have this periodic table link open when answering these questions. You may also need the conversion map. Answers are rounded to the nearest significant figure.
How many molecules are in 16g of CO_{2}?
Answer: 2.2 * 10^{23} molec CO_{2}
If you have 3.0 * 10^{24} molecules of K_{2}S how many grams is that?
Answer: 55 g K_{2}S
How many molecules are in 70g of Sn?
Answer: 3.5 * 10^{23} molecules Sn
If you have 9.2 * 10^{20} molecules of Cl how many grams is that?
Answer: 0.053 g Cl