Some of you may have noticed that the molar mass (or atomic mass) can be written in terms of a ratio. This means we can use it as a conversion. If your molar mass of CH_{4} is 16 g/mol that means we can write it as a ratio like the ones below.
16 g |
1 mol |
or
1 mol |
16 g |
Either way, both are true statements of the relationship between grams and moles of CH_{4}. That means you can write the molar mass for a conversion whichever way you need to arrange the units correctly. For all conversions going forward we are going to start using conversion maps to help guide us. Right now some people may feel they are a waste of time. However, as chemistry gets more complex so do the conversion maps. Our first conversion map is how to go between grams and moles. It is the map below.
On a side note, I tend to abbreviate the units of moles as ( mol ) for reasons of saving space and to help distinguish it from molecule which is something different.
VIDEO Converting Between Grams and Moles Demonstrated Example 1: If we have 12 g of He how many moles of He is that? You will need the periodic table for this question.
Step 1:
What information are we given?
Answer: 12g He
Step 2:
What units does the question ask for?
Answer: ? mol He
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 1 arrow when we go from Grams —> Moles. 1 arrows = 1 conversion
Step 4:
How do we set up the problem?
Answer: First box is info given, second box is the 1 conversion, last box (third box) is what the question asked for.
12g He | mol He | |
1 |
Step 5:
What is the first conversion?
Answer: molar mass (grams to mole ratio) of Helium found on the periodic table
Step 6:
What is the molar mass of He?
Answer: about 4 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)
12g He | mol | mol He |
g |
Step 8:
What comes next?
Answer: fill in the numbers and cross out units
12g He | 1 mol = | mol He |
4 g |
Step 9:
Simplify by removing all crossed out units.
12 He | 1 mol = | mol He |
4 |
Step 10:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case mol and He
12 He | 1 mol = | 3 mol He |
4 |
Step 11:
How do I do the calculations?
Answer: 12 * 1 / 4 = 3
12 He | 1 mol = | 3 mol He |
4 |
COMPLETE ANSWER: 3.0 mol He
VIDEO Converting Between Grams and Moles Demonstrated Example 2: How many grams of SiH_{4} is 3.7 moles of SiH_{4}? You will need the periodic table for this question.
Step 1:
What information are we given?
Answer: 3.7 mol SiH_{4}
Step 2:
What units does the question ask for?
Answer: ? g SiH_{4}
Step 3:
How many conversions must we do?
Answer: Look at the conversion map. We pass through 1 arrow when we go from Moles —> Grams. 1 arrows = 1 conversion
Step 4:
How do we set up the problem?
Answer: First box is info given, second box is the 1 conversion, last box (third box) is what the question asked for
3.7 mol SiH_{4} | g SiH_{4} | |
1 |
Step 5:
What is the first conversion?
Answer: molar mass (grams to mole ratio) of Aluminum found on the periodic table
Step 6:
What is the molar mass of SiH_{4}?
Answer: about 32 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)
3.7 mol SiH_{4} | g = | g SiH_{4} |
mol |
Step 8:
What comes next?
Answer: fill in the numbers and cross out units
3.7 mol SiH_{4} | 32 g = | g SiH_{4} |
1 mol |
Step 9:
Simplify by removing all crossed out units.
3.7 SiH_{4} | 32 g = | g SiH_{4} |
1 |
Step 10:
How do I know I am done with conversions?
Answer: The only units left are the units that match the answer. In this case mol and SiH_{4}
3.7 SiH_{4} | 32 g = | g SiH_{4} |
1 |
Step 11:
How do I do the calculations?
Answer: 3.7 * 32 = 118.4
3.7 SiH_{4} | 32 g = | 118 g SiH_{4} |
1 |
COMPLETE ANSWER: 120 g SiH_{4}
PRACTICE PROBLEMS: Solve the conversions between grams and moles. Make sure you have this periodic table link open when answering these questions and also the conversion map. Answers are rounded to correct number of significant figures.
If you have 4.5 moles of potassium how many grams is that?
Answer: 180 g K
If you have 0.32 grams of silicon how many moles is that?
Answer: 0.011 mol Si
Given 8.2 moles of Ar how many grams will you have?
Answer: 330 g Ar
Given 9.7 grams of Ca how many moles is that?
Answer: 0.24 mol Ca
If you have 4.6 grams of NH_{3} how many moles is that?
Answer: 0.27 mol NH_{3}
Given 1.2 mol of CaF_{2} how many grams would you have?
Answer: 94 g CaF_{2}