## Chem – How to Convert Between Grams and Molecules

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 * 1023 things, whereas a dozen is 12 things. So this is where we get the conversion between 1 mole and 6.022 * 1023 molecules.

 6.022 * 1023 molecules 1 mol

or

 1 mol 6.022 * 1023 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?

Step 2:

What units does the question ask for?

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?

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 * 1023 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 * 1023 molec = molec Cr 52 1 mol

Step 13:

Simplify by removing all crossed out units.

 5.4 Cr 1 6.022 * 1023 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 * 1023 molec = molec Cr 52 1

Step 15:

How do I do the calculations?

Answer: (5.4 * 6.022 * 1023) / (52) = 6.25 * 1022

 5.4 Cr 1 6.022 * 1023 molec = 6.25 * 1022 molec Cr 52 1

Step 16:

COMPLETE ANSWER: 6.3 1023 molecules of Cr

VIDEO Converting Between Grams and Molecules Demonstrated Example 2: When you have 7.4 * 1025 molecules of O2 how many grams is that? You will need the periodic table for this question.

Step 1:

What information are we given?

Answer: 7.4 * 1025 molecules O2

Step 2:

What units does the question ask for?

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 * 1025 molec O2 g O2 1

Step 5:

What is the first conversion?

Answer: Avogadro’s number (moles to molecules ratio) found on the conversion map. 6.022 * 1023 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 * 1025 molec O2 mol g O2 molec

Step 7:

What comes next?

Answer: fill in the numbers and cross out units

 7.4 * 1025 molec O2 1 mol g O2 6.022 * 1023 molec

Step 8:

Simplify by removing all crossed out units.

 7.4 * 1025 O2 1 mol g O2 6.022 * 1023

Step 9:

What is the next conversion?

Answer: molar mass (grams to mole ratio) of O2 found on the periodic table

Step 10:

What is the molar mass of O2?

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 * 1025 O2 1 mol g = g O2 6.022 * 1023 mol

Step 12:

What comes next?

Answer: fill in the numbers and cross out units

 7.4 * 1025 O2 1 mol 32 g = g O2 6.022 * 1023 1 mol

Step 13:

Simplify by removing all crossed out units.

 7.4 * 1025 O2 1 32 g = g O2 6.022 * 1023 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 O2.

 7.4 * 1025 O2 1 32 g = g O2 6.022 * 1023 1

Step 15:

How do I do the calculations?

Answer: (7.4 * 1025 * 32) / (6.022 * 1023) = 3932

 7.4 * 1025 O2 1 32 g = 3932 g O2 6.022 * 1023 1

Step 16:

COMPLETE ANSWER: 3900 g of O2

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 CO2?

Answer: 2.2 * 1023 molec CO2

If you have 3.0 * 1024 molecules of K2S how many grams is that?

How many molecules are in 70g of Sn?

Answer: 3.5 * 1023 molecules Sn

If you have 9.2 * 1020 molecules of Cl how many grams is that?  