Imagine an atom. Well, most likely you can't even begin to grasp how small an atom even is⚛️. How do scientists then perform laboratory work when it is nearly impossible to count the atoms they are working with🤔? This is where the concept of a mole emerged from. A mole relates the mass of an element to the number of particles there are. Let's begin with the molar mass.

**🎥Watch: AP Chemistry - **__Moles and Molar Mass__

The **molar mass** of a substance is the amount of grams there are in a mole (the units for molar mass are grams/mole). Finding the molar mass of an element or compound is not as hard as it might seem: the only things that you need to know are which elements are involved and how many of them are present. Also, there's one huge important thing that you'll need: a **periodic table!**

👉 __Cool Interactive Periodic Table__

Reference table you will have with you during the AP

For example, let’s say that we have the compound CO2. To calculate the molar mass, you simply have to multiply the atomic mass of each specific element by it’s subscript, and then add it all together. Simple, right?

So we're working with CO2. Carbon has a subscript of 1 and has an atomic mass of 12.01 grams. Oxygen has a subscript of 2 in this element and has an atomic mass of 15.99 grams. Always multiply the subscript by the atomic mass, and when it's done where, it equals 31.98 grams. Finally, we add 31.98 grams to 12.01 grams to get 43.99 grams. Therefore, Carbon Dioxide has a molar mass of 43.99 grams per mole.

One of the most *fundamental *takeaways from this unit is **dimensional analysis**! What this means is that you can change the units that a number is represented in. For instance, you can convert from miles per hour to meters per second.
We can apply the concept of moles and perform dimensional analysis with Avogadro's number. **Avogadro’s principle** tells us that 1 mole is equal to 6.022x10^23 atoms of a pure substance. This will be important to know when converting from grams to moles to atoms.

Now that we have learned the basics, let’s try converting a sample of 50.0 grams of CO2 between units.

When doing dimensional analysis, you want to put the number that you know first, which in this case, is 50.0 grams of CO2. Then, you want to multiply 50.0 by the molar mass in order to convert to the moles of CO2. The unit that you have (grams CO2) should always be on the bottom of the next ratio in order for the units to cancel out. Here, the grams of CO2 cancel out and you are left with a measurement in moles.

💡**Tip: **It is good to memorize that moles = grams/molar mass. The conversion step in this problem is actually using this concept since you are ultimately dividing the number of grams you have by the molar mass to get the number of moles.

Now let's convert 1.13 moles of CO2 into atoms using Avogadro's number.

Here, you are once again taking the number that you have and putting it first. Then, you are putting the unit of measurement that you want over the unit of measurement that you have, making that step the unit conversion. This enables the moles of CO2 to cancel out, leaving you with just 6.80x10^23 atoms of CO2.

Since the subscript on Carbon is 1, the number of atoms of CO2 is equivalent to the number of carbon atoms in CO2. There is nothing to multiply by because of this 1 to 1 ratio, therefore the number of carbon atoms in this 50.0g sample of CO2 is 6.80x10^23.

Unlike carbon, oxygen has a subscript of 2. This makes the ratio of CO2 atoms to oxygen atoms 1:2. Therefore, we have to use dimensional analysis:

Since there are two atoms of O in one atom of CO2, we had to multiply by 2 to get the number of atoms of O.

You got this! Once you practice multiple problems involving dimensional analysis, it'll seem like a piece of cake. Sadly, these problems become more difficult as the course progresses but as always, practice makes perfect.

Download our ap chem survival pack and get access to every resource you need to get a 5.

ap chem

✍️ Free Response Questions

AP Chemistry Free Response Questions

- Overview of AP Chemistry FRQs
- Exam Format
- How to Ace the FRQ - Step by Step
- AP Chemistry FRQ Tips
- Concise and Clear is Key
- Stay Consistent
- Cash In On Credit
- Words and Symbols
- Know Your Chemistry
- Example FRQ - Short Answer (4 points)
- Break Down
- Scoring Guidelines
- Example FRQ - Long Answer (10 points)
- Break Down
- Scoring Guidelines

⚛️ Unit 1: Atomic Structure and Properties

🤓 Unit 2: Molecular and Ionic Compound Structures and Properties

🌀 Unit 3: Intermolecular Forces and Properties

3.6Deviations from the Ideal Gas Law

- When Do Gases Deviate From The Ideal Gas Law?
- Gas particles can become attracted to each other➡️⬅️
- Gas particles can make up a significant portion of a gas samples’ volume
- Graphically
- Correcting the Ideal Gas Law using the Van der Waals Equation
- Practice Question
- Diffusion and Effusion
- Diffusion
- Effusion
- Graham's Law of Effusion

🧪 Unit 4: Chemical Reactions

👟 Unit 5: Kinetics

🔥 Unit 6: Thermodynamics

⚖️ Unit 7: Equilibrium

🍊 Unit 8: Acids and Bases

Unit 9: Applications of Thermodynamics

🤺 AP Chemistry Essentials

🧐 Multiple Choice Questions

✏️ Blogs

continue learning

Fiveable Community students are already meeting new friends, starting study groups, and sharing tons of opportunities for other high schoolers. Soon the Fiveable Community will be on a totally new platform where you can share, save, and organize your learning links and lead study groups among other students!🎉

*ap® and advanced placement® are registered trademarks of the college board, which was not involved in the production of, and does not endorse, this product.

© fiveable 2021 | all rights reserved.

2550 north lake drive

suite 2

milwaukee, wi 53211