Calculating Moles: How to Find the Molarity of a Solution

calculating molesWhen you’re doing chemistry, the balances that are given are read in grams. Unfortunately, the standard unit of measurement is in moles, not grams. Calculating moles or the molarity of a solution involves measuring how many moles of a solute exist per liter of that solution. Fortunately, the process of solving molarity problems is pretty simple, and with a bit of practice, you will undoubtedly be able to do mole calculations with ease. The biggest pitfall you have to avoid is conversions. Conversions can easily make a problem much more difficult.

Calculating moles is something that you will do as a beginner in chemistry, and as you take more advanced classes mole calculations will become increasingly difficult. If you really want to learn how to calculate moles properly, then you should check out the Udemy course, Chemistry 101 – Part 1 Principles of Chemistry. There’s also the Chemistry 101 – Part 2 course, which you can check out if you’re ready to move on to more advanced studies in the subject.

The Importance of the Mole 

Although everything is calculated in grams, you may be wondering why everything has to be calculated using moles. In chemistry, it’s important to know how many atoms are in each element when you’re considering the different reactions and properties of those elements. Even when measuring the smallest amount in mass possible, you will still have several trillion atoms. In order to be more accurate with these numbers, chemists have the mole unit. 

What is the Mole? 

A mole is defined as the number of atoms in 12 grams of carbon-12. This number is also referred to as Avogadro’s number, and it has been measured to be around 6.022 x 1023. The important thing about moles is that it makes calculations much easier. Imagine trying to calculate a chemistry problem in the millions. Instead of saying the number of atoms in a solution is several trillion, you could say that it is a few moles. This doesn’t just make your calculations more precise, but it also helps in making them easier to perform. 

Solving a Mole Problem 

Solving a mole problem is pretty simple. For example, you can solve how many moles are in 20 grams of copper by following four simple rules. The first thing that you have to determine is how many steps you’re going to need to convert the unit you’re given into moles. Since the 20 copper is in grams, you only need to convert the units in one step. Another important thing to remember is that since you’ve been given the number in grams, you have to use the atomic mass of your element, which in this case is copper, as your converting factor.

The next step involves putting your problem in a chart. You’re trying to calculate the moles of copper in the problem. What you’re going to do is multiply 20 grams of copper by x moles of copper, divided by x grams of copper. You aren’t sure what x is in either situation, but that is what you’re going to find out. The problem will look something like what is shown below.

20 grams Cu x (X moles Cu)/x grams Cu

Now that you’ve set up the problem, you can take care of the penultimate step. To do this you just have to replace the x in “x grams Cu” with the atomic mass of copper. Remember that you can always find the atomic mass of an element on the periodic table. In this case, the atomic mass of copper is 63.5. For the mole you will just replace x with 1. In these problems, your goal is to find how many moles are in the solution, so you will always start off with 1 mole unless instructed to do otherwise. Now the problem looks like what you will see below.

20 grams Cu x (1 mole Cu)/63.5 grams Cu

The final step is the easiest. All you have to do is cancel out the units that are on the top left and bottom right of the problem. In this case these units are grams Cu, but remember to leave the numbers. What you will have left is:

20 x 1 mole Cu/63.5

Now you can just multiple and divide as if you’re doing simple math. You will end up with 20 moles Cu/63.5, which equals 0.31 moles. That’s all there is to the problem.

This means that 20 grams of copper equals out to be around 0.31 moles of copper. 

This is just a one-step problem. There are other calculations that you can do that would require you to break down the problem into atoms. These are called two-step equations. Taking the same problem you just did, you would have to add another section to the equation.

22 grams Cu x (1 mole Cu) / (63.5 grams Cu) x (6.022 x 1023 atoms of Cu)/1 mole Cu

This time the calculation is for atoms. Since 1 mole is equal to Avogadro’s number or 6.022 x 1023 atoms, you can easily calculate the number of atoms needed in the problem. Just like before, cross out the units that are the same on the top and the bottom. This means that grams Cu and mole Cu are both eliminated.

What you’re given left is 22 x 1/63.5 x 6.022 x 1023 

Simply multiply and divide across, and you should get the number that you want. Here’s a hint. You’ve already done 22/63.5 in the previous problem. So just multiply that by Avogadro’s number. Your answer should be something similar to 2.09 x 1023.

This is how you do basic mole problems. As you can see, they don’t require more than just a little knowledge of multiplication and division. What makes mole problems complicated is remembering the units and making sure that you have everything set up probably before you start the problem.

If you’re trying to find out what to study for Chemistry Regents Exam, then you can rest assured that mole problems will be there. If you’re having trouble understanding how to do these calculations or any other form of chemistry, then you should check out the Udemy course on Regents Chemistry.

Calculating the Mole of a Large Solution

Now that you understand how to calculate moles, you can explore calculating the mole of solutions that consist of more than one element. A good solution to practice is KMnO4. In this solution, the first thing you have to do is find the molar mass of the equation. As mentioned before, you do this by calculating the atomic mass of the problems, which in this case is 39.1 grams for K, 54.9 grams for Mn, and 16.0 grams for O.

It’s important to note that there is 4 oxygen or O’s in the problem, so you have to multiply O by 4. Add all of the numbers together, and you will get the molar mass for the entire solution to be 158.0 grams. Say that you have 24 grams in the solution.

You should be able to calculate the mole for the solution based on your previous knowledge. First set up the problem like you did for the previous ones.

24 grams KMnO4 x (1 mole KMnO4/ 158.0 grams KMnO4)

Now cancel out the like terms, and do the basic mathematics. You should end up with 0.15 moles for the solution. If you didn’t get this answer, go back through your work.

Taking Your Chemistry Lesson Further 

Now that you’ve learned about moles, you can go deeper into your study of chemistry. Try the Udemy course Chemistry 101 – Part 3, which teaches you all about advanced concepts in organic and physical chemistry.