The most common way to measure chemical substances is through moles . . .
Well no, not that kind of mole . . . a chemical mole is a unit of measure, just like a gram or an ounce. It is used internationally so that all chemists speak the same measurement language. The mole was invented because, well, it made sense. Scientists were having a hard time converting between atoms of an element and grams of an element (grams were the previous standard of measure), so scientists came up with a "mole" of substance, which is defined as anything that has 6.02x1023 particles in it.
There we go, the mole is showing us the value of a mole!
This is based off of a mole of carbon, which has 6.02x1023 carbon atoms in it. Carbon is often used to define chemical principles and constants, because it is one of the oldest and most easily observed elements, and it occurs naturally. You might recognize 6.02x1023 as Avogadro’s number; this number is used as a constant throughout chemistry, and here we’re going to use it to define the mole. Usually, moles refer to particles that make up a certain amount of an element, and we use moles to measure how much of a substance is reacting in a chemical equation. However, you can also measure other things in moles—for example, a mole of hippos would be 6.02x1023 hippos . . . which is actually quite a lot. When you think about a mole as 602,000,000,000,000,000,000,000 hippos, it seems like way too big of a number to be describing something that fits in a beaker in the chem lab! However, because atoms are so small (remember, we can’t even see an atom with just the human eye) there are bunches of atoms in everything we’re measuring. Therefore, a mole is actually a very appropriate way to measure chemical substances.
Another benefit of using moles to measure substances is that it directly correlates to the number of atoms and molecules and grams. A mole tells you what fractional part of Avogadro’s number you’re working with; for example, if you have .25 mol, you would have .25 (or 25%) of Avogadro’s number, which is 1.505x1023 (.25 * 6.02x1023=1.505x1023).
Now we’ll show you how to do common conversions from grams to moles to atoms, and back again.
Converting from grams to moles is really quite easy. First, locate a periodic table. (You can find one in your textbook or review book, or there are many versions available on the internet. Then, read over the specific problem you want to complete, and decide if you need to convert from grams to moles, or from moles to grams. We will show you one example of each, and then supply practice problems for you to do. After you’ve identified the problem and the conversion, find the element you are working with on the periodic table. Then, locate the atomic weight (this is the decimal number listed under the element’s symbol). If you are converting an amount in grams to mol, you will be dividing by the atomic weight. If you are converting an amount in mol to grams, you will be multiplying by the atomic weight. However, if you use dimensional analysis (shown in the problems below) you will not have to worry about figuring out the operations before you do the problem. Dimensional analysis is similar to setting up a table, and it will eventually become your best friend for long, chemical conversions.
Convert 15.5 grams C to mol.
First, locate your periodic table. Then, locate C (carbon) on the periodic table. It is in row 2, column IVA. Be sure to locate the atomic weight of this element, which will most likely be listed under the symbol for the element, but could also be near the upper right hand corner of the element’s box on the periodic table. Please note that the atomic weight is also referred to as relative atomic mass. For carbon, it should read 12.01, or 12.011 depending on how many significant figures your periodic tables uses. We are going to use 12.01 in this calculation.
Now that we’ve found all of our information, we’re going to set up dimensional analysis. You’ll start with a table, like this:
And then you can begin filling in the information. You’ll always start in the upper left hand corner with the information given to you in the problem. In this case, we were given 15.5 g C to convert, so we’re going to put in 15.5 g C in the upper left hand corner. Notice that we can abbreviate grams as g, but also note that we always include the symbol for the element we’re working with. Although it may not seem important now, it will be in future conversions. Here’s what the first step would look like:
Now, we have to fill in the conversion part, which is the two boxes on the right hand side. We want the items with the same units (grams, moles, etc) to be diagonal to each other, because we want similar units to cancel. In other words, if g C are across from each other, we can cross out g C and use the units we're converting to. Technically, we are performing division, which is what cancels the units. We’re going to use 12.01 g/mol as our conversion factor. Any of the atomic weights you get from the periodic table will have the units of g/mol, which stands for grams/mole. Now, one thing that we should explain before we get started is that by giving 12.01 g/mol that unit label, we are technically saying that there are 12.01 grams for every 1 mole. This is important to remember as we fill out our dimensional analysis box. Now, remembering that we are going to place amounts with the same units diagonally from each other, we’re going to put 12.01 g C in the bottom right hand corner of our dimensional analysis, like this:
Now, we have to put in the "1 mol." We're going to put it above the 12.01 g C, because we know that those two are equal. 12.01 g C = 1 mol C. That’s why we can write them as a grams per mole fraction, because they express the same value. Here’s what our dimensional analysis looks like now:
Now, notice that the units we don’t want, grams, are across from each other. We’re going to circle them in red so that you can see them. We have the units we do want, mol, in the upper right hand corner. No matter how many columns of conversion factors you may have, the units you want to end up with should always be furthest to the right, in the last column. The units you want can be on the top or the bottom, but they should never cancel with other units. Notice that in this problem, we have grams canceling (because that's not what we want) whereas moles do not cancel, because we want our answer in moles. We’re going to circle moles in green, so that you can see it more easily.
At last, we’re ready to do our calculation. In our dimensional analysis box, we are going to multiply by any numbers on the "top" row, and divide by any numbers on the "bottom" row. It would look like this:
Now, the calculation would go as follows:
15.5 g C * 1 mol C / 12.01 g C = 1.29 mol C
Notice that we multiplied the top numbers, and divided by the bottom number in order to get our final answer of 1.29 mol C. Notice also that we rounded the answer to three significant figures (sig figs) because the least accurate number contains 3 sig figs and the answer needs to follow the sig fig rules.
Convert .798 mol O into grams of O.
This example should go much more quickly than the first one, now that we know how to set up the dimensional analysis. First, find O (oxygen) on the periodic table. Its atomic weight is 16.00 g/mol. Now that we know the atomic weight, we can set up dimensional analysis. This time, we are given a measurement in moles, and want to convert it to grams, so we place the mol number given to us in the top left corner of the dimensional analysis box. Then, we have to set up our conversion factor. Our conversion factor this time is 16.00 g/mol O, which means 16.00 g O = 1 mol O. Since we want the same units placed diagonally from each other, so they can cancel, we’re going to place 1 mol O in the bottom right hand corner, which means 16.00 g O is placed in the upper right hand corner. This is because we want to find the amount of grams in the specified amount. Our dimensional analysis looks like this:
So, we multiply across the top, and divide by the bottom number. In this case, the bottom number is 1, so we do not need to divide (because anything divided by 1 is that same number). After performing the multiplication, we get 12.8 g O. (Notice that we rounded to three sig figs, because our least accurate number had 3 digits.) Therefore, our final answer is 12.8 g O.
Now, here are some for you to try! We’ll give you two to start with.
Practice Problem 1
The dimensional analysis looks like this:
To get the answer, we multiplied across the top and divided by the bottom number. After our multiplication and division, we rounded the number to two digits because the least accurate number we were given had two sig figs as well.
Final answer: 50. g N
Practice Problem 2
The dimensional analysis looks like this:
To get the answer, we multiplied across the top and divided by the bottom number. After the multiplication and division, we rounded the number to three digits because the least accurate number we were given had two sig figs as well.
Final answer: 3.06 mol F