The Mighty Mole
By Lydia from SLN More Blogs by This AuthorFrom the Science Bits in a World of Bytes Blog Series
If you hang around with people interested in science, you have probably heard some of the following terms: mole, molar mass, molarity, or molality. They are not talking about animals tearing up their yard or how much their teeth weigh. They are rather talking about a certain number and its applications in chemistry. The number is 6.022 x 1023, or written out in longhand, 602,200,000,000,000,000,000,000. It’s huge but has good reason for that: it is used in scientific calculations to keep track of atoms and molecules, which are nearly unfathomably small.
The value of this number has been determined and refined by experiments over the years. The particular value for the mole has been defined as the number of atoms in 12 grams of carbon-12, the most common form of that element. The advantage of that, as said by my general chemistry professor, is that you can count atoms by weighing. The concept of the mole leads into the concept of molar mass. The molar mass is how much 6.022 x 1023 atoms or molecules of a substance weigh. This is often listed on the periodic table under an element’s symbol. For example, aluminum has a molar mass of 26.98 grams, or about equal to two 12-ounce pop cans. There are 6.022 x 1023 aluminum atoms in those two cans.
I want to impress on you how big the mole is. As part of my high school chemistry class, I had to do some calculations with a mole of something. I chose chocolate chips. A chocolate chip is about one-quarter of an inch across. A mole of chocolate chips placed in a line would be 2 376 104 798 000 000 000 miles long. This is 404,470 light years long, or four times the diameter of the Milky Way galaxy. Each person on Earth could have 86 028 571 428 571 chocolate chips. Instead of falling into a vat of chocolate, we’d all be crushed.
Compounds also have molar masses, also known as formula weight or molecular weight. These are simply the molar masses of the constituent atoms summed. A simple example is water, H2O. Look up the molar masses of hydrogen and oxygen. They are 1.0079 grams/mole and 15.9994 grams/mole, respectively. Because there are two hydrogen atoms, remember to multiply the molar mass of hydrogen by two. The molar mass of water is
molar mass=2×1.0079 (grams/mole)+ 15.9994 (grams/mole)
molar mass=18.0152 (grams/mole)
This value is commonly rounded to 18.02 grams per mole. If you weigh out 18.02 grams of water, there will be 6.022 x 1023 water molecules in there. It sounds like a lot, but it’s only a little more than a tablespoon.
The mole is used to express concentration. The most common way is to use molarity (M), which is moles of stuff dissolved (solute) in a liter of solvent. Sodium chloride (NaCl) is a large part of what makes seawater salty. Its molar mass is 22.99 g/mol + 35.45 g/mol = 58.44 g/mol. In 100 milliliters (mL) of typical seawater there are 2.7 grams NaCl. What is the molarity of typical seawater? First, we need to find how many moles of NaCl we have. The formula used is moles=mass/(molar mass) ; plug in the numbers.
moles=(2.7 grams)/(58.44 (grams/mole))
We get 0.046 moles of NaCl. One hundred mL of water is one-tenth of a liter (milli- is a prefix meaning “one-thousandth”), so we can figure out the molarity of seawater as follows:
molarity=(moles solute)/(liters solvent)=(0.046 moles)/(0.100 liters)=0.46 M
The other main expression of concentration is molality. This is similar to molarity but is not the same. Molality is moles of solute per kilogram of solvent. Why would this be needed? The density of the solvent decreases as its temperature increases, so molarity can vary. With molality, you can use a scale to measure both solute and solvent and not be bothered by the effects of ambient temperature.
Moles are also used to predict how much product a reaction will form. Take a look at this reaction:
C3H8+ 5O2→ 3CO2+ 4H2O
If you use a gas grill, this is what happens when your fuel is burned. Notice the numbers in front of the oxygen, carbon dioxide, and water. These coefficients are there to balance the equation because matter must be conserved in the overall reaction. They have another purpose; they show the relative amounts of products made and reactants required. Complete burning of one mole of propane requires five moles of oxygen. When it’s completely combusted, the mole of propane has been turned into three moles of carbon dioxide and four moles of water.
This would happen only for an unlimited supply of oxygen; if there are fewer than 5 moles of oxygen for each mole of propane, then the amount of oxygen controls how much CO2 and H2O are made. In that case, oxygen is called the limiting reagent. The concept is analogous to a situation in cooking. You have a recipe calling for 2 cups of flour. You want to make two batches, but you have only three cups of flour. Although you might have enough of all the other ingredients, you are limited to one batch because you have enough flour to make only one. If you were given two arbitrary values for the amounts of propane and oxygen, you can use the coefficients to find out which one is the limiting reagent. Divide the amount you have by the coefficient; the reactant with the smaller result is the limiting reagent.
I learned about the mole and its applications starting in high school. I used it a lot in general chemistry, analytical chemistry, and the first semester of physical chemistry. It is a staple of chemistry courses. The equations are easy to remember but powerful. The ability to predict to how much product will be formed has a large practical application. Companies know how large to make their reaction and storage vessels to ensure safety. Moles in the yard annoy. Moles in the lab work.
White. "Review of Chemical Concentrations." . University of Utah, 16 Jan 2010. Web. 3 Apr 2013. <http://www.chem.utah.edu/faculty/white/chem3000/Ch3000LN513.pdf>.