You will need the following resources to complete this assignment.

• Calculating Molar Mass

When biologists talk about moles, they’re usually referring to the tiny, grey rodents that dig underground to find tasty earthworms. When chemists talk about moles they’re usually referring to something entirely different. The term ‘mole’ represents a number in chemistry the same way the word ‘dozen’ represents 12 of something to a baker. In this case, one mole represents 6.02 x 10²³, a humongous number!

To help you and any bakers or hungry biologists reading this get a sense of just how many things are in one mole, we can use an analogy with something that is often found in dozens (Figure 1). One yeast-raised doughnut weighs approximately 40 grams. One dozen yeast-raised doughnuts weigh about 500 grams. One mole of yeast-raised doughnuts weighs about 301,000,000,000,000,000,000,000,000 grams—more than four times the mass of the moon. Imagine the whole moon made of doughnuts, yummy...

Figure 1: A dozen yeast-raised donuts

Obviously, the mole is not a term we need for most things in daily life. Instead of being used for things we encounter in daily life, the mole is used by scientists when talking about enormous numbers of particles like atoms or molecules. Atoms and molecules are very tiny things. A drop of water the size of the period at the end of this sentence would contain 10 trillion water molecules. Instead of talking about trillions and quadrillions of molecules (and more), it's much simpler to use the mole. However, the mole does more than represent a big number: it provides a key link for converting between the number (amount) of a substance, and its mass.

The International Committee for Weights and Measures defines one mole as the number of atoms in exactly 12 grams of carbon-12 (Figure 2). Experiments counting the number of carbon-12 atoms in a 12-gram sample have determined that this number is 6.02214076 x 10²³. Regardless of whether the substance is carbon-12 or doughnuts, one mole represents the same number of each of these things.

Figure 2: Carbon-12

Scientists have then defined the molar mass of a substance as the mass of 6.02214076 x 10²³ units of that substance. So, the molar mass of yeast-raised doughnuts is 301,000,000,000,000,000,000,000,000 grams. With doughnuts, this is not very useful. However, it is quite useful if we apply it to other substances, especially elements. By standardizing the number of atoms in a sample of an element, we also get a standardized mass for that element that can be used to compare different elements and compounds to one another. Carbon's molar mass is 12.01 grams, which represents the combined mass of 6.02 x 10²³ carbon atoms. However, other elements have different molar masses; for example, 6.02 x 10²³ sulfur atoms have a mass together of 32.06 grams, which is sulfur's molar mass.

Along with telling us the mass of one mole of an element, molar mass also acts as a conversion factor between the mass of a sample and the number moles in that sample. For example, 24 grams of carbon atoms would be equal to two moles since 24 grams divided by the mass of one mole (12.01) equals 2. Further, Avogadro’s number acts as the conversion factor for converting between the number of moles in a sample and the actual number of atoms or molecules in that sample. Extending our example, two moles of carbon atoms contains 2 times 6.02 x 10²³ atoms, which equals 12.04 x 10²³ atoms, which can be written as 1.204 x 10²⁴ atoms.

Look on a periodic table in order to answer the following questions.

Your instructor will provide you with a handout so you can practice calculating molar mass.