What is molar mass?
Molar mass is the weight of one mole (or 6.02 x 1023 molecules) of any chemical compounds. Molar masses of common chemical compounds that you might find in the chemistry laboratory can range between 18 grams/mole for compounds like water to hundreds of grams per mole for more complex chemical compounds.
The lightest possible chemical that one can have under normal conditions is hydrogen gas, or H2. There is no limit to how heavy a chemical compound can be - it is not uncommon for macromolecules (large organic or bioorganic compounds such as DNA) to weigh thousands of grams per mole.
How can I find the molar mass of an element?
The molar mass of elements is found by looking at the atomic mass of the element on the periodic table. For example, if you want to find the molar mass of carbon, you would find the atomic mass of carbon on the periodic table, and this is equal to the molar mass in grams per mole. So, in our example, carbon has a molar mass of 12.01 grams per mole.
There are a few exceptions to this rule. In some cases, the element is usually found in a different form than just one unbonded atom. In the case of hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, the element is diatomic, meaning that each molecule of the element has two atoms of that element stuck together. As a result, the formula of hydrogen is H2, nitrogen is N2, etc.
This gets weirder for a couple of cases... phosphorus is normally found in clumps of four atoms, P4, and sulfur is found in clumps of eight atoms, or S8.
Still, aside from the exceptions above, all elements have the same molar mass as the atomic masses on the periodic table.
For any chemical compound that's not an element, we need to find the molar mass from the chemical formula. To do this, we need to remember a few rules:
1. Molar masses of chemical compounds are equal to the sums of the molar masses of all the atoms in one molecule of that compound. If we have a chemical compound like NaCl, the molar mass will be equal to the molar mass of one atom of sodium plus the molar mass of one atom of chlorine. If we write this as a calculation, it looks like this:
(1 atom x 23 grams/mole Na) + (1 atom x 35.5 grams/mole Cl) = 58.5 grams/mole NaCl
2. If you have a subscript in a chemical formula, then you multiply the number of atoms of anything next to that subscript by the number of the subscript. For most compounds, this is easy. For example, in iron (II) chloride, or FeCl2, you have one atom of iron and two atoms of chlorine. The molar mass will be equal to (1 atom x 56 grams/mole Fe) + (2 atoms x 35.5 grams/mole of chlorine) = 127 grams/mole of iron (II) chloride.
For other compounds, this might get a little bit more complicated. For example, take the example of zinc nitrate, or Zn(NO3)2. In this compound, we have one atom of zinc, two atoms of nitrogen (one atom inside the brackets multiplied by the subscript two) and six atoms of oxygen (three atoms in the brackets multiplied by the subscript two). The molar mass of zinc nitrate will be equal to (1 atom x 65 grams/mole of zinc) + (two atoms x 14 grams/mole of nitrogen) + (six atoms x 16 grams/mole of oxygen) = 189 grams/mole of zinc nitrate.
For all other compounds, the general idea is the same. Basically, you should know how to find the molar masses of any chemical compound now. In the next and final section, I'll give you some practice problems, followed by a solution key...
Some sample problems:
These are the kind of molar mass calculation probems I might ask you on a quiz. The solutions are given at the end.
Give the molar masses of the following compounds:
1. sodium fluoride
2. potassium hydroxide
3. copper (I) chloride
4. manganese (IV) oxide
5. calcium sulfate
6. magnesium phosphate
1. In sodium fluoride, there is one atom
of sodium and one atom of fluorine. The molar mass will then be:
2. In potassium hydroxide, there is one atom of potassium, one atom of hydrogen, and one atom of oxygen. The molar mass will then be (1 x 39 grams) + (1 x 1 gram) + (1 x 16 grams) = 56 grams/mole of potassium hydroxide
3. In copper (I) chloride, there is one atom of copper and one atom of chlorine. The molar mass is then (1 x 63.5 grams) + (1 x 35.5 grams) = 99 grams/mole of copper (I) chloride
4. In manganese (IV) oxide, there is one atom of manganese and two atoms of oxygen. The molar mass is then (1 x 55 grams) + (2 x 16 grams) = 87 grams/mole of manganese (IV) oxide
5. In calcium sulfate, there is one atom of calcium, one atom of sulfur, and four atoms of oxygen. The molar mass is then (1 x 40 grams) + (1 x 32 grams) + (4 x 16 grams) = 136 grams/mole of calcium sulfate
6. In magnesium phosphate, there are three atoms of magnesium, two atoms of phosphorus, and eight atoms of oxygen. (The formula is Mg3(PO4)2). The molar mass is then (3 x 24 grams) + (2 x 31 grams) + (8 x 16 grams) = 262 grams/mole of magnesium phosphate
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