Steps on How to Calculate Molar Mass

written by: Keren Perles โข edited by: Amanda Grove โข updated: 1/17/2012

How do you calculate molar mass? The molar mass of a compound is the mass of exactly one mole of that compound. Read this study guide for more information about how to find molar mass.

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Definition of Molar Mass

Are you wondering, "How do you calculate molar mass?" In order to understand how to find molar mass, it is important to define the word "mole." One mole of a compound consists of exactly 6.02 X 1023 molecules of any given compound. The molar mass of an compound is the mass of exactly one mole of that compound. The compound with the least molar mass is hydrogen gas (H2), and there are some macromolecules, both organic and bioorganic, that have an extremely large molar mass. One example of a macromolecule is a DNA base pair, which has a molar mass of 325 grams.

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Finding the Molar Mass of an Element

The steps of how to find molar mass of an element is surprisingly simple. Just take a look at the periodic table, find the element in question, and locate the atomic mass of that element. The atomic mass is equal to the molar mass of the element in grams per mole. For example, the atomic mass of carbon would be 12.01 grams per mole, because its atomic mass is 12.01.

Of course, in elements that only exist in nature as two or more bonded atoms, the number of atoms will have to be taken into account. For example, hydrogen does not occur in nature as just one atom, only has the diatomic H2, or two hydrogen atoms bonded together. Therefore, the molar mass of hydrogen would be 1.01 X 2, or 2.02 grams per mole. The same would hold true for oxygen, nitrogen, flourine, bromine, chlorine, and iodine, all of which only exist as two bonded atoms. Additional exceptions include phosphorus, which is made up of four atoms (P4), and sulfur, which is made up of eight atoms (S8).

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Finding the Molar Mass of a Chemical Compound

To find the molar mass of a chemical compound, you need to add up the molar masses of each of the elements in the compound. As a simple example, sodium flouride is made up of an atom of sodium and an atom of flourine. Sodium has a molar mass of 23 grams per mole and flourine has a molar mass of 19 grams per mole. Therefore, sodium flouride has a molar mass of 42 grams per mole.

It can be more difficult, however, to figure out how to find molar mass for more complex compounds. To do this, you will need to find out how many atoms of each element are in the compound, similar to the technique you use in the early stage of balancing an equation. For example, to calculate the mass of magnesium phosphate, you have to know that the formula is Mg3(PO4)2. That means that there are three atoms of magnesium, two atoms of phosphorus, and eight atoms of oxygen in every molecule of magnesium phosphate. Adding up the molar mass of each element gives you (3 x 24) + (2 X 31) + (8 X 16) = 262 grams per mole.

Now that you know the basics, you can answer the question, "How do you calculate molar mass?" for any given compound.

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References

What is molar mass?: http://misterguch.brinkster.net/molarmass.html