How Do You Calculate Gram Molecular Mass?

Gram molecular mass of an element is equal to the atomic mass of the element. For elements that are comprised of several atoms, the gram molecular mass is the atomic mass of the element times the number of molecules in the element. For chemical compounds that have several different elements, the gram molar mass is the summation of the gram molecular masses of those elements.

1. Find the atomic mass of the element

The atomic mass of any element can be found from the periodic table. For example, the atomic mass is 1.007 for hydrogen, 12.01 for carbon and 15.9994 for oxygen.

2. Convert the atomic mass to gram molecular mass

Gram molecular mass of an element, also known as molar mass, is the weight of 1 mole of the element. It is calculated by multiplying the atomic mass by the molar mass constant, which is 1 gram per mole. For example, the molar mass of hydrogen becomes 1.007 gram per mole.

3. Calculate the gram molecular mass of the compound

For molecules that have only one atom of an element, the gram molecular mass is easily found from the periodic table. However, many elements found in nature are comprised of two or more atoms. A hydrogen molecule has two hydrogen atoms, so the gram molecular mass of a hydrogen molecule is 2.014 gram per mole. For chemical compounds that are comprised of more than one element, the gram molecular mass is calculated by adding the gram molecular mass of the individual elements. For example, a molecule of water has two hydrogen atoms and one oxygen atom. The gram molecular mass of water is (15.9994 + 1.007 + 1.007) gram per mole = 18.0134 gram per mole.

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