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Weight and Mass
Atoms and Atomic Structure

How do you convert from grams to moles and also from moles to grams?


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September 24, 2015 9:58PM

You must first find the molar mass of the element or compound. Use the periodic table (see the link to the left of the answer). If the chemical is an element, just read off the atomic mass from the periodic table. If it is a compound, you must know the molecular formula, and then you find the total molar mass of the compound by adding up the atomic masses of each atom in the compound. The unit of the molar mass will be in grams per moles (g/mole)

Once you have the molar mass, you can easily convert from grams to moles, and also from moles to grams.

Number of moles = (# of grams) / (molar mass)

Number of grams = (# of moles) * (molar mass)

Here are a two examples:

Example 1:

How many moles are in 5 grams of O2?

The molar mass of O2 = 16.00 g/mole x 2 (for 2 atoms of oxygen) or 32.00 g/mole.

5 g of O2 / (32 g/mole) = 0.15625 moles

Example 2:

How many grams does 4 moles of NH3 weigh?

The molar mass of NH3 = 14.01 + (3 * 1.01) = 17.04 g/mole

4 moles * 17.04 g/mole = 68.16 grams Grams in a MoleIn chemistry a mole is considered to be Avogadro's number (6.02 x 1023)of molecules (or atoms) of a substance - so depending on the density of the substance, the mass of that amount of the substance could vary widely.

One mole of hydrogen, for example, would have a different mass than one mole of lead. The question compares two different measurements of a given substance, rather like asking "how heavy is a gallon?". The question becomes "A gallon of what"?

*On the other hand, the Eastern Mole (Scalopus aquaticus) a small, burrowing mammal, weighs between 54 and 99 grams at adult size.
To convert grams to moles simply multiply the molar mass by the number of moles.

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December 20, 2014 4:07AM

Gram is a measurement of mass. A mole represents a number of Avogadro Constant. The relationship between them: The mass of a mole of a substance is equivalent to the relative molecular mass in grams.