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How to Calculate Mass When You Are Given the Number of Atoms

Horses race in furlongs; ships sail in nautical miles; atoms come in moles. The mole (mol) is a scientific unit of measurement for quantities of objects like atoms and molecules. Because atoms and molecules are very small objects, the mole is a very large number: 6.022 x 10^23. Scientists often refer to this number as Avogadro's number in honor of Italian mathematician and physicist Amedeo Avogadro (1776-1856). Moles are useful when you want to calculate the mass of a given number of atoms.

Things You'll Need

Periodic table of the elements
Scientific calculator

Instructions

- 1
  Write the number and type of atoms you want to analyze. For example, write "11 x 10^23 atoms of mercury (Hg)."
- 2
  Find the quotient of the number you have written and Avogadro's number, or 6.022 x 10^23, using a scientific calculator. For example, 11 x 10^23/6.022 x 10^23 = 1.8266. Your sample represents 1.8266 moles of mercury atoms.
- 3
  Consult a hard-copy or online periodic table of the elements or table of chemical properties and learn the molecular weight of the substance you are analyzing. For example, you can access the National Center for Biotechnology Information PubChem Compound website and learn the molecular weight of mercury is 200.59 grams per mole (g/mol).
- 4
  Multiply the number of moles of atoms you have calculated by the molecular weight of the substance you are analyzing. For example, 1.8266 x 200.59 = 366.4048. Your sample of 11 x 10^23 atoms of mercury has a mass of approximately 366.4 grams (12.92 ounces).

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