How to Calculate the Total Moles of Ions


Because atoms are so very tiny, it is inconvenient, if not impossible, to perform chemistry calculations involving numbers of atoms. To address this difficulty, scientists invented the unit known as the mole, which is equal to 602.2 billion trillion atoms. Ions are regular atoms that carry a few extra electrons, or have fewer electrons than normal. Calculating moles of ions is exactly the same as calculating moles of normal atoms.

Things You'll Need

  • Periodic table of the elements, listing atomic masses
  • Calculator
  • Weighing apparatus (optional)
  • Multiply the volume that the ions take up by the density of the substance that the ions are made out of. The result is the total weight of the ions. Make sure that the density is measured in grams per cubic centimeter or grams per cubic meter, so that the weight will be expressed in grams.

  • Calculate the molar mass of your ions. This is the number of grams a mole of those ions weighs. To do this, look up the elements that make up your ions on the periodic table of elements. Write down the atomic mass of each element that makes up the ions, and add them up. The atomic mass for each element is the same as the molar mass for that element. Physicists developed one term, and chemists developed the other, but they describe the same thing. If an element occurs two or more times in the chemical formula of your ions, then add its atomic mass in two or more times. For example, if you are working with ionized carbon dioxide, there are two atoms of oxygen for every atom of carbon. The molar mass of oxygen is 16 grams per mole. The molar mass of carbon is 12 grams per mole. The molar mass of the ion is 12+16+16, or 44 grams per mole.

  • Divide the weight of your ions by the molar mass of those ions. The result will be the number of moles of ions in the sample you are investigating. In the example of the carbon dioxide ions, if you have 4.4 grams of carbon dioxide, then 4.4 grams divided by 44 grams per mole equals 1/10 of a mole.

Tips & Warnings

  • If you do not know the density of the ions, then you can still determine how many moles you have if you have the weight of the ions, or can at least weigh them yourself.


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