So you’re given the amount of grams a heap of a certain element weighs and asked to find the number of moles present. Have no fear. It’s pretty simple. See, the atomic mass (pull out your periodic tables, kids) of any given element is the weight of one mole of that particular element. So you take number of grams you were given in the problem and divide that by the element’s atomic mass. Bingo. Done.

If you’re given the amount of moles of an element present and asked how many grams there are, you do just the opposite. Multiply the number of moles you were given in the problem by the atomic mass of the element and there you have the mass in grams. Not too bad, is it? Monatomic elements are easy. After this it gets a bit harder.

But if you remember the rules you used for this you can easily convert back and forth from moles to grams with diatomic elements and compounds too. Let's look at the 3 steps:

Moles to Grams

  1. Find how many moles are given in the problem.
  2. Calculate the molar mass of the substance (the atomic mass when we’re dealing with these monatomic elements, the atomic mass multiplied by two when we’re playing with diatomic elements, and the sum of the atomic mass of each single atom represented when we’re dealing with a compound).
  3. Multiply step one by step two.

Grams to Moles

  1. Find the number of grams given in the problem.
  2. Calculate molar mass.
  3. Divide step one by step two.

Okay, now just let me clear up that hellishly long bracket in step two up there. When we’re talking compounds you want to make sure each individual atom is represented. If there are two hydrogen atoms in a compound, you gotta add the atomic mass of hydrogen twice. If there are eight sulfur atoms in a compound, you want your molar mass to have been found by multiplying eight by the atomic mass of sulfur.

Hopefully this helps any other E2 students out there who are, like me, frantically studying for their chemistry midterm.

For those interested, one can convert moles to atoms by multiplying the number of moles by Avogadro’s Number (6.02 x 10^23). One can likewise divide the amount of atoms by Avogadro's Number to discover the number of moles.

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