Working out oxygen
Put the formula to work once. Two numbers are needed: the mass of an oxygen atom and the mass of a carbon atom.
Problem
The real mass of an atom of oxygen (O) is 2.66 × 10-23 g. The real mass of an atom of carbon is 1.99 × 10-23 g. Find the relative atomic mass of oxygen.
Solution
Ar = mass of one oxygen atom ÷ (1/12 × mass of one 12C atom)
= (2.66 × 10-23 g) ÷ (1/12 × 1.99 × 10-23 g)
= (2.66 × 10-23 g) ÷ (1.66 × 10-24 g)
= 16.02
The 10-23 appears on the top and the bottom, so it cancels; what is left is 2.66 divided by 0.166. The grams cancel too, which is why the answer carries no unit.
16.02, or 16?
Tables give oxygen as 16. The calculation came out at 16.02 only because the masses it started from had been rounded to a few decimal places. Use the table value when solving problems.
What the answer is telling you
An oxygen atom is 16 times as heavy as a twelfth of a carbon-12 atom. That is not a number of grams; it is a comparison. Being able to put every element on one scale is the whole power of the method.