You'll need to have a periodic table and a calculator handy. Remember the atomic mass is the larger of the two numbers on the periodic table. If you're struggling with this page make sure you can decode chemical formulae.
Water (H2O) has a relative formula mass of 18. Look at the periodic table symbols for oxygen and hydrogen to the right to see if you can work out why.
H2O means that there are two hydrogens and 1 oxygen. The symbols to the right show that the mass of hydrogen is 1 and that the mass of oxygen is 16. So.... Two hydrogens have a mass of 2 and one oxygen has a mass of 16, 16+2=18. So the relative formula mass of water is 18! WOW!!!!
Showing your working is important - it helps you to earn marks when you make silly mistakes. This question is guaranteed to be in the exam and is almost always followed by a question on persentage mass which will be easier if you have the working for this question to hand(trust me on this).
The image to the right shows how I teach my students to layout their work.