You are given 750 mL of 0.80 M HCl. Calculate how many moles of HCl are contained within this volume of solution.

If you have the atomic number it means that is the number of protons and electrons.
if you the atomic mass than you have to subtract the atomic number from the atomjc mass to get the number of neutrons.
when you add the atomic number and the number of nutrons it will give you the atomic number