Is a strong acid the same as a concentrated acid?

They sound like they should mean something similar! However, chemistry uses words very carefully, and these words have very specific meanings. Strong acids completely dissociate in water. Hydrochloric acid (HCl) is an example - if you mixed this with water, the hydrogens and chlorines wouldn't stay together - you'd find them separate, as H+ and Cl- ions. The hydrochloric acid is said to have therefore been completely 'ionised' in water. Given that there will be more free H+ ions, the pH of the solution will be lower. Strength is measured in pH, and strong acids will have a low pH.If an acid is concentrated, there is a lot of the acid per set volume of water, regardless of how much it has dissociated. It is measured in mol/dm^3. You can have a weak acid (has a pH around 5, is not that ionised in solution) that nonetheless is very concentrated as there is a lot of it per volume.