Question

Which solution has the lowest pH?

Answer 1

Ok. So the first step is to identify whether the solution is an acid or a base. That is, wherer it contains the H+ ion (acid) or the OH- ion (base). There are two rules that follow. 

For an acid. pH = -log(H+), where log is log base 10 and H+ is the concentration (molarity) of H+ ions in the solution. 

For a base. pH = 14 + log(OH-), where log is log base 10 and OH- is the concentration (molatity) of OH- ions in the solution. 

Basically, if you have a calculator, you can work this out.