Concentration and the pH scale
Concentration is moles per litre, and pH is the negative log of hydrogen ion concentration, so each unit is tenfold.
A worked example
Diluting 25.0 mL of 1.2 mol/L acid to 300.0 mL gives c = (1.2 x 25.0) / 300.0 = 0.10 mol/L, because the moles of solute do not change.
The mistake people actually make
Convert millilitres to litres before using c = n/V. At 25 degrees C, pH + pOH = 14, so a pH of 11 means a pOH of 3.