Recall this : Acid solutions contain hydrogen ions. The higher the concentration of hydrogen ions, the lower the pH. Hydrochloric acid is a strong acid and ethanoic acid is a weak acid. Strong acids are fully ionised but weak acids are only partly ionised in solution.
Dilute Acid is not the same as Weak Acid
Concentrated Acid is not the same as Strong Acid
Strength of Acids refers to the extent of dissociation (or ionisation) of an acid. We will see words like “STRONG” and “WEAK.”
Concentration of Acids refers to how much of an acid (the solute) is dissolved in the solution. We will see words like “CONCENTRATED” and “DILUTE”
Concentration is a concept covered when you study Mole Concepts or Mole Calculations.
The concentration of a solution is given by the amount of a solute dissolved in a unit volume of the solution. Concentration of an acid can be changed. You can decrease the concentration of an acid solution by adding more solvent to it. You can also increase the concentration of an acid solution by adding more solute to it.
How can a weak acid solution have the same pH as a strong acid solution?
It is possible. The weak acid would have to be more concentrated. pH is determined by the concentration of hydrogen ions in solution.
By the way, you must know this fundamental question :
How is pH related to a solution’s acidity?
If the pH of a solution is less than 7, the solution is called acidic; if the pH is about 7, the solution is neutral; if the pH is greater than 7, the solution is is called basic. In an acidic solution, then, the concentration of hydrogen ions is greater than the concentration of hydroxide ions (commonly tested in MCQ)
Chemistry Notes 