[JohnTan]

Hello everyone,
in this question 10cm^3 of an aqueous solution of a monoprotic strong acid is added to 90cm^3 of water. I now have to work out what this will cause the pH of the solution to do. I proposed that it would decrease the pH of the solution by 1, being a strong acid, however, the answer says it will increase by 1. why is this?
thank you

[cpncoop]

pH is defined as the -log[H+] ions.  If you are adding water to the solution, you are decreasing the concentration of H+ ions, and thus increasing the pH.  The problem could be in how you are reading the question:  the question is asking what will happen to the pH of the monoprotic aqueous acid solution when it is diluted (it will increase), not what will happen to the water being used to dilute the solution (pH will decrease).  I hope this helps....

[JohnTan]

Ahh, yes that does help. I was indeed interpreting it as the pH of the water changing. Thanks for clearing that up.