pH level after water change

[redsilver]

if my tap water has 6.5 pH and my tank water has 8 pH, after 50% water change what will happend to the pH level on the tank, will it go down or its the same or it will be the average of the remaining water and the new added water...
will it be like this :
6.5(tap water) + 8(tank water) = 14.5(total pH)/2 = pH 7.25

 

[famman]

pH is the negative logrithm of the number of free H ions per mole (?). A simple average will not give the resultant pH. The resultant pH will partly depend on the buffering after the water change, in your case it should go down, but by how much only experience will show.
good luck