That's interesting. I tried it on Grok and it gave the correct answer straight away:
"The pH of the mixed water depends on the volumes mixed. Assuming equal volumes (e.g., one liter of each, as pitchers are typically similar in size and no volumes are specified), the resulting pH will be approximately 8.8."
Along with an explanation of how pH is a logarithmic scale and the calculations involved. It used its code execution tool and wrote a program to do the calculation.