From the equation:

HCl + H20 --> H3O+ + Cl-

 

How much H30+ is needed to create a 2.2 decrease in pH for a 50 000L volume of water currently at pH 9.7???

What have you got so far?

From the equation:

HCl + H20 --> H3O+ + Cl-

 

How much H30+ is needed to create a 2.2 decrease in pH for a 50 000L volume of water currently at pH 9.7???

 

You want to achieve a pH of 9.7 - 2.2. The question is how much HCl should be added to obtain that pH. We can find the original concentration of H3O+ by taking 10^(- orig. pH). We can also find the new concentration of H3O+ the same way by taking 10^(- new. pH). The difference in these amounts will be the amount of HCl needed for the decrease. You'll probably have to express your answer in grams or moles, so dont forget to multiply the concentration found by the amount of solution in liters. I hope that helps.