## Calculating theoretical pH of a solution while taking pH of tap water into account

I am solving a lab problem that involves calculating a theoretical pH value, then testing the experimental pH in a lab with tap water that is not the expected pH 7, and comparing the values.

I am determining the theoretical pH value of a 0.1M concentration of NaNO2.
Based on my calculations using the Kb value of 1.96*10-11, the result is pH 8.15.

Then, after completing the test experimentally by creating a 0.1M solution from solid NaNO2 and 0.250L of pH 4.95 tap water, I I recorded an experimental value of pH 6.75.

How do I take into account the pH of the tap water to calculate the proper "theoretical pH value"?

Thanks!
-Rob
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