Calculating Theoretical & Experimental pH Values of NaNO2 Solution

[robspg]

Hello, thanks for reading.

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

 

[jbsteele]

To take into account the pH of the tap water, you will have to calculate the pH of the solution after the addition of the tap water. This can be done using the Henderson-Hasselbalch equation, which states that the pH of a solution is equal to the pKa (the acid dissociation constant) plus the logarithm of the ratio of the concentrations of the conjugate base and the acid. Since you already know the pKa (1.96*10-11) and the concentration of the acid (0.1M) you can use the equation to calculate the theoretical pH of the solution with the addition of the tap water.