Calculate the [H+] and pH of a 0.0040 M hydrazoic acid solution. Keep in mind that the Ka of hydrazoic acid is 2.20×10−5. Use the method of successive approximations in your calculations or the quadratic formula.

Explanation:

In this, we can with the ionization equation for the hydrazoic acid (). So:

Now, due to the Ka constant value, we have to use the whole equilibrium because this is not a strong acid. So, we have to write the Ka expression:

For each mol of produced we will have 1 mol of . So, we can use "X" for the unknown values and replace in the Ka equation:

Additionally, we have to keep in mind that is a reagent, this means that we will be consumed. We dont know how much acid would be consumed but we can express a subtraction from the initial value, so:

Finally, we can put the ka value and solve for "X":

So, we have a concentration of 0.000285 for . With this in mind, we can calculate the pH value:

I hope it helps!