Calculate the pH of a solution containing 2.0 molar of nitrous acid and 1.48 molar of lithium nitrate. Here we're told that the KA of our weak acid is 4.6×10-4. Now here, because the K value is less than one, we know that nitrous acid is a weak acid.
Lithium nitrite looks similar to nitrous acid except it has one less H+ ion. Because of this, this has to be the conjugate base. So we have a weak acid. We have a conjugate base. This is the pairing that helps to make a buffer. So we're going to use the Henderson-Hasselbalch equation to calculate the pH of this buffer.
Here we're going to say pH equals. Now because they give us KA, we can say pH equals pKA plus log of conjugate base over weak acid. Here pKA remember is just negative log of KA. So now of 4.6×10-4 plus log of conjugate base amount which is going to be 1.48 molar divided by weak acid amount which is 2.0 molar.
When we plug this in, we get 3.21 as the pH for this buffer solution.