The point of this problem is to correctly calculate the pH of a buffer system using formonitrile, or hydrogen cyanide. Here, we use the dissociation constant and the Henderson-Hasselbach equation.

Given Problem: Calculate the pH of a buffer that’s .250 M HCN (kª= 4.9*10^-10) and 0.170 M KCN. 

2. This is to remind us of the equations we’re using for this problem. First is the dissociation constant kª, which we can use to calculate the pH of a dissociated acid. The reaction was also written out to show the products seen in solution. This is to show that every molecule of HCN/KCN has exactly one cation (H+/K+) and one anion (CN- in both cases). From this, we can assume that the molar ratio is equal. For instance, the dissociation of 3 molecules (or moles) or HCN will yield three molecules of both H+ and CN-, or “three moles”. Use this to help you figure out the H-H equation.

3. The Henderson-Hasselbach equation is used specifically for finding the pH (or pOH) of a buffer system. It can be used for reactions that contain both an acid and it’s conjugate base. Remember, the KCN is a conjugate base of HCN, so that’ll go on the numerator of the equation. The HCN will go in the denominator. Luckily, we also have the pKa given to us, so we can plug that into the equation as well.

4. So I skipped a few steps! But hopefully it makes sense. The important thing is to keep track of your A-/HA molecules. Remember, when you put a ‘p’ in front of either the Ka value, or the [H+], you need to take the -log of it in base 10.

Feel free to check my numbers!

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