Calculate the pH of 0.750 M pyridinium chloride, CH NHCI?
Given K of CSH. N is 1.70x10
-9

Explanation:

We have to remember that pyridinium chloride is an acid. So, this compound will produce hydronium ions . Additionally, is an ionic compound, so we have to start with the ionization reaction (the production of the cation and anion) first and then with the reaction with acid reaction (the production of hydronium ions).

Ionization reaction

Acid reaction

Now, if we want to calculate the pH value we have to know the concentration of (let's remember that ). To calculate this concentration we have to use the ICE table:

I

C

E

The initial concentrations are:

With this in mind:

I     0.75                 0              0

C  

E

In the change we will have +X for the products and -X for the reagents, so:

I    0.75                   0              0

C   - X                    +X           +X

E

In the equilibrium we only have to add "I" and "C":

I    0.75                  0              0

C   - X                   +X           +X

E   0.75-X               X             X

Now, we have equilibrium values  that we can use in the Ka expression:

Finally, we can use the Ka value and solve for . (The reported Ka value for pyridinium is , see reference). So:

When we solve for X we will obtain:

Finally, we can calculate the pH value:

I hope it helps!

Reference for the Ka value:

http://clas.sa.ucsb.edu/staff/Resource%20Folder/Chem109ABC/Acid,%20Base%20Strength/Table%20of%20Acids%20w%20Kas%20and%20pKas.pdf

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