Solving the pH of a solution given its concentration

[Jasmine]

First, thanks for checking out my topic!

 

The last time I had to deal with pH problems was in Biochemistry last year, so this is probably a simple question that I'm over analyzing because I haven't done this type of problem in a while.

The question asks: Calculate the pH of 0.1 M solution of glycine hydrochloride.

 

I know that HCl is a strong acid and so it should have completely dissociated in the solution, and I know the two pKa values for glycine are 2.34 and 9.6 (should I be using the pI or should I use the more acidic group's pKa?). I looked up Glycine-HCl and found out it can be used as a buffer. I also know that in the HH eqn, [HA]=0.1 M because it's the concentration of the initial solution. When I looked online, I found that the pKa of Glycine-HCl is 2.34 on a site about how to create buffers.

I think this is the dissociation equation: C₂H₅NO₂*HCl <---> C₂H₅NO₂Cl^- + H⁺ (but I'm not totally sure that this is what I need to solve the problem).

 

I'm stuck on how I'm supposed to find [A-] in this case to solve for pH. If HCl completely dissociated, does that mean that [A-]=[HA], so pH=pKa? If so, then I don't quite understand how HCl's pH affects the pH of glycine, and I would appreciate an explanation on that.

[mississippichem]

First, thanks for checking out my topic!

 

The last time I had to deal with pH problems was in Biochemistry last year, so this is probably a simple question that I'm over analyzing because I haven't done this type of problem in a while.

The question asks: Calculate the pH of 0.1 M solution of glycine hydrochloride.

 

I know that HCl is a strong acid and so it should have completely dissociated in the solution, and I know the two pKa values for glycine are 2.34 and 9.6 (should I be using the pI or should I use the more acidic group's pKa?). I looked up Glycine-HCl and found out it can be used as a buffer. I also know that in the HH eqn, [HA]=0.1 M because it's the concentration of the initial solution. When I looked online, I found that the pKa of Glycine-HCl is 2.34 on a site about how to create buffers.

I think this is the dissociation equation: C₂H₅NO₂*HCl <---> C₂H₅NO₂Cl^- + H⁺ (but I'm not totally sure that this is what I need to solve the problem).

 

I'm stuck on how I'm supposed to find [A-] in this case to solve for pH. If HCl completely dissociated, does that mean that [A-]=[HA], so pH=pKa? If so, then I don't quite understand how HCl's pH affects the pH of glycine, and I would appreciate an explanation on that.

 

I think they're just talking about the hydrochloride salt of glycine so don't worry about any HCl.

 

Remember that:

 

[math] K_{1}K_{2}=\beta[/math]

 

So you can multiply two equilibrium constants to obtain an average value for a multi-protic molecule. I would use the Henderson-Hasselbalch equation:

 

[math]pH=pK_{a}+log\frac{[A^{-}]}{[HA]}[/math]

 

Or you can just set up both equilibrium expressions and solve accordingly.

Edited February 13, 2011 by mississippichem