Please help! Calculating Mw of amino acid from pH change?

[jasmine97]

I am a 2nd year Biochemistry student and have been given this supposedly 'basic' question, but I'm really struggling with it.

 

"1.068 g of an amino acid (pKa1 = 2.4, pKa2 = 9.7) was dissolved in 100 ml of 0.1 M NaOH to produce a solution with a final pH of 10.4. Calculate the molecular weight of the amino acid. What is this amino acid?"

 

Does anyone have any ideas on how to tackle this?

Any help would really be appreciated.

[BabcockHall]

What are your thoughts?

[BabcockHall]

I would start with a chemical equation showing what is going on.

[jasmine97]

do you have any ideas for what equation? I thought they could be hinting at using the Henderson-Hasslebach equation

[BabcockHall]

I would use the H-H equation. Which pK_a value should you use and why?

[hypervalent_iodine]

Why would you use the HH for this? I can't see how that answers the question at all. You have a reaction between a strong base and a weak acid, meaning that all of the amino acid has reacted. You have the final pH, and you have the concentration of the NaOH initially, so the question is really just one of stoichiometry.

 

OP: the first thing you need to figure out is what value you need to get the final answer. If it is asking you molar mass, what is the equation for that? What part of that equation do you have, and what do you need? Once you identify that, you then need to identify how to get that number (sorry, I know that sounds patronizingly obvious). I gave a hint above, but it would be helpful if you wrote out a general equation for the reaction between an amino acid (you will need to figure out if it is mono or diprotic based on the pKa information given) and NaOH.

[BabcockHall]

Because we are less than one pH unit away, we are still in the buffering region of the second pK_a, which is 9.7. Putting it another way, not all of the weak acid is being consumed by hydroxide. It is actually a nice problem.

Edited September 20, 2016 by BabcockHall

[BabcockHall]

Try drawing the chemical reaction of sodium hydroxide reacting with an amino acid first.