jasmine97 Posted September 10, 2016 Share Posted September 10, 2016 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. Link to comment Share on other sites More sharing options...
BabcockHall Posted September 13, 2016 Share Posted September 13, 2016 What are your thoughts? Link to comment Share on other sites More sharing options...
BabcockHall Posted September 13, 2016 Share Posted September 13, 2016 I would start with a chemical equation showing what is going on. Link to comment Share on other sites More sharing options...
jasmine97 Posted September 16, 2016 Author Share Posted September 16, 2016 do you have any ideas for what equation? I thought they could be hinting at using the Henderson-Hasslebach equation Link to comment Share on other sites More sharing options...
BabcockHall Posted September 17, 2016 Share Posted September 17, 2016 I would use the H-H equation. Which pKa value should you use and why? Link to comment Share on other sites More sharing options...
hypervalent_iodine Posted September 18, 2016 Share Posted September 18, 2016 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. Link to comment Share on other sites More sharing options...
BabcockHall Posted September 20, 2016 Share Posted September 20, 2016 (edited) Because we are less than one pH unit away, we are still in the buffering region of the second pKa, 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 Link to comment Share on other sites More sharing options...
BabcockHall Posted September 21, 2016 Share Posted September 21, 2016 Try drawing the chemical reaction of sodium hydroxide reacting with an amino acid first. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now