Please Help... These two problems are tough

[twith89]

You have prepared a 0.2 M histidine solution. Calculatethe molar concentration ofisoelectric histidine at a) pH 5.5 B) pH 7.0. An "isoelectric aminoacid" is the structurein which the molecule has no net charge.

 

 

Calculate the pHof a solution obtained by adding 20 ml of 0.20 M KOH to 480 ml of 0.02M isoelectric glycine.

 

 

[cypress]

I assume this is homework. You'll have to start it off by suggesting how you would solve these questions and also take a initial try. It won't help you much if I do your work for you.

 

I assume this is homework. You'll have to start off by suggesting how you would solve these questions and also take a initial try. It won't help you much if I do your work for you.

[twith89]

These aren't homework problems... They are practice problems... I was given the answers but don't know how my instructor got them.... I know you use the henderson hasselbach equation for the 2nd question...

 

 

I assume this is homework. You'll have to start it off by suggesting how you would solve these questions and also take a initial try. It won't help you much if I do your work for you.

 

I assume this is homework. You'll have to start off by suggesting how you would solve these questions and also take a initial try. It won't help you much if I do your work for you.

 

 

 

[cypress]

Same equation for the first as well, because dissociation and acidity (measured by pH) goes hand in hand for acids and bases. You first need to identify the formula for dissociation of histidine near the indicated pH and the pKa for that dissociation. Can you do that?

[singinglupines]

Hey, I am in the same class as well as my roommate and we are having problems solving the first one as well. I know you need to use Henderson-Hasselbalch and find the isoelectric point. I'm having issues determining what pKa to use when. I'm thinking we need to use H-H twice. Any more pointers you could give cypress? Thanks!

[cypress]

Histidine is a bit tricky because it has three acid groups and thus three pKa's with three conjugate bases plus the H-A form indicated in the question which was identified as the isoelectric form. See if you can identify the three conjugate bases and the three pKa's plus indicate what range of pH you would expect to see predominately the isoelectric form and we will proceed.