I Need Some Chemistry Help!
By Pigglies
@Pigglies (9329)
United States
April 23, 2008 10:12pm CST
Okay, I've figured out most of this homework assignment, except for these two problems. If anyone can help me with these, please let me know!
1. How many milliequivalents of acid would be required to change the pH of 100mL of 1.00M phosphate buffer having a pH of 6.80 to a pH of 5.80? What would be the change in pH if a similar amount of acid were added to a similar buffer (1.00M phosphate) that had a pH of 6.20?
2. How much of each reagent would you use to prepare a phosphate buffer having a volume of 500mL? The buffer is to be 0.25M and have a pH of 7.05. Reagents available are distilled water, 6N HCl, 6N NaOH, and solid NaH2PO4 x 4H2O (MW = 192).
I am so confused on these two. It's been a long time since I took general chemistry and these problems are due in biochemistry, but if you've only taken general chemistry or are currently taking general chemistry they may be more fresh in your mind. As it is, my mind is stale apparently.
Thanks in advance for any help! If you can solve them and explain them to me, I'll definitely give you the best response.
1 response
@docpcp (32)
• United States
24 Apr 08
Hey Pigglies,
It's been a while since I did these, too, but what I remember is that you are dealing with the Henderson-Hasselbalch equation here. Since I can't use HTML yet, it's sort of a pain, but there's a decent explanation at http://en.wikipedia.org/wiki/Buffer_solution and specifically for phosphate buffers at http://biotech.about.com/od/buffersandmedia/ht/phosphatebuffer.htm .
Since pH = pKa + log10 ([A-]/[HA]) and the pKas of phosphoric acid are 2.15, 6.86, and 12.32, you should be able to calculate what the [A-] is at 6.80 and what it needs to be at 5.80. Take the difference in molarity and convert it to equivalents using the volume.
You can check your answer at http://tinyurl.com/4gt4fc (that's a link to Sigma-Aldrich's buffer calculator).
I hope that's helpful to you.
