[yg7s7]

(a) Which of two of the following compounds would you mix to make a buffer of pH 7.45: H3PO4 (FM: 98.00), NaH2PO4 (FM: 119.98), Na2HPO4 (FM: 141.96), and Na3PO4 (FM: 163.94)?

(b) If you wanted to prepare 1.00 L of buffer with a total Phosphate concentration of 0.050 M, how many grams of each of the two compounds would you mix?

(c) If you did what you calculated in part (b) you would not get a pH of exactly 7.45. Explain how you would really prepare this buffer in the lab.

Answer:
(a) NaH2PO4 and Na2HPO4 would be simplest.
(b) 4.55 g Na2HPO4 + 2.15 g NaH2PO4
(c) Weigh out 0.0500 mol Na2HPO4 and dissolve in 900 mL water Add HCl until pH is 7.45, and dilute with water.

[enahs]

I have no desire to go look up the pKa's. If you post your math it can be checked.
But A should be correct, even not knowing pKa's off the top of my head.
C, no. Why would you use HCl and add a chlorine counter ion when you can use a phosphate based acid to lower the pH?

[Borek]

C, no. Why would you use HCl and add a chlorine counter ion when you can use a phosphate based acid to lower the pH?

Probably to not change total phosphate concentration, so that's not completely off. Although it can be done better, just starting from the lower pH.

[wpenrose]

(c) Make phosphoric acid or NaH2PO4 to a concentration higher than .05M. Add NaOH until the pH is 7.45. Dilute to a final phosphate concentration of .05M. The concentration of a buffer always refers to the weak acid or base, not the strong base or acid part.

Dangerous Bill

[JGK]

http://biotech.about.com/od/buffersandmedia/ht/phosphatebuffer.htm

[enahs]

C, no. Why would you use HCl and add a chlorine counter ion when you can use a phosphate based acid to lower the pH?

Probably to not change total phosphate concentration, so that's not completely off. Although it can be done better, just starting from the lower pH.

If I dilute a 1M solution of some phosphate buffer at a high pH with a 1M solution of lower pH phosphate, then the phosphate concentration does not really change.

Maybe I am just being overly sensitive, but in the reaction we do in phosphate buffer, adding a counter ion like Cl- or Na+ will cause some serious problems, as the ionic strength has a huge impact on enzymatic reactions.

[AWK]

If I dilute a 1M solution of some phosphate buffer at a high pH with a 1M solution of lower pH phosphate, then the phosphate concentration does not really change.

This statement concerns a total phosphate content

Maybe I am just being overly sensitive, but in the reaction we do in phosphate buffer, adding a counter ion like Cl- or Na+ will cause some serious problems, as the ionic strength has a huge impact on enzymatic reactions.

No so serious, because in this case the ionic strength only slightly diminishes. A change of ionic strength in the same direction is observed during the enzymatic reaction itself. An addition of small amount of HCl or NaOH (but only a small one) to the phosphate buffer is a common practice in a laboratory work for adjusting pH to the needed value.