1) The phosphate ion (H3PO4) is a polyprotic acid with the following pKa values:

Question

1) The phosphate ion (H3PO4) is a polyprotic acid with the following pKa values: pK1= 2.15, pK2= 7.2, pK3= 12.4. This buffering system is present in biological systems and is also frequently used in biochemistry labs as a buffer for many types of sample preparation and techniques.

(b) Buffers are made containing the proper ratios of weak acid and conjugate base to obtain a desired pH. Stock solutions of weak acid and conjugate base can be prepared in advance, then combined in different volumes to easily vary the pH. Assume you have access to Na2HPO4 and NaH2PO4 solids in the lab. Describe how to prepare 0.5 M solutions of each in 1L.

(C) Make a chart of the volume ratios of your 0.5 M solutions of weak acid and conjugate base for the full buffer range (every 0.5 M pH units) of the phosphate ion you chose in (1) to make 1 L of a 0.5 M buffer

(d) Chose one of the ratios from your chart and prove that the total concentration of your final solution (0.5 M) is the sum of the concentrations of the weak acid and conjugate base in your system.

Explanation / Answer

http://www.science.smith.edu/departments/Biochem/Biochem_353/buffer.html

hope it helps

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