you need a buffer containing 0.1M tris, adjusted to ph 8.2, with HCL. Your lab m

Question

you need a buffer containing 0.1M tris, adjusted to ph 8.2, with HCL. Your lab mate has 0.5M tris, adjusted to pH 8.5 with HCL. you realize that you can take some of their stock and adjust the ph to 8.2 by adding more HCL, and adjust the concentration to 0.1 M with water. since you need 200 ml of the new buffer solution, you start by measuring out 40 ml of the original buffer. pka tris-8.1

a) for the original(ph 8.5) solution, calculate the ratio of concentrations of the basic and acidic forms of tris. be sure to make clear in your answer which species is which in the ratio.

b) make a very approximate estimate of how many moles of HCL you would need to add to the solution to adjust the pH to 8.2. you should not have to do any extensive calculations, but explain how you arrived at your estimate.

Explanation / Answer

a) Using Hendersen-Hasselbalck equation,

pH = pKa + log([base]/[acid])

8.5 = 8.1 + log([acid]/[base])

Thus the ratio of basic form and acidic form of tris would be,

[base]/[acid] = 2.51

b) Difference in pH = original pH of solution - required pH = 8.5 - 8.2 = 0.3

pH = -log[H+] = 0.3

[H+] = 0.50 M

So, moles of HCl to be added = molarity x volume of final solution = 0.50 M x 0.2 L = 0.1 moles

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