You need to make two different buffers. Each buffer should have a buffer concent

Question

You need to make two different buffers. Each buffer should have a buffer concentration of .01M. You have two buffer salts available. Please note that for MES, you only have the acid or protonated form while, for TRIS, ou only have the base or unprotonated form.

BUFFER         Form          pK           molecular weight

MES               acid           6.1              195.24 g/mol

TRIS               base          8.1              121.14g/mol

You also have 2.00M HCl and 2.00M NaOH

For each of the following buffers, choose which buffer, MES or TRIS (not both) is best to use. Then calculate how you will make the buffer. How much of the buffer will weigh and how much of the NaOH or HCl will you have to add to bring it to the desired pH.

A. Buffer A should be 0.100M in buffer and have a pH of 7.9

B. Buffer B should be 0.100M in buffer and have a pH of 6.6

USE 1.00L for each of the buffers.

Explanation / Answer

Preparation of buffer solution

A. Buffer A with 0.1 M buffer and 7.9 pH

We would use, TRIS system

let x amount of 2.0 M HCl is added

using hendersen-hasselbalck equation,

pH = pKa + log(base/acid)

7.9 = 8.1 + log(0.1-x/x)

0.63x = 0.1-x

x = 0.061 M

moles of HCl needed = 0.061 M x 1 L = 0.061 mol

Volume of 2 M HCl needed = 0.061/2 = 0.0305 L

mass of TRIS needed = (0.1 - 0.061) x 1 x 121.14 = 4.72 g

B. Buffer B with 0.1 M n buffer and pH 6.6

We would use, MES buffer system

let x amount of NaOH is added

6.6 = 6.1 + log(x/0.1-x)

0.316 - 3.16x = x

x = 0.076 M

Volume of 2 M NaOH to be added = 0.076 M x 1 L/2 M = 0.038 L

mass of MES to be taken = (0.1 - 0.076) M x 1 L x 195.24 = 4.68 g

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