Calculate the weight of the buffers you will use to make thebuffers for Part A f

Question

Calculate the weight of the buffers you will use to make thebuffers for Part A for all the buffer possibilities listed underprocedures in Part A, step 1. In other words, how many grams do youneed to make 100 mL of a 0.1 M buffer?

Buffer        pKa1     pKa2     pKa3         formulawt.(g/mol)

Acetate     4.76                                            136.1
CAPS      10.4                                             221.3
Citrate      3.06       4.74        5.40             294.1
HEPES     7.55                                            238.3
Phosphate   2.12    7.21        12.32          142.0
Tricine        8.15                                          179.2
TRIS           8.3                                            121.1

Explanation / Answer

All buffers must contain the a weak acid and a conjugate base. they call it an acetate buffer. but it also contains acetic acid as well.

Let x be the molarity of Hac (acetic acid) and y be the molarity of acetate ac- ,The sum of the two equals to 0.1,( you want to make a 0.1M buffer).

According to henderson - hasslebach eqaution,

pH = pKA+ Log ( Ac- / Hac)

5.6=4.76 + log y/x

0.84= log y/x

10^0.84= y/x = 6.92 or y = 6.92 x, substitute the expression of y into our first equation.

x+y = 0.1

x+6.92x= 0.1

7.92 x = 0.1,

x=0.0126 M and y =0.1-x = 0.0874 M

Moles Hac needed = M Hac x L of solution = (0.0126) (0.100) = 0.00126 moles of Hac)

Moles ac needed = Mac - x L of solution = (0.0874) ( 0.100) = 0.00874 moles Ac = 0.00874 moles Nac

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