There are several calculations that need to be done in order to prepare a specif

Question

There are several calculations that need to be done in order to prepare a specific volume of buffer solution at a particular concentration and pH. There is a worked example in your lab manual on p. 119. Once the correct ratio of A' to HA is known, determine the actual concentration of A^- needed to make a buffer at a particular concentration. The ratio of acetate to acetic acid in a buffer is 0.176. That is, the concentration of acetate should be 0.176 times that of the concentration of acetic acid. What is the concentration of acetate (in M) needed to prepare a 0.416 M buffer using acetic acid and acetate? There are several calculations that need to be done in order to prepare a specific volume of buffer solution at a particular concentration and pH. There is a worked example in your lab manual on p. 119. What volume of acetate solution (in mL) is needed to make 250.0 mL of a buffer solution? The concentration of acetate in the buffer must be 0.0594 M, and the concentration of the acetate stock solution is 0.500 M.

Explanation / Answer

Q6

acetate/Acid = 0.176

find concentration of acetate if

Mtotal = 0.416M

so...

A+HA = 0.416

and

A/HA = 0.176

A = 0.176*HA

A+HA = 0.416

0.176*HA + HA = 0.416

HA = 0.416/(1.176) = 0.35374

and

A- = 0.35374*0.176 = 0.06225824 M

Q7

V = for 250 mL solution

ratio = 0.0594

A + HA = 0.5

so..

A/HA = 0.0594

A = 0.0594*HA

A + HA = 0.5

0.0594*HA+ HA = 0.5

1.0594*HA = 0.5

HA = 0.5/1.0594 = 0.47196

A = 0.0594*0.47196 = 0.02803

so..

M = mol/V

V = mol/M = 0.02803 / M

M = 0.5

V = mol/M = 0.02803 / 0.5 = 0.056 L = 56 mL

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