Use the following steps to determine how to make 600 mL of a 0.1M acetate buffer

Question

Use the following steps to determine how to make 600 mL of a 0.1M acetate buffer, pH 5.0, using 0.1M acetic acid and 0.1M sodium acetate. Using the desired pH(5.0) and pKa of acetic acid, use the Henderson-Hasselbalch equation to determine a ratio of [base]/[acid] required for this buffer. Your answer should look something like [base]/[acid] = 2.3 Given the example above, we can set [acid] = x and [base] = 2.3x. Remember that the concentration of acid and base should equal the desired molarity, which 0.1. Based on this, a second equation can be obtained given the example above, the equation would be x + 2.3x = 0.1. Use this to solve for x, which represents the concentration of acid ([acid]), and also for [base]. Use M_1V_1 = M_2V_2 to solve for the volume of 0.1M acetic acid and 0.1M sodium acetate required to create the buffer. M_1 = 0.1M of acetic acid or 0.1M sodium acetate (stock solution) V_1 = what to solve for M_2 = the value for [acid] or [base] solved in 4b V_2 = the desired buffer volume (600mL)

Explanation / Answer

Answer:

a) Henderson - Hasselbalch equation to find pH of buffer is,

pH = pKa + log([Base]/[Acid]

For Acetic acid - Sodium acetate buffer,

Required pH = 5.0 unit, pKa of Acetic acid = 4.76 using these values,

5.0 = 4.76 + log([Sodium acetate]/[Acetic acid]

log([Sodium acetate]/[Acetic acid] = 5.0 - 4.76

log([Sodium acetate]/[Acetic acid] = 0.24

[Sodium acetate]/[Acetic acid] = 100.24.  

[Sodium acetate]/[Acetic acid] = 1.74.

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b) We have, [Sodium acetate]/[Acetic acid] = 1.74.

Let the concentration of [Acetic acid] = 'x' M then

[Sodium acetate] = '1.74 x' M

We want to prepare 0.1 M buffer and hence we write,

Molarity of Acetic acid + Molarity of Sodium acetate = 0.1

So, x + 1.47x = 0.1

2.74 x = 0.1

x = 0.1 / 2.74

x = 0.0365.

Hence,

[Acetic acid] = x = 0.0365 M and [Sodium acetate] = 1.74x = 1.74 x 0.036 = 0.0635 M

[Acetic acid] = 0.0365 M and [Sodium acetate] = 0.0635

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c)We have to take solutions as,

[Acetic acid] = 0.0365 M and [Sodium acetate] = 0.0635.

Let us calculate volume required of the each of the component,

i) For Acid componanant.

M1 = 0.1 M (Stock), V1 = ? M2 = 0.0365 M, V2 = 600.

M1V1 = M2V2

0.1 x V1 = 0.0365 x 600

V1 = 0.0365 x 600 / 0.1

V1 = 219 mL.

Note we can have volume of base componant = 600 mL - 219 mL = 381 mL

but shown calculations for trust on method used.

ii) For Basic componant,

M1 = 0.1 M (Stock), V1 = ? M2 = 0.0635 M, V2 = 600.

M1V1 = M2V2

0.1 x V1 = 0.0635 x 600

V1 = 0.0635 x 600 / 0.1

V1 = 381 mL

Answer :

On mixing 219 mL of 0.1 M Acetic acid to 381 mL of 0.1 M Sodium acetate we get 600 mL of Acetate buffer

with pH = 5.

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