Can someone help me solve the problems showing the calculations so I can learn h

Question

Can someone help me solve the problems showing the calculations so I can learn how to do them? Thank you.

1. You have a 2 M stock solution of monobasic sodium phosphate and a 2 M stock solution of dibasic potassium phosphate. You need 2 liters of 0.05 M phosphate buffer, pH 7.1. Starting with the two stock solutions, explain how you would prepare the desired buffer. 2. You need to prepare 250 mL of 0.1 M sodium acetate buffer, pH 5.2. You have available solid sodium hydroxide and glacial acetic acid. The pKa of acetic acid is 4.76, and the density of glacial acetic acid is 1.049 g/mL. Explain how you would go about preparing the buffer.

Explanation / Answer

acid = monosodium phosphate

salt = disodium phosphate

moles of buffer (salt + acid) = 2 x 0.05 = 0.1

pH = 7.1

pKa2 = 7.2

pH = pKa + log [salt / acid]

7.1 = 7.2 + log [salt / acid]

[salt/acid] = 0.794

salt + acid = 0.1

0.794 acid + acid = 0.1

acid = 0.056 moles

salt moles = 0.044 moles

0.056 moles sodium monophosphate and 0.044 moles sodium diphosphatemixed to make 2 L , 0.05 M buffersolution

(2)

buffer milimoles = 250 x 0.1 = 25 = acid + salt

acid = acetic acid, salt = sodium acetate

pH = pKa + log [salt/acid]

5.2 = 4.76 + log [salt/acid]

salt/acid = 2.75

salt = 2.75 acid

2.75 acid + acid = 25

acid = 6.67 millimoles

salt = 18.33 millimoles

6.67 millimoles   acetic acid and 18.33 millimoles sodium acetate mixed to make this buffer

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