Experiment 10: Buffer Preparation Exp 10: Preparation of a Buffer Solution Objec

Question

Experiment 10: Buffer Preparation Exp 10: Preparation of a Buffer Solution Objectives Prepare a buffer two ways o Mixing acid/base with its conjugate to reach desired ratio o Titrating acid/base with strong base/acid to reach desired ratio Introduction In this experiment you will prepare a buffer of a given pH using the materials in the table below. You will need to calculate the relative concentrations of each component and then prepare a solution containing both ingredients using two different methods: 1) prepare the mixture by preparing a solution of an acid or base & its conjugate salt 2) prepare the mixture by preparing a solution of an acid or base and adding HCI or NaOH to form the correct amount of its conjugate in situ. Prior to lab vou should look up the pka of the following acids, You will have these materials to prepare your buffer. Chemicals pKi pK, pkj Acetic acid Sodium acctate Ammonium Hydroxide Ammonium chloride Citric acid monohydrate/ Sodium citrate monobasic Sodium citrate dibasic/Sodium citrate tribasic Phthalic acid /Potassium hydrogen phthalate Sulfuric acid/Sodium hydrogen sulfate/sodium sulfate Phosphoric acid/ Sodium phosphate, monobasic/ Sodium phosphate, dibasic/ Sodium phosphate tribasic n/a Hydrochloric acid IM Sodium hydroxide IM . Acids are sparingly soluble in water, can dissolve in water by first dissolving in small amount of methanol, 5-10 mL or by reaction of the base with strong acid. Alternatively, the acid is prepared in-situ by addition of acid to conjugate base which is the preferred method. 1M solutions are used for preparations Pre-Lab Questions The first two questions pertain to the preparation of buffer using separate amounts of a weak acid and the salt of its conjugate base. Calculate the amounts of each needed to achieve the required concentrationms or pH 1. You need to prepare a 100 mL solution that is both 0.03 M sodium acetate and 0.02 M acetic acid. You have solid sodium acetate trihydrate and I M acetic acid solution. Show calculations and describe how to do this.

Explanation / Answer

1. molarity of buffer is addition of molarities of acid and base. so molarity of this buffer = 0.03+0.02= 0.05M

volume needed is 100ml

0.05=n/0.1; n= 0.005

3. sodium bicarbonate molecular weight 84

you need 0.05M, molarity = moles/vol in litre

0.05= n/0.1

n=0.005

0.005=m/84

m= 0.42g sodium bicarbonate is needed

4. the reaction between sodium bicarbonate and hcl is as follows

Na2HCO3 + HCl ------> NaCl + H2CO3

(the carbonic acid formed is howeevr unstable and further breaks down to carbon dioxide and water)

since we have Na2HCO3 0.05M the carbonic acid produced will be 0.05M too but the molarity can be reduced to 0.02M by adding HCl further.

lets assume you want to take 10ml of sodium bicarbonate

so, M1V1=M2V2

0.05*10= 1*V2

V2= 0.5ml

now, 0.02*10=1*V2

V2= 0.2ml

so you need to add 0.5+0.2=0.7ml HCl

5.total volume will be 10.7ml now so, molarity of HCl will be

M2= M1V1/V2 = 1*10/10.7= 0.93

and carbonic acid is 0.02M as we calculated before

6. H3PO4 <=> H2PO4- + H+

  H2PO4- <=> HPO42- + 2H+

HPO42- <=> PO43- + 3H+

7. since phisphoric acid has 3 step dissociation, it has 3 pKa values

A bufferfrom it cab be made from pH ranging 2-12 (nearby). to make a buffer of pH 12.2 you need to use the handelson-hasselbalch equation takinh th nearest pKa value pKa3= 12.3

Hence, the third chemical equation will be of use here. and the two components needed will be monosodium phosphate and its conjugate base disodium phosphate. NaOH can be used to make the pH more alkaline.

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