xperimen Preparing a Buffer Experiment Inventory Materials 25 mL 4.5% Acetic Aci

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xperimen Preparing a Buffer Experiment Inventory Materials 25 mL 4.5% Acetic Acid (CM02) 5 g Sodium Acetate (C,H,Nao,) Permanent Marker Labware (6) 100 mL Beakers (1) 100 mL Graduated Cylinder (2) Pipetters (1) Weigh Boat (1) Glass Stir Rod (1) pH Meter (1) Digital Scale Distilled Water Note: You must provide the materials listed in "red EXPERIMENT 1: PREPARING A BUFFER In this experiment, you will learn how to prepare a buffer PROCEDURE 1. Place a weigh boat on the digital scale and press the O/T button so that your scale reads 00 g 2. Weigh 4.0 g of sodium acetate (C,H,NaO,) and record the exact mass in Table 2 on the Experiment 1 Data Sheet 3. Transfer the sodium acetate into a 100 mL beaker 4 Use te 100 mLgraduated cylinder to measure and pour 100 mL of distiled water into the beaker to dissolve to sod. um acetate. 5. Stir the solution with a glass stir rod until you have a homogeneous solution. 6. Calculate the molarity of this solution and record it in Table 2 on the Data Sheet. Then label the beaker with the correct concentration and name of the solution. Hint: The molar mass of C,HNao, is 82.03 g 7. Use a permanent marker to label tive clean 100 mL beakers as A, B, C, D, and E 8. Use a pipette to measure 9. Use a clean pipette to transter 5 mL of sodium acetate into Beaker A as well 10 Folow Table 3 on the Experiment 1 Data Sheet to prepare the remaining buffer solutions for Beakers B and transfer 5 mL of acetic acid into Beaker A Use separate pipeties for the sodium acetate solution and the acetic acid solution C, D, and E 11. Remove the cap from the end of the pH meter, and rinse the probe with approximately 1 0 mL of distiled water Turn on the pH meter by moving the switch at the top to the right, and then caibrate your pH meter Z 12 Place the meter in each of the buher solutions to obtain the pht. Record your readings in Table 3 on the Data Sheet Hate You should rinse the phi probe well with distililed water in between each pH reading 13 Reserve the remaining sodium acetate sction tr use in Experiment 2 You wil need a minimum of 20mLfor the ex- peniment

Explanation / Answer

acetic acid [HOAc] molarity = 45 g/60 g/mol x 1 L = 0.75 M

sodium acetate [NaOAc]

1. [NaOAc] molarity = 4.0 g/82.03 g/mol x 0.1 L = 0.49 M

2. pH calculation

Buffer A,

[HOAc] = 0.75 M x 5 ml/100 ml = 0.0375 M

[NaOAc] = 0.49 M x 5 ml/100 ml = 0.0245 M

pH = pKa + log(NaOAc/HOAc)

      = 4.75 + log(0.0245/0.0375) = 4.56

Buffer B,

[HOAc] = 0.75 M x 5 ml/100 ml = 0.0375 M

[NaOAc] = 0.49 M x 1 ml/100 ml = 0.0049 M

pH = pKa + log(NaOAc/HOAc)

      = 4.75 + log(0.0049/0.0375) = 3.87

Buffer C,

[HOAc] = 0.75 M x 10 ml/100 ml = 0.075 M

[NaOAc] = 0.49 M x 1 ml/100 ml = 0.0049 M

pH = pKa + log(NaOAc/HOAc)

      = 4.75 + log(0.0049/0.075) = 3.56

Buffer D,

[HOAc] = 0.75 M x 1 ml/100 ml = 0.0075 M

[NaOAc] = 0.49 M x 10 ml/100 ml = 0.049 M

pH = pKa + log(NaOAc/HOAc)

      = 4.75 + log(0.049/0.0075) = 5.56

Buffer E,

[HOAc] = 0.75 M x 1 ml/100 ml = 0.0075 M

[NaOAc] = 0.49 M x 5 ml/100 ml = 0.0245 M

pH = pKa + log(NaOAc/HOAc)

      = 4.75 + log(0.0245/0.0075) = 5.26

3. Percent error

Buffer A,

Percent error = (4.56 - 4.1) x 100/4.1 = 11.22%

Buffer B,

Percent error = (3.87 - 3.7) x 100/3.7 = 4.59%

Buffer C,

Percent error = (3.56 - 2.8) x 100/2.8 = 27.14%

Buffer D,

Percent error = (5.56 - 4.7) x 100/4.7 = 18.30%

Buffer E,

Percent error = (5.26 - 4.6) x 100/4.6 = 14.35%

4. Potential error sources in the experiment. The transfer of volumes of sodium acetate and acetic acid are not accurate and some volumes remained in the initial pipet. The dilution was not accurate. The pH meter was not calibrated well.

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