[alley45]

Hey guys,

I know this lab has already been posted, but i was hoping someone can help me with this, i just seem to got a little lost.

Lab:  Determining Ka of Acetic Acid

Purpose:  The purpose of this experiment is to determine the molar concentration of a sample of acetic acid and to calculate its Ka.

Materials:
25 mL pipet and bulb      
burette        
2x150 mL beaker
125 mL Erlenmeyer flask  
acetic acid solution  
sodium hydroxide solution
phenolphthalein        
pH metre

Procedure:
1.   Record the molar concentration of the NaOH solution.
2.   Produce a table to record your data.  It should have one column for volume of NaOH added and one column for pH.
3.   Obtain 50 mL of acetic acid and place it into a beaker.
4.   Place 50.0 mL of NaOH into the burette.
5.   Pipet 25.0 mL of acetic acid into the Erlenmeyer flask.  Add two drops of phenolphthalein to the acid.
6.   Record the initial pH of the solution.
7.   Add 1.00 mL of NaOH from the burette to the Erlenmeyer until the pH reaches 5.00.  Record the volume to two decimal places.  Measure the pH of the solution each time you add NaOH.
8.   Above pH=5.00, add NaOH in 0.10 or 0.20 mL portions.  Record the volume at which the phenolphthalein turns pink.
9.   Continue to add NaOH until the pH reaches 11.00.  Above pH=11.00, add 0.10 mL portions until the pH reaches 12.00.


NaOH(ml)           pH
0.00           2.52
1.00           3.58
2.00           3.59
3.00           4.92
4.00           4.09
5.00           4.14
6.00           4.55
7.00           4.73
8.00           4.98
9.00           5.07
9.20           5.09
9.40           5.14
9.60           5.19
9.80           5.24
10.00           5.25
10.20           5.37
10.40           5.38
10.60           5.45
10.80           5.59
11.00           5.62
11.20           5.75
11.40           6.18
11.60           6.29
11.80           6.54
12.00           10.68
12.20           11.15
12.30           11.36
12.40           11.43
12.50           11.57
12.60           11.65
12.70           11.67
12.80           11.77
12.90           11.79
13.20           11.83
13.40           11.84
13.60           11.88
14.10           11.92
15.00           12.00


Questions.
1. Write the chemical equation for the neutralization reaction you observed

i got

CH3COOH + NaOH --> NaCH3COO + H2O

 2.Plot a graph of your data, with pH on the vertical axis and volume of NaOH on the horizontal axis.  Your graph should show a steep rise in pH as the volume of NaOH becomes enough to neutralize the acetic acid.  Take the midpoint of this steep rise and read off the volume of NaOH.  This is the volume of NaOH that was needed to neutralize all the acetic acid.  Compare the volume on your graph with the volume you recorded when the phenolphthalein indicator first turned pink.

which i did, my mid-point of the steep rise is approximately 12ml of NaOH and the pH when compared with  the table is at 10.66pH.

3. Find the molar concetration..this is where i got lost.

now i know i use the formula NaOH= M x L. For L i use 25ml and change it to liters, which gives me 0.025L but where do i get the mid point. Would it be the 12ml which i found as my mid point???  

[alley45]

sorry i meant..where do i get the number for mols, would it be from the mid-point from the graph

[Borek]

Steep part of titration curve is the end point.

[alley45]

so the end steep curve of titration is the numer of mols??

[alley45]

that doesn't make sense..how would i be able to get the number of mols from this lab..could you help me?

[Borek]

End point - as the name suggests - is where titration ends because there is stoichiometric amount of titrant added to titrated substance. In your case end point is where amount of added base is that needed to completely neutralize your acetic acid. You know amount of added base, you know reaction equation, you know everything needed to calculate amount of acid - simple stoichiometry.

[shawen]

hi alley
could i see you Determining the Ka for acetic acid lab
please and thank you