1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  6. 6

Determining Ka by the half-titration of a weak acid

Extracts from this essay...


´╗┐Determining Ka by the half-titration of a weak acid To get the Ka of acetic acid, HC2H3O2 I will react it with sodium hydroxide. The point when our reaction is half-titrated can be used to determine the pKa. As I have added half as many moles of acetic , as NaOH, Thus, OH- will have reacted with only half of the acetic acid leaving a solution with equal moles of HC2H3O2 and C2H3O2-. Then I will use the Henderson-Hasselblach equation to get pKa. CH3COOH + NaOH H2O + NaCH3COO Results: Below is a table that summarizes our results for the reaction of 1M of acetic acid with 1Molar of NaOH which 50cm3 was used. The table shows the PH record at ½ equivalence and at equivalence. We also recorded the observations we saw during the reaction. PH ±0.1 Qualitative observations At ½ equivalence 5.0 When I recorded this, as we slowly added NaOH to the acid, there was a change of color from colorless to a very slight pink as the Phenolphthalein indicator changed color. At equivalence 8.9 As I added the acetic acid to 250 cm3 of reaction mixture, there was no color change. Also as we measured the PH, the PH changed slowly but then changed very quickly at the solution approached equivalence. At this time, the indicator turned pink, when equivalence was reached Calculating the PKa To calculate PKa, we will use the Henderson-Hasselbalch equation. Hence the calculations below show how using this we can calculate the PKa = PKa + But at the half equivalence, the concentration of acetic acid and its salt ion are the same.


Thus method 2, has excellent confident level for its extremely low %error. However the first factor that affects my confidence level is uncertainties. From the %error of PH, we got the %uncertainty of the PKa for method 1. Thus, we know, that from the total % error of 5%, 2% was made by systematic errors i.e uncertainties in this case. Thus the other 3% was caused by random error. Similarly, for method 2, we got % uncertainties for the PH by the volume measure of NaOH. This %error was 4.2%, meaning 4.2% of the total error was caused by systematic error of the graph. Clearly this is bigger than the total %error of 0.84%. Thus this means that actually, even if our graph has on the y-axis an uncertainty of ±0.4, this is an over-estimate. This is since, while we can read a value off with this uncertainty, it can still be very close to the actual half-equivalence PH. Thus this increases my confidence level, as it shows, that the systematic error of the graph y-axis uncertainty is very limited. Thus the biggest error is random error. This occurs when estimating the equivalence point from the titration graph, which is random error as itâs an estimate of the steepest point and hence has no uncertainty. Thus as we could underestimate or overestimate this value, it creates error, as we calculated the half equivalence from it. In this case, clearly we overestimated it as; the PKa from this method is higher than the actual one.


Also, as the colorimeter is accurate, systematic error will also be limited. Another way we can improve is in the systematic errors. The first problem was measuring accurately volumes. As the pipettes had big uncertainties, the volume recorded had high %uncertainties. If we however use micropipettes, which have ±0.01 cm3 uncertainties, our volumes will be extremely accurate. Hence %uncertainties will be minimal. Also micropipettes allow much easier for drops of base to be dropped. Thus the significance of this improvement is that when we measure volumes, the equivalence point will occur, more exactly as we will be less likely to overshoot the solution. Finally to solve the inaccurate measurements of PH we can get a PH sensor and data logger. These do real-time measurements and will state the PH with less uncertainty. It will also provide an alternative method for calculating the half-point. As the data logger draws the graph of the titration done, it can calculate the point with the highest gradient. Thus this will be the equivalence point. Hence we can calculate the PH at half the equivalence point of the graph as this is half the volume of base at equivalence. Thus clearly calculating a very accurate PH from the curve. The significance of this will be that it is a major improvement on method 2 and 1 as it is not qualitative. Thus it does not allow for human error. Hence as the sensor is also very accurate systematic error will also be limited as well as random error. Thus this method will get a very accurate PKa with low systematic and random errors. ________________ [1] IB chemistry data booklet pg 13

The above preview is unformatted text

Found what you're looking for?

  • Start learning 29% faster today
  • Over 150,000 essays available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Over 180,000 student essays
  • Every subject and level covered
  • Thousands of essays marked by teachers

Related International Baccalaureate Chemistry

  1. A Comparison of Strong and Weak Acids and Bases

    1.00 2 HCl(aq) 1.00 3 CH3COOH(aq) 1.00 4 H2SO4(aq) 1.00 5 HCl(aq) 0.10 6 HCl(aq) 0.01 Balanced equations: a) 2HNO3(aq) + CaCO3(s) CO2(g) + H2O(l) + Ca(NO3)2(aq) b) 2HCl(aq) + CaCO3(s) CO2(g) + H2O(l) + Ca(Cl)2(aq) c) 2CH3COOH(aq) + CaCO3(s)

  2. alkali titration

    / 0.0125±2.15% MM = 120 ±(1.33+2.15)% MM = 120 ±3.48% m = n x MM m = 1 x 120±3.48% ? mass of one mole of oxalic acid = 120g ±3.48% 6. How many molecules of water of crystallisation are present in one mole?

  1. Acids/Bases Design Lab. How does a change in the pH value of a solution ...

    (The solution was decanted into a waste beaker) The remaining zinc was then placed (with the use of tongs) in another clean, and dry 50cm3 beaker that was already massed, labeled 'Beaker BRT1', in which it was washed with distilled water from distilled water bottle pumps.

  2. Chemistry Titration Acid Base Lab

    This burette would ensure that a considerably small amount of sodium hydroxide would be discharged from the device at most. A button will be pressed to stop the base from entering the beaker containing the acid electronically. The accuracy of this instrument is within ±0.2%, and precision is better than

  1. Strong and Weak Acids And Bases

    ± 0.04ml Observation Hydrochloric Acid 10.00 13.00 22.97 9.97 Pale Pink Ethanoic Acid 10.00 23.30 34.57 11.27 Pink Experimental Values HCl (aq) + NaOH (aq)--> H2O (l) + NaCl (aq) Molarities 0.2M 0.2M Volume 9.97ml ? It is a one-one ratio so: 0.20molHCl= 0.20mol NaOH= 9.97ml NaOH was needed in experiment CH3COOH (aq)

  2. Research Question By measuring the pH value of the acetic ...

    ] / Molarity of solution pKa = - log10 (Ka) Ethanoic acid is a typical weak acid (CH3COOH) and will be used during this experiment. As temperature and the environmental conditions is a major factor and can immensely affect this experiment, this experiment will be carried out at standard lab conditions.

  1. Chemistry: Strong Acid and Weak Base Titration Lab

    Cb = (0.84 ± 1.69%) / (9.2 ± 1.09%) Cb = 0.0913 ± 2.78% ï  0.0913 ± 0.00254M is the concentration of the base for trial 1 Theoretical Base Concentration = 0.1 ± 0.0005 M Experimental Base Concentration = 0.0913 ± 0.00254 M Trial 2 Calculation: First find change of volume of the acid used up in the titration:

  2. Buffers. This experiment lets us to know more about the buffer solutions and how ...

    Record the number of drops needed. Apparatus: Beakers (50ml) 8 Test-tube rack 1 Dropper 2 Graduated cylinder (10ml) 2 Glass stir bar 1 pH meter 1 Chemicals: Distilled water Universal pH paper (pH 1-14) 1M hydrochloric acid, HCl 1Msodium hydroxide, NaOH 0.1M sodium carbonate, Na2CO3 0.1M sodium hydrogen carbonate, NaHCO3 0.1M sodium hydrogen phosphate, Na2HPO4 0.1M

  • Over 180,000 essays
    written by students
  • Annotated by
    experienced teachers
  • Ideas and feedback to write
    your own great essays

Marked by a teacher

This essay has been marked by one of our great teachers. You can read the full teachers notes when you download the essay.

Peer reviewed

This essay has been reviewed by one of our specialist student essay reviewing squad. Read the full review on the essay page.

Peer reviewed

This essay has been reviewed by one of our specialist student essay reviewing squad. Read the full review under the essay preview on this page.