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# Investigate the factors affecting Ka of a weak acid.

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Introduction

Name: Jonathan Tam Class: 12A Chemistry Experiment Report * Planning (a), Planning (b), Data Collection, Data Processing and Presentation, Conclusion and Evaluation Planning (a): To investigate the factors affecting Ka of a weak acid The acidity of an acid is determined by the dissociation of H+ ions in the acid, ie the higher it dissociates the more acidic. In other words, the concentration is not a factor affecting the acidity, but the degree of dissociation. As the dissociation is at equilibrium for all acids, factors such as temperature should have an effect of shifting the equilibrium position, ie temperature affects the degree of dissociation. e.g. If temperature increases I suspect [H+] will increase because the equilibrium will shift to the left (dissociation of ions is endothermic) as temperature increases. So temperature is one factor to investigate. Since the formula of pH is , pH is affected by [HA], meaning the concentration of acid determines pH. So if an increase in temperature can increase [H+], then at the same time it can decrease [HA]. If [HA] decreases, Ka increases (Ka tells us the acidity). pKa decreases (since it is in a negative logarithm relationship). A simple graph can illustrate this hypothesis. Also, organic acids contain a carbon chain and a H+ ion that will dissociate. If the length of the carbon chain increases, the dissociation of H+ might change. ...read more.

Middle

- Added into 1000 cm3 of distilled water Data Processing and Presentation: From our starting formula of pH: So in order to get pKa we can simply substitute in the value of pH and [HA] into the formula where HA is the organic acid used in the experiment. Temperature (?0.5?C) pH (?0.005) Ka pKa 6 2.83 4.38E-05 4.35897 7 2.79 5.26E-05 4.27897 10 2.63 0.00011 3.95897 13 2.55 0.000159 3.79897 15 2.51 0.000191 3.71897 17 2.48 0.000219 3.65897 19 2.45 0.000252 3.59897 20 2.43 0.000276 3.55897 22 2.41 0.000303 3.51897 26 2.35 0.000399 3.39897 Percentage error of concentration of HCOOH: = Percentage error of moles + Percentage error of volume of water = 0.005 / (0.05*1*46) * 100% + 0.2 / 1000 * 100% = 0.22% + 0.02% = 0.24% Highest percentage error of pH: = 0.01 / 1.87 * 100% = 0.53% Highest percentage error of pKa: = -log [ 10^-(percentage error of pH) + percentage error of concentration ] = -log [ 0.03% + 0.24% ] = 0.57% Highest percentage error of temperature: = 0.5 / 6 * 100% = 8.33% It is easier to see the relationship of pKa against temperature by plotting a graph. Conclusion and Evaluation: Conclusoin: pKa decreases as temperature increases. pH decreases as temperature increases. The acidity of acid increases as temperature increases. Evaluation: This proves the hypothesis above. ...read more.

Conclusion

The concentration will then stay relativiely stable. We can also use a higher concentration to make the concentration constant. However the pH formula will only work in a low concentration, we cannot use a high concentration. This means in this experiment we will have to accept a higher percentage error of concentration and a higher percentage of impurities. The reading of temperature is the one with the highest uncertainty. The percentage error of temperature, when approaching zero, will increase (as high as 8.3%). Then, the accuracy of the thermometer is much lower than that of the pH meter. So if we want a more accurate graph, we can use a more accurate apparatus to measure temperature e.g. thermocouple or a mercury-thermometer. As mentioned above, the probe might contain some remaing buffer solution when we put it into the acid. So if we want to improve on this, we should either leave the probe in the acid without taking it out or we clean it every time when we are using it. Since the probe can be easily broken, it is better to leave it in the acid until the whole experiment is finished. Although not likely, the buffer solution might not be exactly pH 4.00. However we assumed that it is 4.00 and calibrated the pH meter using this. If the buffer solution is not exactly 4.00, then it will affect all our readings of pH. Although this will only shift our graph up a little bit, it is still worth noting. 1 ...read more.

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