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Research Question By measuring the pH value of the acetic acid using a pH meter at standard lab conditions, will increasing the concentration of the acid affect its experimental determined Ka and therefore its calculated pKa?

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High Level Chemistry Design Investigate the effect of concentration on the experimentally determined pKa of a weak acid Background Information An acid is a proton donor which means it is able to donate a proton to another substance. The substance that accepts the proton from the acid is known as a base. An acid can be categorised into being a weak or a strong acid. A weak acid is an acid which does not completely dissociates. In other words, a weak acid does not completely ionise when it is dissolved in aqueous solutions. The pKa, also known as Dissociation Constant, is a value that determines the strength of an acid or base. The pKa value of an acid or base is largely related to the pH of the substance. [1] pH is a value given between 1 and 14 to identify the concentration of hydrogen ions in an aqueous solution. pH is found by calculating the negative logarithm of the hydrogen activity in the solution. [3] pH = -log aH where aH denotes activity of hydrogen ions The dissociation can be written as Ka = [A-][H+] [HA] The unit of Ka is molar per decimetres cubed (mol/dm3) Like the relationship between pH and the hydrogen ion activity, pKa is also found bye taken the negative logarithm of Ka. ...read more.


Also ensure that the humidity of the room is kept constant for the whole time the experiment is conducted The temperature of the acetic acid when tested As experiment will be carried out at SLC the temperature of the acetic acid is likely to be 25�C - room temperature. This temperature must be kept controlled for the whole experiment as temperature can affect pH of the solution. The temperature of the solution will be measured before being tested. If acid solution is not 25�C, leave on bench for a while until temperature drops/raises to the required temperature of 25�C Contamination of equipments Distilled water will be used to thoroughly clean out the equipments before starting the experiment. Different equipments will be used for different substances. Equipments that will be reused (thermometer and pH meter) will be thoroughly cleaned with distilled water before coming into contact with the other substances The materials and apparatus used for the experiment All equipments of the same measurements, sizes and brands will be used for this experiment Amount of acetic acid being tested The amount of acetic being tested is 25dm3. A measuring cylinder will be used measure the volume of acetic acid Experimenter/s Only one and the same experimenter will carry out this experiment Materials - 25mL of 0.5 Molar of Acetic acid - 25mL of 1.0 Molar of Acetic acid - 25mL of 1.5 Molar of Acetic acid - ...read more.


Use the thermometer to measure the temperature of the five different acetic acid and ensure that all acid is 25�C (298 Kelvin). Use distilled water to clean the thermometer before measuring the temperature of the next acid 6. If acid solution is not yet 25�C, leave on bench for a while until temperature drops/raises to the required temperature of 25�C 7. Use the distilled water to clean off any residues on the end of the pH meter. Clean the parts that will make contact to the acid solution Carrying out experiment 1. Turn on the pH meter 2. Place the pH meter into the first beaker - 0.5M 3. Wait until a distinct, stable pH value is shown on the screen. Record the pH value onto data table 4. Use the distilled water to again clean the end of the pH meter 5. Repeat step 3-5 until there is three pH values recorded for the specific molarity acetic acid 6. Use the distilled water to thoroughly clean the ends of the pH meter 7. Repeat steps 3-7 for the remaining independent variables (1.0M; 1.5M; 2.0M; 2.5M) 8. After completing experiment, dispose the acetic acid into the sink and wash out all equipments with tap water except for the pH meter 9. Use the distilled water to thoroughly clean the part of the pH meter that made contact with the substances during the experiment ?? ?? ?? ?? Kathy Nguyen Page 3 of 7 ...read more.

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