Introduction and Objective An acid ionization constant Ka also known as acid dissociation constant is a value that can indicate how weak or strong an acid is since it acts as a measure of the acidity of a specified solution It is the equilibrium constant for the break up of an acid or the value of a chemical reaction quotient at equilibrium for a dissociation chemical reaction in the context of acid base reactions The common dissociation equation of an acid when its at equilibrium is represented as HA aq H2O A aq H3O aq And the reaction is considered to be at equilibrium when the concentration of the generic acid HA the conjugate base of the acid A and the hydronium ion H quit on changing during time Ka is characterized by K A H3O H2O HA noting that the concentration of water will remain constant at equilibrium in a dilute solution And since the concentration of water is constant it is allowed to be a part of a new equilibrium constant known as the dissociation constant Ka Ka K H2O A H3O HA For practical reasons it is more convenient to use the logarithmic constant pKa It is commonly used as it facilitate mathematical calculations and turns large numbers into smaller and more suitable ones pKA could be calculated by taking the negative log of a given acid dissociation constant Ka pKa log Ka It presents how strong or weak an acid is
The higher the pKa the weaker the acid and the lower the pKa the more stable the conjugate base is This experiment will determine the acid dissociation constant of Ascorbic acid which is a very known organic acid for its antioxidant properties The determination of the acid dissociation constant is found by figuring out the pH of a solution obtained by mixing special water with the ascorbic acid and then using it to find the concentration and the Ka of the organic acid solution After that the strength of this acid will be compared with another organic acid known as citric acid Reaction C6H8O6 H2O C6H7O6 H3O Materials and Chemicals Safety goggles Lab coat One Digital scale One scoop Two Beakers pH meters 0 01 moles of ascorbic acid 100 ml of special water which is a green mixture of BTB tap water and deionized water Method Start by wearing both the lab coat and the safety goggles Using the formula m n M Convert 0 01 moles of ascorbic acid to grams On the digital scale place one beaker and start scooping out 100 ml of the organic acid When the required amount of the ascorbic acid is taken out move it to the second beaker containing 100 ml of the special water obtained by mixing specific amounts of BTB tap water and deionized water
Now mix properly Finally find the pH of the obtained solution using the pH meter Results pH of solution 2 61 C6H8O6 C6H7O6 H3O Initial Concentration 0 1 Change in concentration x or 2 45 x 10 3 x or 2 45 x 10 3 x or 2 45 x 10 3 Equilibrium 0 1 x or 0 09755 X or 2 45 x 10 3 X or 2 45 x 10 3 C n v concentration number of moles volume 0 01 mol 0 1 dm 3 0 1 M The pH formula pH log H3O H3O 10 PH 10 2 61 2 45 x 10 3 Ka C6H7O6 H3O C6H8O6 Ka x x 0 1 x ka 2 45 x 10 3 2 45 x 10 3 0 09755 6 153 x 10 5 Pka log ka 4 2 Ka pKa Ascorbic Acid C6H8O6 6 153 x 10 5 4 2 Citric Acid C6H8O7 7 244 x 10 4 3 14 Discussion As shown in the data the acid dissociation constant Ka of the ascorbic acid was calculated to be 6 153 x 10 5 resulting in a pKa value of 4 2 pKa logKa To calculate the Ka value the concentrations of the products of the reaction were multiplied and then divided by the product of of the concentrations of the reactants Concerning citric acid the Ka and pKa values were obtained from trusted sources where Ka 7 244 x 10 4 and pKa 3 14 Comparing these values we see that the Ka value for ascorbic acid is less than that of the citric acid In addition the pKa value for citric acid is less than that of the ascorbic acid The strength of an acid refers to the extent that it dissociates into ions when dissolved in water When acids and water react hydrogen ions or hydronium ions are produced by those acids Since Ka is a measure of the strength of an acid the larger this number gets the larger likelihood the reaction has to drive in the direction of the products side and the stronger tendency the acid has to donate its H resulting in a stronger acid Additionally and since the pKa value is calculated by taking the negative logarithm of the Ka value the smaller the pKa value is the more acidic a substance is i e stronger acid All these information signifies that citric acid is stronger than ascorbic acid Sources of errors and improvements Some experiments may be error free but mistakes do happen often in labs Concerning this experiment the accuracy of the results could have been affected by some factors or lack of proper materials For example if a 4 digits scale was used when measuring the weight of the organic acid instead of a 2 digits scale the results could have been more precise during calculations Additionally the accuracy of the pH meter was questionable and agreed to be not very precise as the numbers were moving up and down repeatedly Overall the experiment was successful seeing that intelligible results were acquired
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