Chemistry 151 Weak Acid Titration

Topics: PH, Acid dissociation constant, Titration Pages: 9 (2591 words) Published: November 21, 2010
Weak Acid Titration

Abstract: Our method for determining the unknown weak acid was to determine the equilibrium constant K from the molecular weight of the weak acid from our titration data. In this lab the acid Potassium hydrogen phthalate and two unknown acids were titrated. We determined the molar mass of the Potassium hydrogen phthalate, for the unknown acids we calculated the molar mass and the Ka values. We used NaOH as the known base for titrating in all three of the titrations. Our ka value based on our titration curve of KHP was 6.9 (the equivalence point where the pH equals the pKa) and our unknown acids were determined to be Hexanoic Acid (with an actual molar mass of 116.6g) with a determined mass of 116.1g. The unknown A3 acid turned out to be 2-oxyglutaric Acid with a determined molar mass of 146g/mol (and an actual molar mass of 147.15g/mol).

Introduction: Titration is a general class of experiment where a known property of one solution is used to infer an unknown property of another solution. In acid-base chemistry, we often use titration to determine the pH of a certain solution. By investigating a weak acid titration curve we learned more about the balances involved in equilibrium. The titration curve allows us to see pH versus volume of added titrant, our titrant being a strong base (NaOH). Our plan for the first week was to standardize the sodium hydroxide solution by determining the equivalence point (total moles of HA present=the total moles of base added). We then will perform a weak acid titration with and without a pH meter. We then plotted our titration curve and determined the Ka of the weak acid. At the point of the graph in which the slope of the line is almost vertical the middle of that range is the equivalence point. In this lab we started with the KHP sample and titrated three different trials that were within 1% difference in the ratio of mol NaOH: mol KHP, having this kind of values shows our precision in the titration process. For the unknown acids we determined the equivalence point and using the known moles in the sample and the weight of the sample we can determine the molar mass of that sample. Molar Mass is a unit with sub-units(g/mol) so by dividing the mass of the sample by the titrated moles we can determine the molar mass. To determine Ka values the equation pKa =-log Ka, along with the relationship between pKa and pH at the half equivalence point (they’re equal). This relationship is based on the fact that equilibrium is at the half equivalence point. Some questions we had to answer for this lab were: How is it possible to determine the amount and identity of something based on its acid-base reactivity? How can you determine the molarity of a standardized NaOH solution? What does the endpoint of a titration mean in terms of acid/base and pH? How can pH help determine the pKa of certain acid? Experimental:

1. Using the stock 3M NaOH prepare 1L of 0.1M NaOH following instructions in our notebook. Fill the 1L stock bottle with this solution and store it in the locker to use leftovers for performing the standardization. 2. Weigh out between 0.2g and 0.3g oh Potassium Hydrogen Phthalate (KHP) as accurately as possible. 3. Transfer the solid KHP into an Erlenmeyer flask. Add about 50mL of distilled water. Swirl it gently to dissolve the acid. This is a slow process. Swirl the flask for several seconds every minute or two until solution is complete. Add 2-3drops of indicator solution (Bromothymol Blue irritant). 4. Set up the stir plate and place a stir bar in the solution. Make sure the buret is properly filled with the diluted NaOH solution. Use the wash bottle to wash excess NaOH hanging from the buret tip. Read and record the solution level in the buret. Place a piece of white paper under the flask to better see what is happening in the flask. 5. Begin adding NaOH while the stir bar mixes the solution. At some point a green to blue color will begin...
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