04_Titration_Experiments[1] - TITRATION EXPERIMENTS INTRODUCTION In this experiment we will learn how to prepare aqueous solutions and calculate their

04_Titration_Experiments[1] - TITRATION EXPERIMENTS...

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TITRATION EXPERIMENTS INTRODUCTION In this experiment we will learn how to prepare aqueous solutions and calculate their concentrations (in terms of molarity). An acid solution whose concentration is known with great precision will be made. Then, this solution will be reacted very carefully with a base solution whose concentration is known only approximately. By calculation, we will then determine the base solution's exact molarity. The acid-base reaction will be carried out as carefully as possible. The goal is to combine the exact stoichiometric amounts. In other words, we will try not to have either reactant be in excess. Such a careful addition of one reactant with another is called a titration. We will titrate using instruments called burets. In reality, there may be a slight excess of one reactant, but we try to keep this excess as low as possible. The point at which the stoichiometrically precise amounts have been combined is called the equivalence point. Once we have determined the exact molarity of our base solution, it will be saved and used in the next lab. Any solution whose concentration is known with great precision (usually four or more significant figures) is often referred to as a standardized solution. In this lab, we will first make an "standard" acid solution, which will then be used to standardize the base solution. The reason we do not make a standard base solution to start with is that the most common bases, such as NaOH and KOH, cannot be weighed out on a balance without absorbing some moisture from the air. Therefore, the mass of base cannot be known with great certainty. On the other hand, there are many solid acids which do not absorb moisture from the air in the process of weighing on a balance. The acid we will use is oxalic acid. The solid form of it is a hydrate, specifically H2C2O4·2H2O. The waters of hydration must be included when calculating the molar mass of the solid; however, once dissolved in liquid H2O, the waters of hydration are no longer considered. The base we will use is NaOH. The balanced equation for the reaction that will occur during the titration is: H2C2O4(aq)+ 2 NaOH(aq)Na2C2O4(aq)+ 2 H2O(l)According to this reaction, one mole of oxalic acid reacts with two moles of sodium hydroxide to produce one mole of sodium oxalate and two moles of water. Since there is no visual evidence that this reaction is happening, we will add an acid-base indicator and watch for its color to change to signal the end of the titration. In theory, this "end point" should be the equivalence point. After you have made your “standard” base solution you will use it to titrate a standard vinegar solution to determine the concentration of acetic acid (HC2H3O2) present in molarity.
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