CHEM 2310-210 ANALYTICAL CHEMISTRY Summer 2016 Experiment 2 Reading Assignment:

Question

CHEM 2310-210 ANALYTICAL CHEMISTRY Summer 2016 Experiment 2 Reading Assignment: Lecture Text, chapter 13, and chapters 9 and 14 The principles of acid-base titrations can be found in any standard textbook of analytical chemistry. Briefly described: in aqueous solution, an acid is a proton (H") donor and a base isa proton acceptor. For example, HCl, an acid, donates a proton to water HCI (+H0 (-C (a) O(aq) NHs, a base, accepts a proton from water. NHs(ag)+H:o 0+NH (a)+OIl (aq) Am acid-base titration involves the reaction between the HO and Ot in a systematic manner. Daring the titration, small increments of the reagent solution, called the titrant, are added to the analvte until the reaction is complete. In a volumetric acid-base titration, the titrant is added from a baret in small volume increments to the analyte solution taken in an Erkenmeyer flask. Prior to the beginning of the titration, a few drops of a solution of an indicater is added to the analyte solution. The indicator, which itself is either a weak acid or a weak base, changes color sharply when the acid-base reaction is just complete. The volume of titrant th corresponds to the color change is called the snd-peint of the titration. An acid-base titration is generally performed to determine the unknown concentration of the analyte. If the concentration of the titrant and the stoichiometry of the acid-base reaction is known, the concentration (oe the percent composition) of the analyte can be calculated from the end-point of the titration. In any acid-base titration, either the analyte is an acid and the reagent a base, or vice-versa. In this experiment, you will determine the concentration of a hydrochloric acid solution by titrating with a standardized sodium hydroxide solution. You will report the concentration of the HCI solution in molarity along with its absolute and percent relative standard deviations of your measurement In aqueous solution, HCl is a strong acid and NaOH is a strong base. The reaction between them is called neutralization and can be represented as HCI (ag) +NaOH (ag NaCaq) +HO(0) The reaction stoichiometry is: 1 mole HCI-1 mole NaOH The NaOlH solution (titrant) is dispensed from a buret and the HCl solution (analyte) is taken in a flask, to which a few drops of the indicator is added prioe to the titration. When the titration is complete (i.e. at the end-point),

Explanation / Answer

In a titration, the solution that is neutralized, for example the acid solution is placed in a flask or beaker with a few drops of an acid-base indicator. The other solution used in the titration, the base is added to the burette and is called titrant. The titrant is added to the acid first rapidly and then drop by drop until the equivalence point is reached. The equivalence point is recognized by the change of color of the acid-base indicator. The point where the indicator changes color permanently is called the endpoint of the indicator.

We use this kind of titrations when we need to determine the concentration of an acid solution, and when the concentration of the base solution is known.Considering the stoichiometric ratio between the acid and base such concentration can be calculated. At the equivalence point of the reaction, when all moles of the base have reacted with all moles of the acid, we can say that the moles of acid and the moles of the base are equal, because the reaction has taken place completely. From there and knowing that the moles are the result of multiplying the concentration in moles / L by the volume of acid and base used in the experience, we can calculate the concentration of the acid in units of moles / L.

All this is possible if the concentration of the base is precisely known. First of all we need to perform the titration of the base with a primary standard solution, this solution is usually potassium hydrogen phthalate (KHP), whose concentration is known, it is a substance that is stable and has a high molecular weight.

In the first stage, the sodium hydroxide solution with which we will titrate the hydrochloric acid is prepared. Such a solution of sodium hydroxide is titrated with potassium hydrogen phthalate (KHP) in order to know its exact concentration. An the equivalence point the moles of potassium hydrogen phthalate are equal to the moles of sodium hydroxide, from there you can calculate the concentration of sodium hydroxide that has just been prepared and then this value will be used on the titration of the hydrochloric acid with sodium hydroxide (of known concentration). At the equivalence point the moles of hydrochloric acid are equal to the moles of sodium hydroxide, allowing in this case the calculation of the concentration of hydrochloric acid.

Titrations are performed at least three times in each case to ensure reproducibility in the results and we apply the Q test with 95% confidence to the measurements to eliminate any outcome that might cause unnecessary errors in the measurement.

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