Experiment 16 Rase Titration and Molecular Weight of Acid-Bas an Unknown Acid EX

Question

Experiment 16 Rase Titration and Molecular Weight of Acid-Bas an Unknown Acid EXPERIMENT: Determine the molecular weight of an unknown acid by acid with standardized sodium hydroxide oF os reacts with a base to form a salt plus water. n with a base such as sodium hydroxide. An acid with one reactive Many acids have one proton An acid ule or formula unit is called a monoprotic acid or imo a monobasic acid. A or a dibasic acid has a maximum of two reactive protons per molecule. A e cid or a tribasic acid has a maximum of three reactive protons per molecule. are also examples of acids which have four and even more reactive protons per In contrast, most useful bases have one or two available hydroxide ions per diprotic acid formala unit. rocess for ascertaining the exact volume of a solution that reacts rically according to a balanced chemical equation with a given volume of a ution. One reagent is added by means of a buret until the endpoint is reached Turatio on is the for nd sol The endpoint occurs when stoichiometric quantities of reagents have been mixed. The endpoint of a titration for reactions of acids and bases is usually indicated by a third reagent, the indicator, which has an abrupt and distinctive color change at the hydrogen jon concentration which is present after the stoichiometric reaction has occurred. The ypical indicator for titrations of strong acids and bases is phenolphthalein. Phenolphthalein is colorless in acidic solution and red (pink in dilute solution) in basic solution. Since it is much easier and distinctive to see a color change from colorless to pink rather than from red to pink to colorless, sodium hydroxide is added by means of the buret to the acid, usually contained in an Erlenmeyer flask. In this experiment, you will standardize (determine precisely the concentration) a solution of sodium hydroxide, NaOH, using oxalic acid dihydrate, HC,0,2H,0, as a primary standard acid. A primary standard acid is a solid acid whose mass is an accurate tmeasure of the number of moles of protons the acid will funish. Oxalic acid, H,C,0, 257 EXPERIMENT 16

Explanation / Answer

The size of graduated cylinder you use for this reaction depends on the amount/volumes of the reactants that are being used.

Since the experiment involves the standardization of a NaOH solution, which is a strong base, it is prudent to use a relatively large volume of water to dilute the strong base and make it safe enough to use for the purposes of the practicals. Hence it makes sense to use the 100ml graduated cylinder.

(One more reason to use a larger cylinder is that whenever you perform titrations, it is always a possibility that you require large amounts of the titrants (in this case the standardized NaOH) in the reaction.)

(And you perform a titration repeatedly until you get a concodant reading, same reading for two different tries of the reaction) .

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