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Determination of the molar mass of an acid (and also for citric acid)
Typology: Schemes and Mind Maps
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An acid reacts with a base to form a salt plus water (see Scheme 1). Many acids have one proton available for reaction with a base such as sodium hydroxide. An acid with one reactive proton per molecule or formula unit is called a monoprotic acid or a monobasic acid. A diprotic acid or a dibasic acid has a maximum of two reactive protons per molecule. A triprotic acid or a tribasic acid has a maximum of three reactive protons per molecule. There are also examples of acids, which have four and even more reactive protons per molecule. In contrast, most useful bases have one or two available hydroxide ions per formula units. Titration is the process for ascertaining the exact volume of a solution that reacts stoichiometrically according to a balanced chemical equation with a given volume of a second solution. One reagent is added by means of a burette until the endpoint is reached. 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 ion concentration which is present after the stoichiometric reaction has occurred. The typical indicator for titrations of strong acids and bases is phenolphthalein. Phenolphthalein is colorless in acidic solution and red (pink in dilute solutions) in basic solution. Since it is much easier and distinctive to see a color change from colorless to pink rather than from pink to colorless, sodium hydroxide is added by means of the burette to the acid, usually contained in an Erlenmeyer flask. Phenolphthalein = In HIn In- Colorless Fuchsia
In this experiment, you will standardize (determine precisely the concentration) a solution of sodium hydroxide, NaOH, using oxalic acid dihydrate, H 2 C 2 O 4 " 2 H 2 O, as a primary standard acid. A primary standard acid is a solid with high degree of purity and large molar mass. The solid acid whose mass is an accurate measure of the number of moles of protons the acid will furnish. Oxalic acid, H 2 C 2 O 4 "2H 2 O, is a diprotic acid and provides two reactive protons per molecule according to the following net equation for the neutralization reaction. H 2 C 2 O 4 "2H 2 O(s) + 2 NaOH(aq) $ Na 2 C 2 O4(aq) + 4 H 2 O(l) We must standardize the NaOH solution as sodium hydroxide reacts with carbon dioxide (CO 2 ) from the air to produce NaHCO 3. The concentration of NaOH is then decreased. We must standardize it against a compound that does not easily react with air or decompose, such as oxalic acid. In the second part of the experiment you will titrate an unknown acid with your standardized NaOH solution using phenolphthalein as the indicator. Your goal will be to calculate the molecular mass of your acid. Your instructor will tell you the number of protons your acid furnishes for reaction with base.
PRE-LAB ASSIGNMENT Titration Experiment Name:
EXPERIMENTAL PROCEDURE Part A. Standardization of NaOH solution
LAB REPORT GUIDELINES The lab report section includes work recorded during the lab, your analysis and discussion of data and results, and your conclusions. The discussion and conclusion sections should be word-processed. Other parts of the report - calculations, etc. may be typed. Heading ! Title of experiment and number ! Your name ! Dates of the experiment Data/Observations/ Results ! ORIGINAL QUANTITATIVE DATA (signed data pages from your lab notebook) Calculations Show calculations for the following quantities in your report (use proper format for labeling and showing calculations in a formal report): Part A: ! Calculate [NaOH] based on each titration ! Calculate the average [NaOH] (of the good trials as described in the procedure) ! Calculate the relative range ! Provide a discussion of your choice to omit any bad trials, including any other relative range calculations you may have performed. Part B: ! Calculate the molar mass of the unknown acid for each trial ! Calculate the average molar mass (of the good trials as described in the procedure) ! Calculate the relative range ! Provide a discussion of your choice to omit any bad trials, including any other relative range calculations you may have performed. ! Determine the identity of your unknown acid. Your acid is one of the following: o Oxalic acid dihydrate (H 2 C 2 O 4 • 2 H 2 O) (diprotic) o Succinic acid (C 4 H 6 O 4 ) (diprotic) o Fumaric acid (C 4 H 4 O 4 ) (diprotic) o Tartaric acid (C 4 H 6 O 6 ) (diprotic) o Citric acid monohydrate (C 6 H 8 O 7 • H 2 O) (triprotic) o Based on the molar masses of these compounds (calculate and provide in a table), identify the most likely identity of your unknown. If you cannot definitively determine its identity, suggest the reasonable possibilities. ! Calculate the percent error for the experimental molar mass with the value for the acid you have determined it to be. (If more than one possibility, show the percent error for each of them). Discussion/Theory/ Results/Error Analysis In this section, you will explain the experiment, evaluate, discuss your results, and analyze errors in paragraph form. ! Briefly explain why we standardized the sodium hydroxide
Acid-Base Titration