Experiment 1 Determination of the Acetic Acid Content of Vinegar.

During this experiment you will gain experience in working with volumetric glassware, and gain knowledge on how to determine the uncertainties in the volumes delivered by these. In addition, you will use a suitable titration to determine the concentration of acetic (ethanoic) acid in vinegar samples.

On completion of the laboratory you should be able to:
All measurements (volumes, lengths, weights etc.) have associated errors. Some, called gross errors, arise from mistakes (writing down the wrong number, recording the wrong units, etc.) but these can be easily avoided by working carefully. Others, called systematic errors, arise from equipment or instruments not operating according to their specifications (for example if a pipette always delivered 4.96 cm3 rather than the stated 5.00 cm3) or something was wrong with the measurement procedure (for example there was something unexpected in the sample being studied (called an interferant) that resulted in the measurement being different from what it would be if the interferant was absent). Systematic errors lead to inaccurate results but may be avoided if the cause of the error can be identified. The final type of error is called random error and results from unavoidable fluctuations in the measurement process (for example the possible variations in a burette reading that results from attempts to assess the second decimal place). Random errors cannot be avoided, only minimized by adherence to good laboratory practices. Throughout this semester you will investigate some methods used to assess random and systematic errors.
In this experiment you will start to consider random errors by assessing their magnitudes in the volumes delivered by the pipette, volumetric flask and burette that you use in a titration of an acid (acetic acid) of unknown concentration in vinegar with a base (sodium hydroxide) of known concentration. In subsequent weeks you will calculate the random errors in the determined concentrations.

From the volumes of the acid (pipette) and the base (burette) used and knowledge of the stoichiometry of the reaction that takes place and the concentration of the base, the moles of acid involved in the reaction can be calculated and from that the concentration of acetic acid in the vinegar.

Safety Notes:
During this experiment you will be working with a weak acid and strong base. You must wear eye protection at all times. In the event that any reagent used in this investigation comes in contact with your skin or eyes, wash the affected area immediately with lots of water. Notify your instructor.

Directly record all observations onto your worksheet.

Carefully pipette, using a cleaned pipette and a pipette filler and the procedures demonstrated to you, 5.00 cm3 of vinegar into a 100 cm3 volumetric flask. Be sure to drain your pipette properly (slowly rotate the pipette while it is draining and discharge the final drop by touching the pipette tip to the side of your titration flask; do not blow out the last drop). Add distilled water up to the bottom of the stem of the flask, mix well and then make up to the 100.00 cm3 mark. To ensure the solution is homogeneous, mix thoroughly by slowly inverting the flask at least ten times. Pipette 10.00 cm3 of this diluted vinegar into a 100 cm3 conical flask.

Clean and fill you burette with the provided standard sodium hydroxide solution and titrate the vinegar solution, using phenolphthalein as indicator. Repeat the titration until two titres agree to within 0.20 cm3, i.e. you obtain concordant results. Be sure to read your burette to two decimal places (±0.02 cm3). If there are any doubts, ask your demonstrator.

As indicated on your worksheet
a) Calculate the concentration of acetic acid in your sample of vinegar.
b) Estimate the error in your pipette and volumetric flask volumes.
c) Estimate the error in each burette reading.
d) Calculate the error in each titration volume and the error in the average titration volume.

Apparatus per student
25 cm3 (or 50 cm3) burette, a funnel to fit the burette, 10 cm3 pipette, 100 cm3 volumetric flask, 100 cm3 beaker or conical flask, wash bottle, white tile, burette stand and burette clamp.

Chemicals per student
15 cm3 of vinegar, 50 cm3 of standard sodium hydroxide solution (~ 0.05 mol dm-3) and phenolphthalein indicator.

(a) the molar concentration of acetic acid in the original sample of vinegar.

(b) the mass of acetic acid per 100g of solution (the density of acetic acid solution is 1.01 g/cm3 ).

Compare this value [from (b)] for the three different brand names of vinegar provided. For each brand name use the average of at least three (list them) values obtained from your classmates.

Complete your Lab. worksheet and submit it tomorrow.

Answer the following questions on your worksheet.

1. What is the accuracy of the quantities measured using
(i) the pipette?
(ii) the burette?

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The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Oct 2002. Links checked and/or last modified 31st August 2014.
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