Meeting #4 The Acetic Acid Content of Vinegar PDF

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meeting notes of acetic acid content of vinegar,...

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Meeting #4

Experimental instructions -

Use methods of quantitative volumetric analysis to determine the acetic acid content of commercial vinegar, which involves titrating vinegar with NaOH Acid = proton donor, bae = proton acceptor Product of a reaction of an acid with a base is a salt, salts will be ionic compounds consisting of a cation derived from a base and an anion derived from an acid Titration involves an acid base reaction o To analyze reaction wee, need to have a method of determining when all the acid present initially in a given amount of solution (vinegar) has been consumed by reactions with the added base o Can detect the end point with phenolphthalein, colorless in acidic solution and pink in basic solutions

Standardizing a base -

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Exact concentration of NaOH is found by first preparing an aqueous solution of the base that is approximately the concentration desired and then determining the volume of this base required to neutralize a known number of moles of acid The known moles of acid are provided by weighing the appropriate amount of an acid By weighing the acid, we can calculate the number of moles. The overall neutralization reactions occurs when NaOH solution is standardized using a standard acid. At the equivalence point of the reactions, the number of moles of NaOH will be equal to the number of moles of standard acid calculated The molar concentration of NaOH can be calculated from M (mass) NaOH X V (volume) NaOH = mass of acid / molar mass of acid o M (base) V(base) = mass acid / molar mass acid

Acetic acid content of vinegar -

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Vinegar is produced by bacterial oxidation of dilute aqueous solution of ethanol (ethyl alcohol ) o Household vinegar: 5% acetic acid CH3COOH, by volume in water o Pickling vinegar: 7% acetic acid by volume Concentration is determined by titrating a known volume of vinegar solution with standardized aqueous solution of NaOH. Reaction is: o CH3COOH (aq) + NaOH (aq)  CH3COO-Na+ (aq) + H2O (l)  Acetic acid and NaOH react at a 1:1 ration, thus at the equivalence point of the reactions, the number of moles of acetic acid is equal to the number o moles of sodium hydroxide.  The number of moles of NaOH used can be determined by using given concentration and the volume used to reach the equivalence point  # moles NaOH = M NaOH X VNaOH  From the number of moles of acetic acid, you can determine what mass o acetic acid was in your original vinegar sample  Then calculate the mass percentage of vinegar and com[are it to that quoted of the manufacturer, assume d = 1.00 g/mL  % by mass of acetic acid in vinegar = mass of acetic acid/mass of vinegar X 100  Regular vinegar claims content 5.35% acetic acid by mass (5.25g/100g vinegar)  Pickling acid claims to contain 7.34% acetic acid by mass

Measuring Liquid Volumes -

Erlenmeyer flasks and beakers may have volume markings but are not calibrated for accurate volumemeasuring devices Graduated cylinders are somewhat accurate Burettes ad volumetric pipettes are quite accurate 1

Graduated cylinders -

Hold cylinder at eye level, transfer liquid into cylinder. The top of liquid is curved downward, point is called the meniscus. When reading the volume, me measure from the lowest point of the meniscus. Use of the volumetric pipette o For transferring definite volume of liquid form one container to another Burette are used to accurately dispense a known volume of reagent solution sick as a base required to react with a given amount of another reagent such as an acid. The procedure is called titration, and the burette for up to 25mL, graduated in 0.1mL intervals

Titration -

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The equivalence point of titration is reached when the amount of added reagents is just sufficient to combine chemically with all the reacting constituent on the titrated reagent, usually indicated by some change in physical property of final solution i.e., color. Color does not generally coincide with equivalence point because the color change is often caused by an indicator reagent that changes color near the equivalence point. The point detected by the color change is the end point The equivalence point can be detected by adding a minute amount of some suitable substance that will change color berry near the equivalence point.

Videos -

Molarity measures how concentrated a solution is M=n/v o M = molarity, n = # of moles of solute, V = volume of the solution in liters

Lab PPT Dilutions: M1V1 = M2V2 (do not use for stoichiometric relationships) Materials -

Vinegar (regular/pickling) Standardized NaOH (standardization done already) Acid-base indicator Small beakers for vinegar and NaOH 2.00mL pipette 250mL Erlenmeyer flask 25.00mL burette

Background information -

An acid and a base react together (neutralization) HNO3+ NaOH  NaNO3 + H2O Complete ionic reaction: H3O (aq) + NO3 (aq) + Na (aq) + OH (aq)  Na (aq) + NO3 (aq) + 2H2O (l) Net ionic equation: H3) + OH  2H2O

Acetic acid concentration of vinegar -

Regular 5.25% by mass in water pickling contains 7.34% Neutralization reactions: CH3COOH (aq) + NaOH (aq)  CH3COO-Na (aq) + H2O

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o Equivalence point of reaction will be determined using g phenolphthalein indicator which detects end point of titrations At the equivalence point: # moles acetic aid = # moles of NaOH

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Acetic acid content o Moles of NaOH  moles of acetic acid o From moles acetic acid  mass acetic acid o Calculate the mass percentage of acetic acid in vinegar  Mass % of acetic acid = mass of acetic acid / mass of vinegar sample X 100  In order to calculate m=the mass of vinegar, assume D – 1.00g/mL

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Molarity of NaOH: 0.115mol/L 2.00mL – which vinegar: pickling vinegar 7.34% Initial volume of NaOH: 0.00mL Final volume of NaOH: 22.95mL...