Exp. 10 Vinegar Analysis: Acid-Base Titrations Purpose – To use quantitative analysis and titrations to find the concentration of an acid or base. In this experiment, we shall determine the % by mass of acetic acid in vinegar.

Expressions of Concentrations of Solutions Solution = homogeneous mixture of solute and solvent Solute = the substance dissolved; usually in lesser quantity Solvent = the substance that causes dissolution; usually in greater quantity To express quantities in solution, both solute and solvent (or a measure of it) must be included. Concentration is the amount of solute in a given amount of solvent (or solution)

Expressions of Concentration: Formulas % by mass = _mass solute _x 100 mass solution % by volume = _volume solute _ x 100 volume solution % mass by volume = _mass solute _ x 100 volume soln Molarity, M = moles solute V L solution molality, m = moles solute kg solvent

Density is a useful expression but is it an expression of concentration? Density, D = _mass solution _ volume solution We should be able to convert from one expression to another Density provides a useful relationship Expressions of Concentration: Relationships

Exp. 10 Vinegar Analysis Quantitative analysis –Determining the specific amount (molarity or other concentration) of a particular substance in a sample (qualitative analysis determines the presence w/o specifying the amount) –Examples -Cholesterol levels in serum -Iron in blood -Lead in drinking water -NO in air -Drug levels for transplants-Blood sugar

Exp. 10 Vinegar Analysis cont’d Titration –Determining the amount of a substance by a reaction with another substance with known concentration –Detection of the chemical reaction by some kind of signal, e.g., color change of an indicator Purpose of the Experiment –Determine the amount of acetic acid in vinegar by titration with sodium hydroxide CH 3 COOH + NaOH  CH 3 COONa + H 2 O H + + OH -  H 2 O Neutralization Reaction –Indicator is phenolphthalein Changes from colorless (acidic) to pink (basic)

Analysis by Titration: Steps –Take a known volume of acetic acid –Add sodium hydroxide of a known concentration in small amounts CH 3 COOH + NaOH  CH 3 COONa + H 2 O H + + OH -  H 2 O –As long as there is more acetic acid than sodium hydroxide, the solution reacts acidic Indicator will be colorless Exp. 10 Vinegar Analysis cont’d

Analysis by Titration –When acetic acid initially in solution equals sodium hydroxide added, you have reached the equivalence point (neutral solution) –When more sodium hydroxide than acetic acid is present, the solution turns basic Indicator will become pink When the solution becomes just pink with one drop of NaOH, you have reached the “endpoint” of the titration Exp. 10 Vinegar Analysis cont’d

An Acid-Base Titration Start of titration Excess of acid Point of neutralization Slight excess of base Addition of base until all acid is neutralized Base Acid Indicator

An Acid-Base Titration Point of neutralization (“Equivalence point”) All moles of H + ions present in the original volume of acid have reacted with an equivalent amount of moles of OH - ions form the base added # moles of H + (originally in flask) = # moles of OH - (added from buret) H + (aq) + OH - (aq)]  H 2 O(l) Point of neutralization

@ Equivalence Point CH 3 COOH + NaOH  CH 3 COONa + H 2 O H + + OH -  H 2 O –Every single ion of H + from acetic acid has reacted with an OH - ion from sodium hydroxide to form H 2 O # moles of acid = # moles of base # moles of H + = # moles of OH - Where: # moles of acid = concentration acid * volume acid = (mol/L) * L # moles of base = concentration base * volume base = (mol/L) * L –Calculation of acid-base concentrations for a molar ratio n b /n a = 1:1 M acid * V acid x n b /n a = M base * V base  if you know the volume of acid, the volume of base in a titration and the concentration of one of the components, you can calculate the concentration of the other Exp. 10 Vinegar Analysis cont’d

For this lab: We need %by mass from the formula: % by mass = (mass solute / mass solution ) x100 Vinegar is commercially prepared as a 5% acetic acid solution => (5g acetic acid/100g soln) We expect our answers to be near that value Exp. 10 Vinegar Analysis cont’d

Experimental: follow lab manual!!! Take notes Get 50 mL buret and 10 mL pipet Get 30 mL of vinegar and ~ 85 mL of NaOH solution, 0.10 M in labeled beakers Put buret in clamp and add about 5 mL of NaOH solution by using a funnel Clean buret by carefully decanting the 5-mL NaOH solution into a waste beaker Put buret back in buret clamp Exp. 10 Vinegar Analysis cont’d

Experimental: Trial Titration (WHY?) Put 15.0 mL of vinegar solution in a clean 125 mL Erlenmeyer flask Add 2 drops of phenolphthalein Fill the buret with 50-mL of NaOH 1)Record the molarity of NaOH 2)Use a funnel to fill the buret, not over the 0.0 line 3)Record the initial volume ((put white paper with a black line behind the buret as you make a reading; use SIG FIGS!!!!) Put the tip of the buret in the Erlenmeyer, and a piece of white paper underneath the flask Add the NaOH solution in 1-mL increments to the acetic acid while swirling Note the color after swirling When you get close to the equivalence point, the solution will briefly turn pink and then colorless again Add smaller amounts until the solution stays pink for about 30 seconds RECORD VOLUME OF NaOH ADDED (final reading – initial reading) Exp. 10 Vinegar Analysis cont’d

Experimental: Exact Titration Put 15.0 mL of vinegar solution in a clean 125 mL Erlenmeyer flask Add 2 drops of phenolphthalein Fill the buret with 50-mL of NaOH 1)Record the molarity of NaOH from the label 2)Use a funnel to fill the buret, not over the top line 3)Record the exact initial volume (put white paper with a black line behind the buret as you make a reading) Put the tip of the buret in the Erlenmeyer, and a piece of white filter paper underneath the flask Rinse the wall of the flask with dH 2 O (Why?) Continue adding NaOH drop-by-drop until the solution has a barely visible pink color that does not go away upon swirling Exp. 10 Vinegar Analysis cont’d

Experimental: Exact Titration (continued) Record the volume of the solution in the buret The volume NaOH added is V final - V initial Repeat the procedure Calculate the moles for each trial using the formula 10.1 p.137 for the titration. –The difference must be not more than 2%. Calculate the mass of acetic acid from the equation 10.3 p.137 Calculate the % by mass of acetic acid in the vinegar using equation 10.5 p.137 Exp. 10 Vinegar Analysis cont’d

For next Lab: Vinegar analysis cont’d