Chem Lab Experiment 17

End-Point

A color change occurs indicating that the titration has come to an end.

Titration Curve:

A plot showing the changes of pH of the titrated solution versus the volume of the added standard solution (titrant)

Buffer

A solution that resists change in pH when small amounts of acid or base are added. Buffers contain appreciable amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid)

Equivalence Point

A stoichiometric amount of a reagent (the titrant) has been added to the substance being titrated.

Why is Phenolphthalein used as an indicator?

Because, it is a weaker acid than HPth-(aq), so the phenolphthalein reacts with the OH- from NaOH only after all of the HPth- has reacted with the OH- from NaOH.

Acetic Acid titration reaction (with strong base):

CH3COOH(aq) + OH-(aq) -> CH3COO- (aq) + H2O(l)

How to find the weight percent of acetic acid in vinegar?

Convert the moles of CH3COOH in the vinegar sample to a mass of CH3COOH via it's molar mass. Determine the total mass of the vinegar sample from the vinegar volume and density (assume density is 1.0 g/ml)

How to find the molarity of acetic acid in Vinegar

Find moles of base (NaOH) required to reach equivalence point, obtain moles of acetic acid. Divide by volume of acetate used.

The End-point reaction equation with phenolphthalein is?

HIn(colorless/acid color) + OH- --> In-(pink/base color) + H2O

Titration reaction between NaOH and KHPth is described by the following chemical equation

HPth-(aq) + OH-(aq) -> Pth2-(aq) + H2O(l)

Potassium Hydrogen Phthalate:

KHC8H4O4 abbreviated KHPth

Standardization

Standardization is doing a titration to work out the exact concentration of the titrant you want to use to determine the concentration of an unknown solution (analyte).

Primary Standard and It's 6 Properties

The pure substance used in the standardization process is called a primary standard. Properties Required of a Primary Standard: 1) high Purity 2) low Moisture absorption 3) Availability at low cost 4) Stability in Air 5) good Solubility in the titration medium 6) large Molar mass to minimize weighing error

Buret

Used to deliver solution in precisely measured, variable volumes. Burets are for titration, to deliver one reactant to until the precise end point of the reaction is reached.

What it is the color change of the indicator?

When phenolphthalein does give up an H+ ion, it undergoes a clor change from colorless to pink.

Analyte

solution used in a titration that has the unknown concentration

Titrant

A titrant is a chemical reagent with known concentration, which is added to the analyte during the process of titration, in order to calculate the concentration of the analyte in the solution.

How would each of the following problems in the standardization of KMnO4 affect the measured value of the KMnO4 molarity (make it larger, smaller, both larger and smaller or have no effect). Explain why. Overshooting the end point: The primary standard dissolved in larger than prescribed amount of water: Wet primary standard: A dirty buret (drops of KMnO4 remain on a buret wall): Few drops of titrant didn't make it into the solution: Buret reading errors : Loss of Primary Standard in Transfer: Incomplete Dissolution of the Standard:

Overshooting the end point: Effect: The molarity would be smaller. Overshooting the endpoint would cause the final volume reading to be a higher a higher value in the buret. This would decrease the molarity by adding a larger value to the denominator in the mol/L expression. The primary standard dissolved in larger than prescribed amount of water: Effect: Molarity would not be affected. Explanation: Adding more than the prescribed amount of water to the primary standard would have no effect on the molarity of the titrant because it would not affect the moles of the analyte. Wet primary standard: Effect: The molarity would be larger. Explanation: If the primary standard were wet, it would weigh more. This would cause less of the actual primary standard to be added to the analyte, because a lot of the mass would be due to water when weighing. This would cause less volume of the titrant to be needed to reach the equivalence point. A dirty buret (drops of KMnO4 remain on a buret wall) Effect: The molarity would be smaller. Explanation: Drops of the titrant will stick to the walls of the buret, therefore more solution will need to be dispersed into the analyte in order to compensate. This will cause the volume reading (denominator) to increase, and this would lower the molarity calculation. Few drops of titrant didn't make it into the solution: Effect: The molarity would be smaller than reality Explanation: If a few drops of the titrant did not make it into the analyte then the volume reading for the titrant would reflect a value higher than it should. This will cause the volume reading (denominator) to increase and which would lower the molarity calculation. Buret reading errors : Effect: The molarity would be both larger and smaller. Explanation: Value could be read higher than the true value or lower than the true value. Loss of Primary Standard In Transfer: Effect: The molarity will be smaller than reality. Explanation: By not transferring all of the primary standard you write down there being a higher amount of titrant in the standardization. And so a lower amount of titrant is used to find the equivalence point. Incomplete Dissolution of the Standard: Bad primary standard!

Dessicator and Drierite

Stores solids in a moisture-free environment. In the dessicator, any moisture from the air is absorbed by the drierite. This is particularly imporant during the cooling of a heated sample, because as a hot solid cools, water from the air condenses on it. Keep the dessicator closed except when you take out and return the vial.

Titration of vinegar reaction Equations: W/strong base. Dissolution of conjugate acid, conjugate base.

Titration reaction: CH3COOH(aq) + OH-(aq) -> CH3COO- (aq) + H2O(l) Dissolution with water (weak acid): CH3COOH(aq) + H2O(l) <-> H3O+(aq) + CH3COO-(aq) Dissolution with water (conj base): CH3COO-(aq) + H2O(l) OH-(aq) + CH3COOH(aq)