Experiment 5

% Potassium Hydrogen Phthalate (KHP) in an Unknown

Writing Lab Reports

See Course Website for “Academic Integrity” and

“Writing Lab Reports” Handouts

6 sections Title Introduction Procedure or Methods Results Discussion or Conclusion References

Title Same as in notebook brief statement about what is to be

determined and how. The title should be descriptive enough that you do not need the “purpose” section that you may have written in high school. DO NOT COPY THE HANDOUT TITLE AS IT MAY NOT BE DESCRIPTIVE

Introduction NEW explains the chemical principles behind the

experiment. It should explain what you are trying to

determine by performing the experiment explain why you are using the chosen method. Explain the chemistry of why the method allows

you to determine what you are looking for. It is often helpful to give the chemical reactions

and EXPLAIN what is happening at each step. Third person present tense Best to prepare before performing the

experiment.

What should the intro for this experiment include?

What is the reaction? Be sure to explain the reaction instead of just drawing it out.

What types of chemicals are reacting with each other and what are they forming?

What is the stoichiometry? Important because it is used in calcs

Differences between primary and secondary standards What are the conditions for the system at the

endpoint? What was used to indicate when to stop? Why was it

chosen? How the titration allows for you to find %KHP in the

unknown

Be sure to answer all of these questions in intro What happens during a titration? Define primary standard. What was your primary standard? Define secondary standard. What was your secondary

standard? What type of reaction is being evaluated? What types of

reactants are being used? What is the specific reaction that is occurring? What is the

stoichiometry of the compounds that you need to consider? What is the condition of the system at the end of your

titration? What tells you when you have reached the end of this

titration? What properties of this particular indicator made it a good

choice for this titration? How does the amount of NaOH tell you the %KHP in the

unknown?

Method/Procedure Similar to notebook Complete sentences and

paragraphs Third person past tense

Results Similar to notebook All in Computer-generated Tables and

graphs/charts Tables/Graphs labeled with “Table/Graph

#” & Descriptive Title All paired data kept together All values have units All values have appropriate significant

digits Important observations can be paired

with the appropriate results

Discussion/Conclusion Same as notebook I will be looking very heavily at your

logic for errors and conclusions Must be third person Well structured, grammatically

correct paragraphs – not just a list of results

References MLA/APA/CBE format (learned in

English classes) Cited within the appropriate

section of the text of the report, as well

Experimental Goals To prepare a carbonate-free

solution of ___________ To determine the _________ of the

NaOH solution by titration (called standardization)

To determine the ____ in a solid mixture using the standardized NaOH solution

Titrations Slow addition of reagent of known

concentration is added to analyte to complete reaction in order to determine the amount of analyte present

When you titrate something you are taking it to the equivalence point

How do you add slowly and know exactly how much you added?

By using a Buret Reading a Buret

Equivalence Point vs. End Point End point-

Equivalence point-

LeChatelier’s Principle LeChatelier’s Principle- when a stress is added to an

equilibrium system, the system will shift to remove the stress.

Before titrating, we have acid in water: HA H+ + A-

For this reaction, the stress is the addition of OH-. OH-

+H+ H2O. This removes H+ ions from the right side of the initial reaction, making the equilibrium offset. Therefore, the equilibrium will shift to the right (making more H++A-) to release the stress.

It is the equivalent of putting a bunch of children on a teeter totter. If it is balanced (in equilibrium) and one child gets off from the right side, a small child could leave the left side and go to the right side to make the teeter totter balance again.

So, how do we determine our End Point? Consider the reaction taking place

Start with weak acid: HA In water it has an equilibrium: HAH++A-

conj. base Add strong base: HA+NaOH H2O+A-+Na+

When enough base (an equivalent amount) has been added to use up all of the HA (in the form of H+), you are left with just A- and Na+ in solution. The HA has been neutralized. This is due to the LeChatelier shift.

The A- reacts with H2O: A- + H2O HA + OH-

This will make the solution slightly basic If you add one more drop beyond this, your solution

will be extremely basic (dependent on the pH of the solution that you are adding)

The Titration

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30

Volume NaOH

pH

ACID BASE

HA + OH- A- + H2O

A- + OH-

Buffer Region

All A- in H2O

End Point? Summarize Reaction (What is

happening?) Start

End Point?

Choose an appropriate indicator

Our end point is

What will we see? At first, not much. As more NaOH is added,

you will see pink where it is added.

The pink color will fade As you get closer to the

end point, the color will remain longer

At the end point, the whole solution will turn pink. You want it to be pale.

If you are not sure that it is pink, record volume and add one more drop to test

Observations??????

Hazards Phenolphthalein

KHP

NaOH

1° = a reference standard of known purity (often a dry solid). You can know EXACTLY how many moles you have (by knowing mass and molecular weight)

2° = not initially sure of concentration. You use the 1° standard to determine concentration in order to use this standard to determine amount of something else. (this is our NaOH solution)

Primary vs. Secondary Standards

Calculations Want to determine Molarity (moles/L)

of NaOH At the end of Exp’t. you will know:

Mass (g) KHP used “x” Volume (mL) NaOH titrated “a”

*(convert to L by * by 1L/1000mL) mw KHP (204.22g/mol) from handout Molar ratio: 1molKHP:1molNaOH

Reminder:Tables in Notebooks Put DESCRIPTIVE titles on all tables Pair data and results for each trail

together It is easier to find and compare

data/results if all paired data is kept together in tables with descriptive titles

Experimental Design Variables

What type of standard to use What type of indicator to use How much solid to weigh Concentration of NaOH

Critical Measurements/Steps Weighing KHP Reading buret Seeing the end point stopping titration