Laboratory 1

Determination of KHP in a Sample

Objective

A solution of sodium hydroxide will be prepared and then standardized using the primary standard, potassium hydrogen phthalate (KHP). Then using the standardized NaOH solution, the concentration of KHP in an impure sample will be determined by titration.

Background

• • NaOH is commonly used in the preparation of standard solutions. Standardization of the solution is necessary after preparation as the NaOH used is not highly pure, it is hygroscopic in nature and also reacts with CO₂ from the atmosphere. These shortcomings can be overcome by the use of 50% NaOH solution. 
  • Accurate measuring apparatus such as the volumetric flask and pipette are to be used when a solution's concentration must be known exactly such as with a stock or standard solution; approximate measuring apparatus such as graduated beakers, cylinders and reagent bottles suffice when only approximate concentrations are needed. 
  • The 0.1 M NaOH solution is stored in a polyethylene bottle as the base reacts with glass, forming silicates. 
  • Potassium hydrogen phthalate (KHP) is used as a primary standard to standardize the

0.1 M NaOH. 

• Reverse osmosis (RO) water must be used for all chemical procedures as tap water would cause errors in your results. 

Calculations

Standardization:

 

Standardization is a way of accurately finding the concentration of a titrant solution.  So, for example, we may say that a titrant solution is 0.1 M NaOH, and that is its approximate concentration.  But in order to be useful as a titrant, its concentration needs to be known more precisely, usually to 4 significant figures, e.g., 0.1089 M or 0.09753 M.  Standardization of a titrant is sometimes done by using that titrant to titrate a precisely known mass of a primary standard.  That is how it is done in this experiment.

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CHEM25770 3 Fall 2020 At the endpoint of the titration, the number of moles (n_B) of base titrant (0.1 M NaOH) that you have added is equal to the number of moles (n_A) of acid (KHP) that was in the flask.

????????_???????? = ????????_????????                                     (1)   

The number of moles of acid is equal to the weight of pure KHP that you weighed out divided by the molar mass of KHP.

 

                                  (2)

 

Since the molarity of the base = (no. of moles)/volume, the number of moles of base is equal to the molarity of the base times the volume of the base:

????????_???????? = ????????_????????????????_????????         (3) Combining equations (1), (2), and (3), we get

                                                            

                       (4)

 

Because you know all of the values in this equation except ????????_????????, the molarity of the base, you can solve the equation to find ????????_????????.

You will prepare the NaOH solution in such a way that its molarity is about 0.1 M.  The purpose of the standardization (when you titrate pure KHP with your NaOH solution) is to find out what the NaOH molarity is exactly.  You must take all of your readings (weight of KHP, volumes of titrant) to 4 or more significant figures.  This will allow you to calculate the NaOH molarity to 4 significant figures.

 

Determination of the unknown:

After you have done the calculations for the standardization, you will know the exact concentration of your NaOH solution.  It will be around 0.1 M, but the purpose of the standardization is to find out what it is exactly (to 4 significant figures).  In the second week of the experiment, you use your standardized NaOH to titrate your unknown sample.  Your unknown contains KHP, but it is not pure KHP.  Use equation (4) to find the weight of KHP in your unknown.  Divide that weight of KHP by the weight of the sample (that you titrated) to find the percentage of KHP in your unknown.

Chemicals Required

Week 1: 50% w/w NaOH KHP primary standard  Phenolphthalein indicator

Week 2: Phenolphthalein indicator Standardized NaOH from the previous week KHP unknown

 

Equipment Required

Week 1:

10 mL graduated cylinder 

1 L volumetric flask

1 L polyethylene bottle

50 mL burette

3-250 mL Erlenmeyer flasks

Burette stand

Burette cap

 

Procedure

Week 1

Week 2: 

3-250 mL Erlenmeyer flasks

Burette stand

50 mL burette           

Burette cap

Wash bottle

 

Preparation of 0.1 M NaOH solution

1. Wash a 1 L volumetric flask thoroughly and rinse it with RO water.  Add RO water to the volumetric flask until it is about half full. 
2. Using a graduated cylinder, pour 6 mL of 50% NaOH solution into the 1 L volumetric flask. 
3. Add RO water slowly to the flask up to the base of the neck and then bring it up to the calibration mark. You do not have to get it exactly to the calibration mark.  Why not?
4. Invert the flask 12 - 15 times to ensure good mixing of the solution. 
5. Condition a clean 1 L polyethylene bottle by pouring about 5 - 10 mL of the 0.1 M NaOH solution into the bottle and swirling it around, then discarding the liquid in the sink, while running the tap water.  Repeat this with another 5 – 10 mL of the 0.1 M NaOH.  Store the rest of the NaOH solution in the polyethylene bottle.  Label it properly with contents, concentration, date of preparation, and your initials.
6. Squeeze as much air as possible out of your polyethylene bottle after pouring the NaOH solution in and tightly cap the bottle. 

Standardization of 0.1 M NaOH solution

1. The primary standard KHP will be dried for 2 hrs at 110°C and cooled in a desiccator         ready for use.
2. Weigh accurately (to the nearest 0.1 mg) about 0.7 g of primary standard KHP into a clean and dry 250 mL Erlenmeyer flask.  (If you have washed the flasks, dry them with paper towel.)  Do this two more times so that you have three Erlenmeyer flasks, each containing about 0.7 g KHP, accurately weighed. (Why must the flasks be dry before you weigh the KHP into them?)  Add 50 – 75 mL of RO water to each flask and swirl to dissolve.
3. Condition a clean 50 mL burette with the 0.1 M NaOH solution that you prepared.  
4. Fill the burette with the 0.1 M NaOH solution to a level close to 0.00 mL, but do not waste time trying to get it exactly to 0.00.  Record the initial volume to two decimal places, for example 0.83 mL.  It is not good enough to record it as 0.8 mL.  Since NaOH can absorb CO₂ from the air, your burette should be capped. 
5. Take one of the flasks containing the weighed KHP and add 3 – 4 drops of phenolphthalein indicator.  Titrate with NaOH until a very faint pink endpoint.  The endpoint should not have a strong colour.  The endpoint occurs when you can see the weakest possible pink colour and that colour persists for at least 30 - 45 s.  Record the final volume to two decimal places, for example, 35.17 mL.
6. Repeat steps 4. and 5. for the other two flasks containing primary standard KHP.
7. Calculate the molarity of NaOH from each titration. Determine the mean molarity, the standard deviation, and the % relative standard deviation.  Use the mean molarity of NaOH to determine the % KHP in your unknown sample. 

Week 2

Determination of KHP in an Impure Sample 

1. Your instructor or technologist will provide you with your unknown sample (which has been dried by the lab technologist).  Immediately record in your lab notebook the number of your unknown sample.
2. Weigh out 0.8 g (to the nearest 0.1 mg) of your unknown KHP sample into each of three 250 mL Erlenmeyer flasks and dissolve in about 75 mL of RO water.
3. Fill the burette with the 0.1 M NaOH solution to a level close to 0.00 mL, but do not waste time trying to get it exactly to 0.00.  Record the initial volume to two decimal places.  Since NaOH can absorb CO₂ from the air, your burette should be capped. 
4. Take one of the flasks containing the weighed unknown KHP and add 3 – 4 drops of phenolphthalein indicator.  Titrate with NaOH until a very faint pink colour of the indicator persists for at least 30 - 45 s.  Record the final volume to two decimal places.
5. Calculate the percent by weight of KHP in your unknown sample.  Make sure you have 3 good results from your titrations.
6. When you have finished with your sample, return the remaining unknown to its original container and place in the box near the whiteboard.

 

Lab Report Format

 

This lab report will include:

 

COVER SHEET

TITLE

PURPOSE

OBSERVATIONS

CALCULATIONS

RESULTS

CONCLUSION

POSSIBLE SOURCES OF ERROR

REFERENCES

PHOTOCOPY OF ALL DATA FROM LAB NOTEBOOK

 

COVER SHEET

The cover sheet is to be completely filled in and attached to the front of the report.  If information such as sample number, mean and standard deviation are missing, the analytical results cannot be marked, resulting in a mark of 0 for that part of the lab.

 

TITLE

 

The title should be the same as that in your lab manual.

 

PURPOSE

 

The purpose is to be 1 – 2 short sentences describing the purpose of your lab.

 

OBSERVATIONS

 

This section includes both physical and numerical observations.

 

Physical observations such as colour change may be written in a short paragraph which includes the change in colour of the indicator and the rate of colour change.  Any in between changes must be noted.

Numerical observations are to be put into table form.  The table must have an appropriate title and sub headings. 

  

Each week’s work is to be summarized in 1 table only.

 

Appropriate headings are:

 

Sample number

Sample mass

Initial volume of titrant

Final volume of titrant

Net titrant volume

 

CALCULATIONS

 

At the beginning of this section, the name, chemical formula, and molar mass of each substance involved in the titration are to be listed.  All measurements in this lab are done to four significant figures, and all calculations must be done to four significant figures.

 

The balanced equation for the reaction must be present immediately before any calculations.

 

A sample calculation is to be done for one sample only. 

 

Use the average molarity of the NaOH when calculating the % KHP in your unknown sample.  Calculate the % KHP as the final result for each of the three determinations.  Calculations for mean, standard deviation, and % relative standard deviation are included here.

 

RESULTS

 

The final results for the unknown must be presented in table format.  Two results tables are required, one for the standardization of the titrant and one for the analysis of the unknown.  

The first table should show the three values found for the molarity of the titrant, the mean (with units), the standard deviation (with units), and percent relative standard deviation (with units).

The second table should show the unknown sample number, the three values found for %KHP, the mean (with units), the standard deviation (with units), and percent relative standard deviation (with units).

 

CONCLUSION

The Conclusion is a sentence which states “Sample number ______ was found to contain ____ % KHP with a relative standard deviation of ____%.”

 

POSSIBLE AND ACTUAL SOURCES OF ERROR

At least 3 sources of error are to be discussed here with their effect on the % KHP in the unknown.

 

REFERENCES

List all references used in report.  All web sites must be easily accessed by the instructor or they will be deemed invalid.

 

Any diagrams, charts, pictures, tables, etc, downloaded from the web or copied from a book must be specifically referred to in your text and have a reference site under the diagram. 

Marks will be deducted for lack of proper references.

Some or all marks will be deducted for plagiarism.  Copying from a reference is deemed plagiarism.  When using references to present your answers, use your own words.