CHEM120L Lecture Notes - Lecture 3: Sodium Hydroxide, Titration, Molar Mass

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beigecheetah562

14 Mar 2016

School

University of Waterloo

Department

Chemistry

Course

CHEM120L

Professor

Sue Stathopulos

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Document Summary

Acid- base titraions, the ideniicaion of an unknown solid acid. Generally, the experiment performed in this lab provided a pracical applicaion of acid base reacions. The experiment showed that there is a molar relaionship between acids and bases, and that the relaionship can be calculated as well. In part a, this relaionship was tested between solid oxalic acid and standardized naoh. In part b, through experimental procedures such as standardizaion and itraion, we were able to come to the conclusion regarding the idenity of an unknown solid acid. In part a of the procedure we were required to ind the concentraion of naoh through the itraion mechanism. Because we know the concentraion of the acid involved, as well as its molar mass, we can essenially idenify this concentraion. Through calculaions, which assume perfect condiions of the itraion, the desired concentraion of the base should have been 0. 1016 mol/l. Our average concentraion was found to be 0. 0993 mol/l.

Related Questions

In this experiment, you will identify an unknown weak acid by titration with standardized base. This lab takes place in two parts, which you can do in either order:

1. Standardization of a sodium hydroxide solution to find its actual concentration (approximately 0.1 M)

2. Titration of the weak acid with the sodium hydroxide solution.

You won't know if your acid is monoprotic (HA), diprotic (H₂A), or triprotic (H₃A).

a.) Indicate how you would differentiate between the three different types of acids based on what you observe from your titration curve. Use rough sketches of curves (which you can draw by hand and upload) for the different types of acids to justify your answer.

b.) Which step in your calculations for this experiment will you change to determine the correct molar mass of your acid if it is polyprotic?

1.) average number of moles of NaOH dispensed

2.)average concentration of the acid (mol/L)

3,) Based on your coarse titration volume, do you expect the acetic acid solution to have a higher or lower concentration than the NaOH solution?

higher could you please tell me if number 3 is right

please help with these 2 questions

Part 1: Prepare the NaOH Solution

Lab Results

How many mL of water were used to prepare the NaOH solution?

200mL

Data Analysis

Calculate the molarity of the NaOH solution. The molar mass of NaOH is 39.997 g/mol.

0.1M

Calculate the amount of benzoic acid to be neutralized by 20.00 mL NaOH solution, in both moles and grams. The molar mass of benzoic acid is 122.12 g/mol.

1.207 g

Part 2: Perform a Coarse Titration

Lab Results

Record the following data from your course titration in the table below.

mass of benzoic acid used (g)	0.26614g
pH of benzoic acid solution before titration	2.96
volume of NaOH in the burette before titration (mL)	50.0mL
volume of NaOH in the burette after titration (mL)	28.89mL
volume of NaOH dispensed in the titration (mL)	21.11mL

Data Analysis

How do you expect your coarse titration volume to compare to your fine titration volumes?

I expect the values to be a little off from each other since in the first titration we let out a lot at a time and didnt slowly do the titration. should only be off by a few mL.

Part 3: Perform Fine Titrations

Lab Results

Record the volume of NaOH solution dispensed in the 3 fine titrations.

20 mL dispenced ,20.34mL, 20.30mL

Data Analysis

Calculate the average concentration of the NaOH solution, using the average volume of NaOH solution dispensed in the 3 fine titrations. Report your answer using enough significant figures to distinguish it from the expected concentration of 0.100 M.

V = (V₁ + V₂ + V₃) / 3

V = (20.03 mL + 20.06 mL + 20.02 mL) / 3 = 20.04 mL

M = n / V = 0.00200 moles / 0.02004 L = 0.0998 M

Learning objective: Determine the concentration of a solution.

20.21= ----> 0.0999M

Experiment 2: Use the Standardized NaOH Solution to Determine the Concentration of an Acid

Part 1: Perform a Coarse Titration

Lab Results

What was the pH at the end point of the coarse titration?

3.87ph

Data Analysis

Based on your coarse titration volume, do you expect the acetic acid solution to have a higher or lower concentration than the NaOH solution?

higher

Part 2: Perform Fine Titrations

Lab Results

For the 3 fine titrations of the acid of unknown concentration, fill in the following data.

	Titration #1	Titration #2	Titration #3
volume of acid (mL)	59.11	40.97	40.98
volume of NaOH dispensed (mL)	9.05	9.03	9.02

Data Analysis

Calculate the following quantities and record the data in the table below.

average volume of NaOH solution dispensed (mL)	9.03
average number of moles of NaOH dispensed
average concentration of the acid (mol/L)

Conclusions

Phenolphthalein is pink over the range of pH 8 â 12. Why was it a useful indicator of when the equivalence point was reached?

This was helpful because once I got to the ph of 7 I knew that I would be getting closer to the end point which would change the color to pink.

Suppose a student titrated a sample of monoprotic acid of unknown concentration using a previously standardized solution of NaOH.

volume of 0.125 M NaOH dispensed	24.68 mL
volume of acid solution	50.00 mL

Given the data in the figure above, what is the concentration of the unknown acid?

0.0617M

Lab NOTES:

ex1

Take sodium hydroxide from the Materials shelf and add 0.800 g to the beaker.

Take water from the Materials shelf and add 200 mL to the beaker.

Ph 2.96

inital volume in burett

inital volume 50ml

1.) volume43.78mL

volumes dispenced =6.22ml

2. volume =38.05

volume dispenced =11.95ml

3.)volume = 32.06

volume dispenced=17.94

4.28.89ml

volume dispenced=21.11ml

ph=10.95

--------------------------------------

Ex 2

inital ph= 2.96

volume30.33

dispence 20.05

1. volume 39.92ml

volume dispenced=10.08

2.volume 29.66

voiume dispence 20.34

final ph10.43

initial valume in burett 50ml

--------------------

ex3

final ph9.43

volume 30.28

volume dipenced=20.00

----------------------------------------------------------------------

2nd Ex

initial ph 3.11

volume 50

1. volume 48.95mL

dispenced 1.05mL

ph 3.87

2. volume 45.88

dispenced 4.12

ph 4.67

3 volume 42.99

dispenced 7.01

ph 5.29

4 volume 39.93

dispence 10.07

ph 11.45

--------------------

final ph 9.99

volume 59.11

dispenced 9.05

ph 9.59

volume 40.97

dispenced 9.03

ph 9.50

volume 40.98

9.02

I am unable to solve these questions due to a time constraint. I received help on part 1 of the unknown acid. Need help with the rest! I really appreciate it!

Lab: ACID-BASE TITRATION

Objective: To use titration to determine in Part 1, the concentration (molarity) of an unknown acid solution and in Part 2, the purity of a sample of KHP acid. Background: Titration is a volumetric technique used to determine the concentrations of solutions, molar masses of solids, and purity of samples. A titration inolves the addition of a titrant (a solution of known concentration) to an analyte ( a solution of unknown concentration), or vice versa, using a piece of glassware called a burette. The titration is carried out until it reaches an equivalence point (the exact point at which the reaction between the two solutions is complete). A chemical indicator is often used to aid in the identification of the equivalence point. An indicator changes color upon reaching the equivalence point of endpoint. Since we will be carrying out an acid-base titration, the indicator must change color upon reaching the endpoint at a specific pH. An example of such an indicator is phenolphthalein, which changes from colorless to pink near the equivalence point when pH approaches 7. It should be noted that the indicator selected is dependent upon a given titration, so that the observed color change is close to the ideal equivalence point. Once the titration is completed, we can use the volumes measured for each solution, as well as the concentration of the titrant, to determine the concentration, molar mass, or purity of the unknown analyte. Procedure Note: During lab, record all measurements and data in a clearly labeled table. Part 1: 1. Obtain ~80mL of ~0.1M of base (sodium hydroxide, NaOH) into a labeled beaker. Make sure to report the actual concentration from the bottle. 2. Obtain ~25mL of unknown acid into a second labeled beaker. Record whether your acid is monoprotic, diprotic, or triprotic. 3. Using a funnel, or carefully pouring, fill the burette with sodium hydroxide NaOH. Make sure a labeled waste beaker is under the buretter to catch any drippings. Rinse your burette by filling it with ~5-10mL of NaOH. Turn the knob to let ~2 mL out the tip into the waste beaker. Make sure your waste beaker is under the burette to catch any drips. Next, turn the Burette upside down and pour the remaining NaOH into your waste beaker while twisting the burette to coat the inside with NaOH. 4. Fill your burette with sodium hydroxide. Turn the knob to fill the tip of the burette with NaOH. Record the initial volume to the correct number of significant figures. 5. Using a volumentric pipette, transfer 10.00 mL of your unknown acid into the Erlenmeyer flask. 6. Add 3 drops of pH indicator and magnetic stirrer to the Erlenmeyer. 7. Under the burette, place the Erlenmeyer on a hot plate and turn on the stir knob. Heat should be off. 8. Titrate the unknown acid until the solution turns pink and remains pink. 9. Record the final volume on the burette to the correct number of significant figures. 10. Repeat (4) to (7). Pre-plan the whole lab and who will do which task. MY RESULTS: PART 1 The actual concentration of NAOH is 0.5939, 80 mL Initial Final 24.9 mL 44.5mL 26.1 mL 45.57mL PART 2 Impure KHP (SAMPLE B ) Only Paper: 0.280 g Impure KHP Mass: 0.526 g Initial volume: 22.30 mL + water +26.10 mL Final Volume: 26.80 mL QUESTIONS: 1. Calculate the concentration (molarity) of your unknown acid. 2. Calculate how many grams in your impure KHP sample was indeed KHP. 3. Calculate the percent (by mass) KHP in the sample. Part 2: 1. What would happen to the final result( state what the final result is ) if the tip of the burette was not filled when reading the initial volume? 2. What would happen to the final answer if you used Ca(OH)2 instead of NaOH and DID NOT know it? 3. What would happen to the final result if the Erlenmeyer flask was contaminated with other types of acid? 4. Recalculate the concentration of your unknown acid if it was given as a diprotic acid. 5. Draw a molecular level picture (showing Na+, H+, OH-, H2O, etc...) of what is in your beaker before, at half point, at equivalent point, and at end point of the titration. Part 3: 1. What would have happened to the final result if the impurity in the sample was also acidic? 2. What would have happend to your final result if you used ~25mL of water to dissolve the solid acid instead of 20mL? Summary data: Create a clearly labeled table of all your final data results for each run, and athe average of multiple runs. Part 1: Unknown #, Vol base, M base, Vol acid, M acid. Part 2: Sample #, % KHP (purity)

CHEM120L Lecture Notes - Lecture 3: Sodium Hydroxide, Titration, Molar Mass

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