Acid Base Titration and Acetic Acid Concentration Lab Report

Titrations

Objectives:

  • To perform an acid-base titration
  • Determine the concentration of a sodium hydroxide solution
  • Determine the acetic acid concentration of vinegar

Introduction:

Neutralization is a reaction where an acid reacts with a base to produce a salt and water. Strong acids like HCl and a weak acid like acetic acid, CH3COOH, both react with NaOH, a strong base to produce salt and water.

HCl (aq) + NaOH (aq) NaCl(aq) + H2O(l) (eqn. 1a)

CH3COOH (aq) + NaOH (aq) NaCH3COO (aq) + H2O (l) (eqn. 1b)

The reactants in both the above reactions react in a 1:1 stoichiometric ratio. These reactions can be used in quantitative analysis by titrating the acid with the base. Titration is a technique for accurately measuring the volume of a solution that is required to react with a known amount of another reagent. A solution of known concentration is used to determine the concentration of an unknown solution. In this method, the solution of unknown concentration is added gradually from a burette into the titrant, a solution of known concentration to the point when the titration is complete. The titration is complete, or the equivalence point is reached when stoichiometrically equal amounts of the two reactants have reacted. In an acid-base titration (or vice-versa), equivalence point of the titration is when # moles of acid = # of moles of the base. The end-point of the titration is the point at which an indicator undergoes a color change. Ideally, the equivalence and the end points should be the same, but in practicality, they differ slightly. Phenolphthalein and methyl orange are common acid-base indicators.

Accurate preparation of sodium hydroxide solutions of known concentration is difficult, as solid NaOH is very hygroscopic and absorbs water. Additionally, the resulting solution tends to absorb carbon dioxide from the air, which neutralizes some of the base. Standardization is the process of determining exact concentration of a solution by titrating it with a known amount of a primary standard, a solution of fixed concentration. In this experiment, you will standardize NaOH solution by titrating the solution with a solution of a weak acid potassium hydrogen phthalate, abbreviated as KHP (molecular formula= C8H5KO4, Molar mass=204.2 g/mol.) KHP is a very reliable primary standard. KHP reacts with NaOH according to the following equation:

http://upload.wikimedia.org/wikipedia/commons/e/ea/Potassium-hydrogen-phthalate-2D-skeletal.png

Potassium Hydrogen phthalate

“ KHP”

(C8H5KO4)

Molar mass=204.2 g/mol

Figure 1

KHP (aq) + NaOH(aq) = KNaP(aq) + H2O(l) (eqn. 2)

Phenolphthalein is used as an indicator of pH, a measure of H+ concentration. Phenolphthalein is clear colorless in acidic solutions and changes color to a magenta pink in basic solutions.

Phenolphthalein indicator in acidic solution Phenolphthalein in basic solution

(eqn. 3)

The end point of the titration is indicated by a very faint persistent magenta color of the solution. The titer value is the volume of NaOH required to stoichiometrically titrate a known amount of KHP. This point is the equivalence point. KHP has one ionizable hydrogen ion. At the equivalence point, the number of moles of KHP are equal to moles of NaOH, reacting in 1:1 mole ratio. By knowing the mass of KHP, we can determine the number of moles used in the titration:

(eqn. 4)

At the equivalence point, then:

moles NaOH = moles KHP (eqn. 5)

The molarity of the NaOH solution is determined via the volume of the base used in the titration:

L (of NaOH) X M (of NaOH) = moles of

In this experiment you will also determine the percent acidity of commercial vinegar. Vinegar is a solution of acetic acid (CH3COOH) in water. In this experiment, we will titrate the vinegar solution with NaOH solution, standardized in experiment 1. The neutralization reaction is

NaOH + HC2H3O2 → NaC2H3O2 + HOH (eqn. 6)

The acid and base react in stoichiometrically equivalent amounts, i.e. one mole of acid (H+) reacts with one mole of the base (OH). You will determine the volume of standardized NaOH required to completely react with a known volume of vinegar. The number of moles of NaOH is equal to the molarity of the standardized NaOH solution multiplied by the volume of NaOH required to reach the end point:

L (NaOH) X Molarity of NaOH = moles of NaOH (eqn. 7)

Acetic acid and NaOH react in stoichiometrically equivalent amounts in 1:1 ratio. Therefore,

moles of NaOH = moles of acetic acid (eqn. 8)

The mass of vinegar is calculated as below.

(eqn. 10)

The mass of acetic acid in the vinegar sample is given by:

moles of acetic acid X molar mass of acetic acid = grams of acetic acid (eqn. 11)

The mass% of acetic acid in vinegar is calculated by dividing the mass of acetic acid in the vinegar solution by the mass of the vinegar solution.

(eqn. 12)

Equipment:

  • ~0.5 M NaOH solution
  • Potassium Hydrogen Phthalate (KHP)
  • Phenolphthalein indicator solution
  • Distilled water
  • 100 mL Graduated cylinder
  • 500 mL volumetric flask
  • Plastic storage bottle
  • Burette funnel
  • Burette
  • Burette clamp
  • Ring stand
  • 250 mL Erlenmeyer flask (2)
  • Scoopula
  • 250 mL beaker
  • Vinegar solution
  • Distilled water
  • 2.00 mL pipette
  • Pipette bulb
  • 50 mL Graduated cylinder
  • Burette funnel
  • Burette
  • Burette clamp
  • Ring stand
  • 50 mL beaker

Safety:

Always wear safety glasses during the laboratory. Hydrochloric acid and sodium hydroxide are corrosive. Remember to notify the instructor of any accident or spill, large or small. Do not pick up broken glass yourself. Label all containers before you put chemicals in them, even if the chemical is just water or the chemical will be in the container for only a few minutes.

Procedure:

Part A: Standardization of sodium hydroxide

Prepare 500 mL of 0.1000 M NaOH by dilution of ~0.50M NaOH solution.

    1. Before you begin, calculate the volume of 0.50 M NaOH required for preparing 0.500 L of ~0.1 M NaOH. Measure the required volume using a graduated cylinder.

word image 980Titration apparatus

    1. Transfer the 0.50M NaOH solution into a 500 mL volumetric flask. Dilute the stock solution with deionized water up to the neck of the volumetric flask.
    2. Cap the flask and mix well by inverting the volumetric flask 20 times.
    3. Label the volumetric flask with your name and the contents.
  1. Conditioning the burette:
    1. Rinse the burette with distilled water,
    2. Rinse the burette with 10-15 mL portion of the ~0.1 M NaOH solution you prepared in step 1.
    3. Discard the sodium hydroxide solution to a waste beaker.
  2. Obtain 2, 250 mL Erlenmeyer flasks, label the flasks 1 & 2
  3. Weigh between ~0.4500 – 0.5000 g of KHC8H4O4 and transfer the KHP to an Erlenmeyer flask labeled 1, repeat this for the Erlenmeyer flask labeled 2 . Record the exact mass displayed on the balance of the KHP on your data sheet.
  4. Using a graduated cylinder measure ~50 mL of deionized water and transfer it to the flask 1, repeat this for flask 2.
  5. Add 2-3 drops of phenolphthalein to each of the flasks, add a stir bar to flask 1, and set the flask on the stir plate, turn on the stir plate. Dissolve the KHP completely.
  6. Fill the burette below the 0.00 mL level. Record the initial volume on the burette on your data sheet.

http://www.phschool.com/science/biology_place/labbench/lab2/images/burette.gifReading a meniscus

  1. Open the stopcock and titrate the KHP solution with NaOH, constantly stirring. Stop adding the NaOH solution once a steady pale pink color is observed. Record the volume on your data sheet to 0.01 mL
  2. Repeat step 8 for flask #2. Refill the burette as needed with your NaOH solution
  3. Calculate the molarity of NaOH

Cleanup- discard your titrated solutions down the sink with plenty of water, wash your glassware with soap and water.

Part B: Determination of Mass% of acid in Vinegar.

  1. Obtain 2, 250 mL Erlenmeyer flasks label as flask 1 and flask 2
  2. Pour about 20 mL of vinegar into a 50 mL beaker

word image 981Pipette filler

  1. Condition the volumetric pipette as below:
    1. Press the button A while simultaneously squeezing the pipette bulb to release the air; once the air is removed release the A button
    2. Insert the 2.00 mL pipette (large end) into the bottom hole of the pipette bulb, place the pointed tip of the pipette into the vinegar from the 50 mL beaker. Press the S button on the bulb until the water rises in the pipette to the line indicating 2.00 mL.
    3. Dispense the solution using button E into the waste beaker. Repeat step b one more time.
  2. Use the conditioned 2.00 mL pipette, dispense 2.00 mL of vinegar into flask 1. Dispense 2.00 mL of vinegar into flask 2.
  3. Add ~ 50 mL of distilled water to flask 1, add ~ 50 ml of distilled water to flask 2.
  4. Add 2-3 drops of phenolphthalein indicator solution to each Erlenmeyer flask along with a stir bar.
  5. Using your standardized NaOH solution from part A. fill the burette with your standardized NaOH solution.
  6. Record the initial volume on the burette on your data sheet,
  7. Place the flask on the stir plate and begin stirring.
  8. Open the stopcock and titrate the vinegar solution with the standardized NaOH, constantly stirring the flask. Stop adding the NaOH solution once a pale pink color is observed. Record the volume on your data sheet.
  9. Repeat steps 7-10 for Erlenmeyer flask 2. Refill the burette as needed with your NaOH solution. Discard the pink solutions in the sink with copious amounts of water down the drain.
  10. Wash the burette and pipette with tap water, rinse with deionized water twice, and invert the burette on the burette stand.
  11. Calculate the % of acetic acid in the vinegar solution.

Cleanup- discard your titrated solutions down the sink with plenty of water, wash your glassware with soap and water. Return your glassware to the appropriate location. If you have left over 0.1M sodium hydroxide, place this in the container labeled 0.1 M sodium hydroxide located in the fume hood.

Name: _________________________________ Date: ________________

Data Sheet

Part A: Standardization of sodium hydroxide

  

Flask #1

Flask #2

Mass KHP (g)

  

End Volume (mL)

  

Initial Volume (mL)

  

Volume of NaOH Used (mL)

  

calculations for ”volume used (mL)” and ”volume used (L)”

Vol. Used. (L)

  
 

Moles of KHP

  

Show calculations for moles of KHP

Show calculations for molarity of NaOH

Molarity of NaOH (M)

  

Average Molarity of NaOH (M)

 

Name: ______________________________________ Date: ____________

Part B: Determination of Mass% of acid in Vinegar.

 

(Trial)

Sample #1

Sample # 2

Sample # 3

Sample # 4

Volume of Vinegar (L)

     

Final burette reading (mL)

     

Initial burette reading (mL)

     

Volume NaOH (mL)

     

Sample calculation for Volume of NaOH (mL) for two samples

Show calculation for Volume of NaOH (L) for two samples

Volume NaOH (L)

     

Sample calculation for moles of NaOH for two samples

Moles of NaOH

     
 

Moles of CH3COOH

     

Sample calculation for molarity of CH3COOH for two samples

Molarity of CH3COOH

     

Average Molarity of CH3COOH =

 

Calculations:

  1. Calculate the average molarity of CH3COOH in the vinegar solutions for samples 1-4
  2. Using the average molarity of acetic acid calculate moles of CH3COOH in 2.00 mL of the vinegar solution
  3. Determine the mass of acetic acid in the vinegar sample (Molar mass of acetic acid = 60 g/mol)
  4. Assuming the density of vinegar is 1.005g/mL, determine the mass in grams of 2.00 mL of vinegar
  5. Determine the mass percent of acetic acid in the vinegar solution %(m/m)
  6. The commercial vinegar you titrated is 5% acetic acid by mass, calculate the percent error using the equation below

% Error = |True Value-Experimental Value| x 100%

True Value

 

  1. Give two possible sources of error

NAME: ________________________________________ DATE: ______________

Post-Lab

  1. The concentration of a NaOH solution is 0.1050 M. A KHP sample required 20.15 mL of NaOH solution. How many grams of KHP were titrated?
  2. Sodium carbonate (Na2CO3) reacts with 25.5 mL of 0.325 M hydrochloric acid. How many grams of solid sodium carbonate will react completely with the acid?

Na2CO3 (aq) + 2HCl 2NaCl (aq) + H2CO3 (aq)

  1. During the titration the student observed an air bubble in the burette tip and it gets removed during the titration by flow of the titrant. Is the calculated molarity of NaOH less than, more than or equal to the correct molarity of NaOH? Explain your answer.
  2. During the titration, you delivered too much NaOH from the burette. Is the calculated molarity of NaOH less than, more than or equal to the correct molarity of NaOH. Explain your answer.
  3. A 20.0 mL of an 5.00 M stock solution of NaOH is diluted to 200.00 mL.
    1. What is the concentration of the resulting solution?
    2. Write the balanced equation and net ionic equation for the neutralization of sulfuric acid (H2SO4) with sodium hydroxide.
    3. What volume of 0.200 M H2SO4 is required to titrate 20 mL of the diluted NaOH solution described above? Show all calculations.
  4. Would the percent acetic acid change if 20.00 mL of vinegar had been titrated in place of 2.00 mL? Explain your reasoning.
  5. How would the percent mass of vinegar change if the density of the vinegar sample was higher than our assumption that the density of vinegar is 1.005g/mL?
  6. How would the mass % of vinegar change if a student used 0.50 M sodium hydroxide instead of 0.10 M sodium hydroxide (not knowing that a mistake was made)?

NAME: _______________________________________ DATE: ______________

Pre-Lab

  1. Describe any hazards associated in with working with 0.5M Sodium hydroxide.
  2. Calculate the volume of 0.5 M. NaOH required for preparing 500 mL of 0.1000 M NaOH.
  3. Define the following terms:
    1. Titration –
    2. Indicator –
    3. Primary standard –
    4. Equivalence point
    5. End point
    6. Meniscus –
  4. A student standardized a NaOH solution and obtained the following data:

Mass of KHP = 0.4755 g

End Volume = 25.10 mL

Initial Volume = 0.00 mL

Calculate the following (show your work):

    1. Total volume of NaOH solution required for titration (mL) and (L)
    2. Moles of KHP
    3. Moles of NaOH
    4. Molarity of NaOH
  1. Why is the burette rinsed initially with NaOH?
  2. What is the formula for calculating the %( m/m) of a solution?
  3. A student determined that 5.5 mL of acetic acid is neutralized by 20.1 mL of 0.110 M NaOH. Based on the data, answer the following questions. Show all calculations.
    1. Write the balanced equation for the above reaction
    2. Calculate the # of moles of sodium hydroxide reacted with acetic acid
    3. Calculate the # of moles of acetic acid reacted with sodium hydroxide
    4. Calculate the molarity of acetic acid solution
    5. Assuming the density of vinegar used is 1.005g/mL, determine the mass of vinegar used in grams
    6. Determine the mass of acetic acid in the vinegar sample (Molar mass of acetic acid = 60 g/mol)
    7. Determine the mass percent of acetic acid in the vinegar solution %(m/m)
    8. Calculate the percentage error in the experimental value of mass % of acetic acid.
Order a unique copy of this paper
(550 words)

Approximate price: $22