Standardization of NaOH and Acid-Base Titration
Adapted from Denise DeMartino, Westlake High School
Introduction:
A titration is a method of volumetric analysis to analyze an unknown. By this method we can accurately determine the concentration of acid or base in an unknown solution.
In this experiment, you will titrate an acetic acid solution with a basic sodium hydroxide solution, NaOH to determine the concentration of acetic acid. We will FIRST need to determine the concentration of the NaOH solution by standardizing it with Potassium Hydrogen Pthalate (KHP = weak acid). The standardization will determine your NaOH concentration which is necessary because NaOH likes to react with the water in the air and decompose! In order for us to be sure of its’ concentration, you must first standardize the NaOH solution. You will then use the NaOH solution to determine the unknown concentration of your acid. Hydrogen ions from the CH3COOH react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:
Complete ionic eqn: H+(aq) + CH3COO–(aq) + Na+(aq) +OH–(aq) H2O(l) + Na+(aq) + CH3COO–(aq)
Net ionic eqn: ______
When an acid solution is titrated with a basic solution, the pH of the acidic solution is initially low. As base is added, the change in pH is quite gradual until it approaches the equivalence point, where acid and base have been mixed in equimolar amounts. Near the equivalence point, the pH increases very rapidly, as shown in Figure 1. The change in pH then becomes more gradual again, before leveling off with the addition of excess base forming a sigmoidal curve. In this experiment, you will use a pH sensor and LABQUEST to monitor pH as you titrate. The region of most rapid pH change will then be used to determine the equivalence point. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the CH3COOH.
Figure 1 Figure 2
OBJECTIVES
In this experiment, you will
· Carefully standardize NaOH and titrate an unknown concentration of acetic acid.
· Use a pH Sensor to monitor changes in pH as sodium hydroxide solution is added to an acetic acid solution.
· Plot a graph of pH vs. volume of sodium hydroxide solution added.
· Use the graph to determine the equivalence point of the titration and calculate the concentration of the acetic acid solution.
PreLab (in notebook): Name, Date, Title, Purpose, Procedural outline
PreLab Questions 1-7
1. What is the molar mass of KHP?
2. If 0.563 g of KHP are measured and placed into 230 mL of water, what is the molarity of your KHP
solution?
3. What is the difference between an endpoint and the equivalence point in a titration?
4. Titration reveals that 11.6 mL of 3.0 M hydroiodic acid are required to neutralize the sodium hydroxide in 25.00 mL of NaOH solution. What is the molarity of the NaOH solution?
5. If 15.0 mL of 0.50 M NaOH is used to neutralize 25.0 mL of HBr, what is the molarity of the acid
solution?
6. HELP! I found an unlabeled bottle of CH3COOH in the back of the chemical storage room. I asked my Chem nerd husband Bob to analyze the CH3COOH. I asked him to determine the concentration of the acid with a known concentration of NaOH using the same procedure you will be doing in this lab.
Unlike your acid, Bob’s acid much too concentrated to use as it he found it in the original bottle. He pipetted 10 mL of the CH3COOH into a 250mL flask and then diluted it to the 250mL mark.
Bob then pipetted 25mL of the diluted acid into a beaker and proceeded to titrate it with 0.1M NaOH. He created the titration curve below.
a. Help Bob out! Calculate the concentration of the diluted CH3COOH.
b. Now calculate the concentration of the concentrated (undiluted) acid in the original bottle.
c. Glacial CH3COOH has a density of 1.05g/mL. Calculate the % by mass of the acid in the original bottle.
(hint: part mass/whole mass x 100%)
7. Copy data table with units.
PROCEDURE:
Day 1
Part 1. Standardization of NaOH
1. Goggle up!
2. Obtain a sample of KHP that has been dried in an oven overnight.
3. Use a balance to weigh 0.4 to 0.6 g of KHP and record the precise mass in your Data Table.
4. Transfer the KHP into a beaker and add about 40 mL of distilled water and swirl until ALL the solid is dissolved.
5. Obtain approximately 75 mL of NaOH solution from bottle A or B and record bottle name.
6. Rinse a 50 mL buret 3 times with approximately three small portions (5-7 mL each) of the NaOH solution and discard wash solution in a beaker.
7. Fill the buret above the 0 mL mark with NaOH
8. Open the buret and close the stopcock at the 0 mL mark, making sure there are no air bubbles in the mouth of the buret.
9. Measure the precise volume in the buret and record this value as the “initial volume”.
10. Add 3 drops of phenolphthalein solution to KHP in the beaker.
11. Begin your titration! Add about 1 mL of the NaOH solution to the beaker at a time and let
the solution stir for a few seconds. Repeat the additions until about 15 mL are added.
12. Reduce your additions to 0.5 mL until the pink color doesn’t “stir away”. (See how faint you
can get the pink color…this takes skill but you can get a really light pink color if you are careful!)
13. Measure the volume of NaOH in the buret once the pink color persists and record this number as your “final volume”.
14. Repeat titration once more (switch jobs so everyone gets a chance to work with the buret) and remember to rinse your beaker thoroughly between the titrations.
Part 2. Calibration of pH meters
1. Obtain a LABQUEST and pH probe as well as two labeled beakers, one containing a pH =7.00 buffer and the other containing a pH = 10 buffer.
2. Plug in the pH probe to CH1 in the back of the LABQUEST and turn on the device.
3. Note the initial pH reading and make sure there is some movement in this value. If you have a single value that isn’t wavering a bit, the pH probe may not be registering correctly.
4. Go to the Sensor menu at the top and choose Calibrate à CH1.
5. Rinse the probe with distilled water and then completely immerse the probe into the pH = 10.00 buffer. Don’t let the bottom of the probe be scratched by the bottom of the beaker so keep the probe suspended in the buffer solution. Let the probe register for ~30 seconds and type in 10.00 into the 1st pH box. Press KEEP.
6. Wash the probe with distilled water and repeat step 4 for the pH = 7.00 buffer and press KEEP, and SAVE.
YAY! Now our pH meters are calibrated for the next step…you can check the pH by washing the probe and placing it back into the pH = 10.00 buffer, what does it read after it completes its measurement???
7. CLEAN UP!!!!!! CALCS FOR HOMEWORK!!
Day 2. Titration of an Unknown Concentration of Acetic Acid.
1. Goggle up!
2. Obtain approximately 50 mL of your acetic acid solution in a beaker.
3. Connect the pH Sensor to LabQuest and choose New from the File menu.
4. Set up the data-collection mode.
a. On the Meter screen, tap Mode. Change the data-collection mode to Events with Entry.
b. Enter the Name (Volume) and Units (mL). Select OK
5. Use a utility clamp to suspend a pH Sensor on a ring stand as shown in Figure 2. Position the pH Sensor in the acid solution and adjust its position so that it is not struck by the stirring bar.
6. Set up the buret as you did before (washing, initial volume reading) with your known
standardized NaOH from Day 1!
8. You are now ready to perform the titration. This process is faster if one person manipulates and reads the buret while another person operates and enters volumes.
a. Start data collection by pressing the green signal at the bottom left of your screen.
b. Before you have added any drops of NaOH solution, tap Keep and enter 0 as the buret volume in mL. Select OK to store the first data pair for this experiment.
CALL ME OVER TO CHECK YOUR SETUP! Okay, now proceed…
c. Add the next increment of NaOH titrant (usually 1 mL is safe to start). When the pH stabilizes, tap Keep, enter the current buret reading (to the nearest 0.01 mL), and then select OK. You have now saved the second data pair for the experiment.
d. Continue adding NaOH solution in increments. When a you notice the pH value change is getting larger (about 3.5 units) change to a 1-2 drop increment. Enter a new buret reading after each increment. After a pH value has risen significantly and begins to level off, again add larger increments (1-5 mL), and enter the buret level after each increment.
e. Continue adding NaOH solution until the pH value remains constant or the graph flattens out
9. Stop data collection.
10. Examine the data on the displayed graph of pH vs. volume to find the equivalence point—that is the largest increase in pH upon the addition of 1 drop of NaOH solution. To examine the data pairs on the displayed graph, tap any data point. As you tap each data point (or use the ► or ◄ keys on LabQuest), the pH and volume values are displayed. Move to the region of the graph with the largest increase in pH. Find the NaOH volume just before this jump. Record this value in the data table. Then record the NaOH volume after the drop producing the largest pH increase was added.
11. Repeat the procedure two more times.
12. Rinse the pH Sensor and return it to me. Clean all equipment, return to proper place and wipe down tables.
Data Analysis
1. Determine the volume of NaOH added at the equivalence point. To do this, add the two NaOH values determined above and divide by two.
2. Calculate the number of moles of NaOH used.
3. See the equation for the neutralization reaction given in the introduction. Determine the number of moles of HCl used.
4. Recall that you pipeted out 50.0 mL of the unknown CH3COOH solution for the titration. Calculate the acid concentration.
data Table and calculations trial 1 trial 2 trial 3
Concentration of NaOH / M / M / MNaOH volume added before largest pH increase / mL / mL / mL
NaOH volume added after largest pH increase / mL / mL / mL
Volume of NaOH added at equivalence point / mL / mL / mL
Moles NaOH / mol / mol / mol
Moles CH3COOH / mol / mol / mol
Concentration of CH3COOH / mol/L / mol/L / mol/L
M
PostLab:
DATA Table: Fill in with all information but do calculations in separate section.
Calculation Section: Calculate the CH3COOH concentration for all three trials. Show work for ALL calcs for ALL THREE TRIALS!!!
PostLab questions:
1. Why did we need to standardize the NaOH? What is the purpose of standardization?
2. What is the function of an indicator? Which indicator was used in the standardization?
3. What is the concentration of your acetic acid solution?
4. Chem nerd Bob repeated your experiment and instead of using 50 mL of acid to start, he only put 10 mL of acid in the beaker and diluted it with 40 mL of water. What did he find as his acid concentration?
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