Acid base titration lab advanced chemistry with vernier answers

LabQuest

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Chemistry with Vernier 24 - 1

Acid-Base Titration

A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. In this experiment, you will titrate hydrochloric acid solution, HCl, with a basic sodium hydroxide solution, NaOH. The concentration of the NaOH solution is given and you will determine the unknown concentration of the HCl. Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:

H + (aq) + Cl

– (aq) + Na

+ (aq) +OH

– (aq)  H2O(l) + Na

+ (aq) + Cl

– (aq)

When an HCl solution is titrated with an NaOH solution, the pH of the acidic solution is initially low. As base is added, the change in pH is quite gradual until close to the equivalence point, when equimolar amounts of acid and base have been mixed. 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.

In this experiment, you will use a pH Sensor 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 HCl.

Volume NaOH (mL)

pH

Figure 1

OBJECTIVES

In this experiment, you will

 Use a pH Sensor to monitor changes in pH as sodium hydroxide solution is added to a hydrochloric 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.

 Use the results to calculate the concentration of the hydrochloric acid solution.

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MATERIALS

PROCEDURE

1. Obtain and wear goggles.

2. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. Add 40 mL of distilled water to a 100 mL beaker. Use a pipet bulb (or pipet pump) to pipet 5.00 mL of the HCl solution into the 100 mL beaker with distilled water.

3. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Obtain approximately 40 mL of ~0.1 M NaOH solution in a 250 mL beaker. Record the precise NaOH concentration in your data table.

4. Obtain the plastic 60 mL reagent reservoir. Note: The bottom valve will be used to open or close the reservoir, while the top valve will be used to finely adjust the flow rate. For now, close both valves by turning the handles to a horizontal position.

Rinse it with a few mL of the ~0.1 M NaOH solution. Use a utility clamp to attach the reagent reservoir to the ring stand. Add the remainder of the NaOH solution to the reagent reservoir.

Drain a small amount of NaOH solution into the 250 mL beaker so it fills the reservoir’s tip. To do this, turn both valve handles to the vertical position for a moment, then turn them both back to horizontal.

5. Connect the pH Sensor to CH 1 of LabQuest. Lower the Drop Counter onto a ring stand and connect it to DIG 1. Choose New from the File menu. If both sensors are identified, proceed to Step 6.

If you have older sensors that do not auto-ID, manually set them up:

a. Choose Sensor Setup from the Sensors menu.

b. Select Drop Counter from the DIG 1 sensor list, or pH Sensor from the CH 1 sensor list.

c. Select OK.

6. Calibrate the Drop Counter so that a precise volume of titrant is recorded in units of milliliters.

a. Choose Calibrate ► Drop Counter from the Sensors menu:

LabQuest magnetic stirrer (if available) LabQuest App stirring bar or Microstirrer (if available) Vernier pH Sensor wash bottle HCl solution, unknown concentration distilled water ~0.1 M NaOH solution ring stand pipet bulb or pump 1 utility clamp 250 mL beaker Vernier Drop Counter

60 mL reagent reservoir 5 mL pipet or graduated 10 mL pipet 100 mL beaker 10 mL graduated cylinder

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b. Select Calibrate Now.

c. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter, lining it up with the tip of the reagent reservoir. The reservoir tip should be aligned so that a drop of titrant will make a clean pass through the opening on the drop counter and land in a 10mL graduated cylinder placed below.

d. Open the bottom valve on the reagent reservoir (vertical). Keep the top valve closed (horizontal).

e. Slowly open the top valve of the reagent reservoir so that drops are released at a slow rate (~1 drop every second). Note: Your flow rate is now set. Do not adjust the top valve for the remainder of the experiment. Use the only the bottom valve to turn reservoir flow on and off. You should see the drops being counted on the screen. If drops are not shown on your screen, ask your TA for assistance. There may be an issue with your drop counter, your LabQuest handheld, or both.

f. When the volume of NaOH solution in the graduated cylinder is between 9 and 10 mL, close the bottom valve of the reagent reservoir.

g. Enter the precise Volume of NaOH. Select Stop. Go to the “equation” tab at the top of the screen. Record the number of drops/mL displayed on the screen for possible future use. Default value is 28.00 drops/mL. Your value is certain to be different. If this is not the case, the calibration has failed or reset. Select OK.

h. Test your calibration. Place a 10mL graduated cylinder under the drop counter as before. Prompt the LabQuest to record data (no data will record until a drop has passed through the drop counter). Open the bottom valve on the reservoir to begin titrant flow. You should see a volume (in mL) recorded on the LabQuest. Close the bottom valve when the volume has reached between 6 and 7 mL on the graduated cylinder. Write down the actual volume in your graduated cylinder. Compare this value to the volume recorded on the LabQuest. If they are identical, your calibration needs no further adjusting. If they are not, divide the volume recorded in the graduated cylinder by the volume recorded on the LabQuest. This is your new conversion factor. Multiply all future LabQuest volume recordings by this conversion factor.

7. Discard the NaOH solution in the graduated cylinder into the aqueous waste bin. Ask your

TA for its location.

8. Assemble the apparatus.

a. Place the magnetic stirrer (optional) on the base of the ring stand. (You can also use a copper wire stirrer).

b. Insert the pH Sensor through the large hole in the Drop Counter.

c. Attach the Microstirrer to the bottom of the pH Sensor, as shown in the small picture. Rotate the paddle wheel of the Microstirrer and make sure that it does not touch the bulb of the pH Sensor.

d. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up with the center of the magnetic stirrer.

e. Lift up the pH Sensor, and slide the beaker containing the HCl solution onto the magnetic stirrer. Lower the pH Sensor into the beaker.

f. Adjust the position of the Drop Counter so that the Microstirrer on the pH Sensor is just touching the bottom of the beaker.

g. Adjust the reagent reservoir so its tip is just above the Drop

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Counter slot.

9. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.

10. You are now ready to begin collecting data. Check to see that the pH value is between 1.5 and 2.5. Start data collection. No data will be collected until the first drop goes through the Drop Counter slot. Fully open the bottom valve—the top valve should still be adjusted so drops are released at a rate of about 1 drop every second. When the first drop passes through the Drop Counter slot, check the graph to see that the first data pair was recorded.

11. Continue watching your graph to see when a large increase in pH takes place—the equivalence point is contained within this data jump. When this sharp rise in pH occurs, let the titration proceed for several more milliliters of titrant, then stop data collection to view a graph of pH vs. volume. Your screen should resemble the titration curve depicted in Figure 1.Turn the bottom valve of the reagent reservoir to a closed (horizontal) position.

12. Dispose of the beaker contents in the aqueous waste bin.

13. Examine the data on the displayed graph of pH vs. volume to find the equivalence point. 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. Note: Another method for determining the equivalence-point volume is described in the Alternate Equivalence Point Method of this experiment.

14. (optional) Print copies of the graph.

15. If time permits, repeat the procedure.

ALTERNATE EQUIVALENCE POINT METHOD

An alternate way of determining the precise equivalence point of the titration is to take the first and second derivatives of the pH-volume data.

1. Determine the peak value on the first derivative vs. volume plot.

a. Tap the Table tab. Choose New Calculated Column from the Table menu.

b. Enter d1 as the Calculated Column Name. Select the equation, 1st derivative (Y,X). Use Volume as the Column for X, and pH as the Column for Y. Select OK.

c. On the displayed plot of d1 vs. volume, examine the graph to determine the volume at the peak value of the first derivative.

2. Determine the zero value on the second derivative vs. volume plot.

a. Tap the Table tab. Choose New Calculated Column from the Table menu.

b. Enter d2 as the Calculated Column Name. Select the equation, 2nd derivative (Y,X). Use Volume as the Column for X, and pH as the Column for Y. Select OK.

c. On the displayed plot of d2 vs. volume, examine the graph to determine the volume when the 2nd derivative equals approximately zero.

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PROCESSING THE DATA

1. Use your printed graph and data table to confirm the volume of NaOH titrant you recorded before and after the largest increase in pH values upon the addition of 1 drop of NaOH solution.

2. Determine the volume of NaOH added at the equivalence point. To do this, add the two NaOH values determined above and divide by two.

3. Calculate the number of moles of NaOH used.

4. See the equation for the neutralization reaction given in the introduction. Determine the number of moles of HCl used.

5. Recall that you pipeted out 10.0 mL of the unknown HCl solution for the titration. Calculate the HCl concentration.

DATA AND CALCULATIONS

Concentration of NaOH M M

NaOH volume added before largest pH increase mL mL

NaOH volume added after largest pH increase mL mL

Volume of NaOH added at equivalence point

mL

mL

Moles NaOH

mol

mol

Moles HCl

mol

mol

Concentration of HCl

mol/L

mol/L

Average [HCl]

M

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KEY LEARNING POINTS  titration  acid-base neutralization reaction  equivalence point  end point  titration curve  titrant  pH