EXPERIMENT 1

PRACTICE WITH QUANTITATIVE TECHNIQUES

Objective: To introduce students to the precise use of the analytical balance, pipets and a burette. The accuracy and precision techniques that are developed in this experiment are essential quantitative skills and will determine the result of future experiments.

Experimental

Equipment

The Analytical Balance

Burets

Pipets

Statistics

Timesaver Hint: There are four stations throughout the laboratory. I recommend that the groups divide themselves among those stations to minimize time.

Procedure

Part A: Balance Sensitivity

The analytical balances can measure differences in this laboratory as little as 0.1 mg (0.0001 g) which means that the presence of fingerprints, the evaporation of water from a beaker or pencil markers can greatly influence the accuracy and precision of a measurement.

1) Clean the balance and the surrounding area using the appropriate brush.

2) Take an index card located near the balance, place it on the pan and weigh it.

Reweigh it twice by removing it from the pan with a pair of forceps and replacing it. Be sure to close the doors of the balance during each weighing to prevent air currents from influencing the reading. Calculate and record the mean weight of the paper.

3) Next, remove the paper from the balance pan and write your full name on the paper.

Be sure to record the type of pen or pencil that you used for this. Place the paper back on the balance pan and reweigh this paper three times (Again using the forceps)

Calculate and report the mean weight of just your name. Also calculate the 95% confidence interval for the weight of your name. (Show work)

Part B: Calibration of a Glass Pipet

1) Clean the balance and the surrounding area using the appropriate brush. Clean all

glassware using acetone and Kim Wipes. Label all containers appropriately.

2) Ask the instructor to visually demonstrate the proper use of dispensing water from a

glass pipet. Practice this technique before moving to the balance area.

3) Measure and record the temperature of the water used for this experiment with a thermometer.

4) Weigh a 50-ml beaker. Remove the beaker from the balance pan using forceps, paper

towels or Kimwipes. Pipet 5.00 ml of water into the beaker and reweigh it. Note that the weight should be recorded immediately since the balance is sensitive enough to measure the rapid evaporation of water.

5) Dispense another 5.00 mL of water into the same beaker and reweigh it. Record the

total weight value. (This value is recorded directly from the balance = the weight of the water and the beaker) Calculate the values in the net weight column which should consecutively be 5.XX,10.XX,15.XX, 20.XX etc.) Calculate the weight of the additional 5.00-ml of water and record this as the additional weight Value. (The values in the Net weight column should consistently be 5.XX)

6) Repeat this procedure until you have performed 5 different pipetings.

7) Using the density of water (Refer to Appendix) calculate the average volume of water

dispensed.

Part C: Calibration of a Push-Button Pipet:

1) Clean the balance and the surrounding area using the appropriate brush.

2) Ask the instructor to demonstrate the proper use of dispensing water from a

push button pipet. Practice this technique before moving to the balance area.

2) Dispense another 100 uL of water into the same tube and reweigh it. Record the

total weight value. (This value is recorded directly from the balance = the weight of the water and the tube) Calculate the values in the net weight column which should also change consecutively) Calculate the weight of the additional 100uL of water and record this as the additional weight Value. (The values in the Net weight column should also be consistently.)

3) Repeat this procedure until you have performed 5 different pipetings. Be sure to use gloves during handling of the tube to prevent the contamination from fingerprints.

Part D: Calibration of a Buret

1) Clean the balance and the surrounding area using the appropriate brush. Clean all glassware using acetone and Kim Wipes. Label all containers appropriately.

2)Weigh one (1) empty labeled glass-weighing bottles with tops. Only handle the bottle with forceps or paper towels to avoid fingerprints.

3) Fill a buret with water. Be sure to 1) remove all air bubbles from the tip.2) check that when full the meniscus is below the 0.00 reading of the buret and 3) check that there are no drops on the end or tip of the buret. To avoid this touch the tip of the buret to the side of a beaker to remove any remaining drops.

3) Nominally (to measure approximately but know exactly) dispense 1 ml of water into

the first weighing bottle. By subtracting the buret reading in the end from the buret reading in the beginning the volume of water dispensed can be measured to 0.01 ml) Quickly cap the weighing bottle to prevent evaporation. Be sure to record the beginning and ending volumes of the buret.

4) Repeat this process by dispensing 2, 3, 4, and 5 ml. Reweigh the bottle with water

after each addition. Calculate the weight of water in each weighing bottle.

5) Calculate the linear regression line for the weights of water versus the volumes.

Calculate the slope of the graph.

DATA SHEET

PRACTICE WITH QUANTITATIVE TECHNIQUES

Part A: Balance Sensitivity

(20 points)

Type of pencil/pen used:

Weight of your name (show work):

(1) (2) (3)

Mean

Standard Deviation: Relative Standard Deviation:

95% Confidence Interval (show work):

Part B Calibration of a Glass Pipet:

(20 points) Total Weight Net Weight Additional weight

(after)

0 pipetings

1 pipeting

2 pipetings

3 pipetings

4 pipetings

5 pipetings

Average Volume Dispensed

Standard Deviation

Relative Standard Deviation

95% Confidence Interval (show work)

What is the measured value for the pipet? Does the 95% confidence interval include this value?

Which factor contributes more to the error: the imprecision in the balance or the imprecision in of the pipetings? Why?

Part C Calibration of a Push-Button Pipet:

(20 points)

Temperature Density

Total Weight Net Weight Net Volume

(after) (ml)

0 pipetings

1 pipeting

2 pipetings

3 pipetings

4 pipetings

5 pipetings

Average Volume Dispensed

Standard Deviation

Relative Standard Deviation

95% Confidence Interval (show work)

What is the rated value for the pipet? Does the 95% confidence interval include this value?

Which factor contributes more to the error: the imprecision in the balance or the imprecision in of the pipetings? Why?

How does the relative standard deviation for the push-button pipet compare to the relative standard deviation for the glass pipet?

Part D: Calibration of a Buret

(20 points)

Weight of Bottle:

Initial reading Final Reading Total Volume Total Weight

(mL) (mL) of H₂O of H₂O

Reading 1

Reading 2

Reading 3

Reading 4

Reading 5

Attach a graph of data and regression line.

Additional Questions:

(10 points)

1) What sort of errors do you expect from analytical balance measurements?

2) What precision can you expect from the analytical balance?

3) What is the best way to read a buret?

4) How is the 95% Confidence Interval calculated?

5) How is the linear regression calculated from a set of experimental data?

6) What is the significance of the slope of the line calculated in Part D. In other word, should this value agree with any already established property of water.

Notebook : Final grade: