EXPERIMENT #1 - INTRODUCTION TO THE ANALYTICAL BALANCE AND VOLUMETRIC GLASSWARE | Cheat Sheet Chemistry

Vers 2015-01-29

Experiment 1

INTRODUCTION TO THE ANALYTICAL BALANCE AND VOLUMETRIC GLASSWARE

2 lab periods

Reading: Chapter 2, Quantitative Chemical Analysis, 7th or 8th Edition, Daniel C. Harris and

CHEM 253 Quantitative Analysis Laboratory Experiments, 7th Edition, pg 4-11.

Objective

This lab will introduce the basic laboratory equipment and skills needed for accurate and precise

quantitative analysis.

Schedule

Lab 1 Practice using the analytical balance and clean your glassware. A brief soaking in warm

detergent solution should be enough to remove grease and dirt. If not, consult your TA.

Become familiar with your glassware and begin calibrating it.

Lab 2 Calibrate your glassware. During this lab period you will need to check out one or more

items from Lab Services. Sometimes long lines can develop at Lab Services, especially in

the afternoon sections. If there is a long line, start working on the parts of the experiment

you can do without the checked out equipment. Usually the lines are much shorter once the

initial “rush” dies down.

Use of the Analytical Balance

The ability to determine masses accurately is fundamental not only to analytical chemistry

but to all of modern science. All measurements that we will perform in this course relate either

directly or indirectly to weighing using an electronic analytical balance. The analytical balances in

Chem 253 can be used to obtain four or five digits to the right of the decimal point. They have a

precision of ±0.1 mg. These are delicate and precise instruments, which, if kept scrupulously clean

and handled correctly, will continue to yield reliable data during the course of their usage. It is

important that you become well acquainted with your balance. You should be confident that the

balance is working properly and that your technique is correct before you proceed with any

experiment. You should also read the section in your text on the analytical balance (Section 2.3 in

Quantitative Chemical Analysis, 8th Edition, pp. 31-34).

1. Introduction to the balance: Attend a demonstration by your instructor on how to operate the

balance and how to handle objects for weighing. After this demonstration, familiarize yourself

with the controls on the balance and learn to read the optical scale to the nearest 0.1 mg (0.0001

g). Zero the optical scale with the pan clean and empty.

Report any defects or poorly functioning controls to your TA immediately; do not try to make

any adjustments until you have received specific instructions. Check the level of the balance.

If the balance is out of level, please report it to the TA. Do not try to adjust the level by yourself.

The floor and pan of the balance should be scrupulously clean. The balance doors should be

closed except when you are loading your sample.

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Experiment 1 INTRODUCTION TO THE ANALYTICAL BALANCE AND VOLUMETRIC GLASSWARE 2 lab periods Reading: Chapter 2, Quantitative Chemical Analysis, 7th^ or 8th^ Edition, Daniel C. Harris and CHEM 253 Quantitative Analysis Laboratory Experiments , 7 th^ Edition, pg 4-11. Objective This lab will introduce the basic laboratory equipment and skills needed for accurate and precise quantitative analysis. Schedule Lab 1 Practice using the analytical balance and clean your glassware. A brief soaking in warm detergent solution should be enough to remove grease and dirt. If not, consult your TA. Become familiar with your glassware and begin calibrating it. Lab 2 Calibrate your glassware. During this lab period you will need to check out one or more items from Lab Services. Sometimes long lines can develop at Lab Services, especially in the afternoon sections. If there is a long line, start working on the parts of the experiment you can do without the checked out equipment. Usually the lines are much shorter once the initial “rush” dies down. Use of the Analytical Balance The ability to determine masses accurately is fundamental not only to analytical chemistry but to all of modern science. All measurements that we will perform in this course relate either directly or indirectly to weighing using an electronic analytical balance. The analytical balances in Chem 253 can be used to obtain four or five digits to the right of the decimal point. They have a precision of ±0.1 mg. These are delicate and precise instruments, which, if kept scrupulously clean and handled correctly, will continue to yield reliable data during the course of their usage. It is important that you become well acquainted with your balance. You should be confident that the balance is working properly and that your technique is correct before you proceed with any experiment. You should also read the section in your text on the analytical balance (Section 2.3 in Quantitative Chemical Analysis, 8th^ Edition , pp. 31-34).

Introduction to the balance: Attend a demonstration by your instructor on how to operate the balance and how to handle objects for weighing. After this demonstration, familiarize yourself with the controls on the balance and learn to read the optical scale to the nearest 0.1 mg (0. g). Zero the optical scale with the pan clean and empty. Report any defects or poorly functioning controls to your TA immediately; do not try to make any adjustments until you have received specific instructions. Check the level of the balance. If the balance is out of level, please report it to the TA. Do not try to adjust the level by yourself. The floor and pan of the balance should be scrupulously clean. The balance doors should be closed except when you are loading your sample.

A further note of caution is that all objects or samples to be weighed must be brought to thermal equilibrium with the room (ambient temperature) before a weighing can be attempted. If you are unsure whether your sample is at ambient temperature, put the sample on the balance and observe whether the reading changes over a two-minute period. Temperature gradients within the balance can cause convection currents and a hot sample will lead to buoyancy errors, both of which can degrade your results.

Weighing out samples: Weighing by difference is the most accurate method we can use to quantitatively determine the mass of a solid sample. This procedure involves repetitive weighings of a weighing bottle containing a quantity of solid reagent. As the solid is removed to another vessel, the mass of the weighing bottle plus contents decreases. Upon reweighing, a lower mass is found. The difference in the two masses represents the mass of solid reagent transferred to the vessel. Hence, the phrase "weighing by difference". In this experiment you will use common table salt (NaCl) to practice weighing by difference. The goal is to weigh out three 0.7 g aliquots of NaCl as accurately as possible. Practice weighing by difference
Place a pproximately 3 g of NaCl (plain table salt) in a clean, dry glass weighing boat.
Record the mass of the weighing boat in your notebook.
Remove the weighing boat and transfer some of the NaCl to the beaker or flask.
Replace the weighing boat on the balance pan and record the new mass. Subtract this new, smaller, mass from the previous mass. Your sample should weigh somewhere in the range of 0.6800 to 0.7200 g. Don’t worry if the mass is slightly over or under these values. You may find it hard at first to estimate the point at which you have transferred ~0.7 g into your flask or beaker. You might have to try the transfer several times until you get it right. If you overshoot, don’t transfer any reagent back into the weighing bottle. Simply discard the solid reagent in the proper manner (see the TA for instructions), rinse out the vessel, and begin the weighing and transfer process over again. Once you have an appropriate amount of reagent transferred, use this amount as a visual guide to estimate subsequent samples.
Repeat these steps for each of your three samples. Record your masses in your notebook as follows. Make sure to use proper mass units. _Initial (larger) mass (g)

Final (smaller) mass (g) Difference (g)_

Clean up your balance station. Calibration of Volumetric Glassware Quantitative analysis is a very precise science and can only be completed successfully with exact measurements and data. Glassware used in quantitative experiments is of the highest grade, but it is not perfect. Therefore, every piece of volumetric glassware must be calibrated in order to allow for the slight variations present in every piece of glassware.

Make a calibration curve for your buret like the one shown in Figure 1 below. Table 1 Calibration of a 50.00 mL buret, Trial 1 10 mL 20 mL 30 ml 40 mL 50 mL Mass of the bottle (g) 30. Mass of the bottle plus water (g) 39.895 49.876 59.856 69.883 79. Mass of water (g) 9.895 19.876 29.856 39.883 49. Final Reading, buret (mL) 10.22 20.23 30.23 40.24 50. Initial reading, buret (mL) 0.23 10.22 20.23 30.23 40. Volume delivered, buret (mL) 9.99 10.01 10.00 10.01 9. Total volume delivered, buret (mL) 9.99 20.00 30.00 40.01 49. True volume calculated from mass of water (mL)

Correction (mL) - 0.07 - 0.08 - 0.08 - 0.04 - 0. Example calculations Volume delivered = final volume – initial volume 10.22 mL – 0.23 mL = 9.99 mL True volume = mass /density 9.895 g (1 mL / 0.99789 g) = 9.916 mL (at 21.5°C) Correction factor, 10 mL = true volume – volume delivered 9.915(8) mL – 9.99 mL = - 0. Making the calibration curve Calculate a correction factor for each volume. Plot the correction (mL) vs total volume delivered (mL) using Excel or a similar plotting program. Make sure your plot has a title, labeled axes, and units.

Figure 1 How to use the calibration curve: The buret used to acquire the above data doesn’t really deliver the exact volume that we read from the buret. The true volume delivered (calculated from the mass) is consistently less than it should be. However, it is not convenient to always weigh out whatever analyte is being delivered from the buret. Therefore, we use the calibration curve to correct the volumes delivered from the buret. The tolerance of a Class A 50 ml buret is ±0.05 mL. If your buret is far out of this range, you’ll want to trade it in for a new one from Lab Services. Remember to calibrate your new buret, too. How to apply the correction: Amparo used the above buret to deliver 30.21 mL of a solution. She calculates the true volume delivered in the following way: volume read from buret + correction factor at a particular volume = corrected volume. 30.21 mL + - 0.08 mL = 30.13 mL The corrected volume is 30.13 mL. This is the value Amparo will use in her calculations. Always use the calibration curve when you use your buret. Be careful when acquiring the data for your calibration curve. Make sure you read the buret carefully. Calibration of 5, 10 and 25 mL pipets. Calibrate your pipets in the same way as for the buret. Make three separate measurements for each pipet. Calculate the average and the standard deviation for your data from the averages. Calculate the true volume delivered from the pipet from the mass of the water, as you did in the buret calibration procedure. Calculate a correction factor for each pipet. Calibration of your 50 and 100 mL volumetric flasks. Calibrate your 50 and 100 mL volumetric flasks in the following way: Place the empty volumetric flask on the top-loading balance and write down the weight of the flask. Remove the flask from the balance. Then, carefully add DI water just until the bottom of the meniscus rests on the top of the mark on the neck of the flask. Reweigh. Discard the water and refill the flask. Repeat three times. Calculate the mean and standard deviation, the true volume, and the correction factor.

Report Sheet Experiment 1 Name/Date ________________________ Calibration of Volumetric Glassware Experimental Purpose Data and Calculations Weighing by difference Mass of Weigh boat___________________ Measurement 1 Measurement 2 Measurement 3 Initial Mass (g) Final Mass (g) Difference (g) Calibration of a 50 mL buret (MAKE SURE YOU ARE READING THE BURET BASED ON THE CORRECT SIGNIFICANT FIGURES!) 10 mL 20 mL 30 ml 40 mL 50 mL Mass of the bottle (g) Mass of the bottle plus water (g) Mass of water (g) Final Reading, buret (mL) Initial reading, buret (mL) Volume delivered, buret (mL) Total volume delivered, buret (mL)

11.46 11.46^ 11.46 11.

Water temperature:___________________^ 24.

Water Density:______________________0.99718 g/ml

True volume calculated from mass of water (mL) Correction (mL)

Attach the graph of average correction factors vs. apparent volumes (total volume

delivered) to the Report Sheets.

Calibrate Pipets 5mL, 10mL and 25mL (USE A BEAKER TO MASS THE

WATER)

5 mL 5 mL 5 mL Mass of the beaker (g) Mass of the beaker plus water (g) Mass of water (g) True volume calculated from mass of water (mL) Total volume delivered, pipet (mL) Correction (mL) 10 mL 10 mL 10 mL Mass of the beaker (g) Mass of the beaker plus water (g) Mass of water (g) True volume calculated from mass of water (mL) Total volume delivered, pipet (mL) Correction (mL)

47.82 47.64^ 47.

52.8 54.43^ 54.

Water temperature:___________________ Water Density:______________________

0.99718 g/ml

5 ml 5 ml 5ml

47.87 47.91^ 47.

10ml 10ml 10ml

0.23 0.12ml*

0.0008ml

- 0.04** -0.11^ -0.54^ -0.65-0.96

100 mL 100 mL 100 mL Mass of the flask (g) Mass of the flask plus water (g) Mass of water (g) True volume calculated from mass of water (mL) Total volume delivered, flask (mL) Correction (mL) Calibration of volumetric flasks 50 mL 100 mL Mean Correction (mL) Mean true volume ± standard deviation (mL) Show both numbers in data table

EXPERIMENT #1 - INTRODUCTION TO THE ANALYTICAL BALANCE AND VOLUMETRIC GLASSWARE, Cheat Sheet of Chemistry

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