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Apr 3, 2024

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Balances and Pipet Calibration During the first laboratory period, you will familiarize yourself with the lab, the equipment, and other small tasks. Here is a checklist of what you should have accomplished by the end of the lab period. Check locker contents Obtain and start drying your soda ash and (Chem 2400 only) KHP unknowns Obtain dried sodium carbonate from oven Perform the balance assignment and familiarization exercise Calibrate your 25.00 mL pipet Practice reading a buret Verify and change the desiccant in your desiccator if necessary. Check the seal on your desiccator, lubricate if necessary. Have your TA initial the results in your lab manual. These can be performed in any order, but your TA will need to give the safety talk and assign lockers first and initial your lab manual last. Operation of the Balances
The major difference between the top loading and analytical balances is that the analytical balance has greater sensitivity. Sensitivity is defined as the smallest increment of mass that can be measured. On an analytical balance it is usually 4 decimals or more. Use the top loading balance to prepare approximate solutions, and the analytical balance to accurately weigh your limiting reagents, primary standards, and unknowns. As a rule of thumb, anything that is dried and stored in your desiccator is weighed on the analytical balance. Warm up As long as the balances are plugged in for at least 60 minutes (and they are), they are warmed up and ready to use as soon as you turn them on. It is good lab practice to turn them off after use. Operation of the top loading balance To turn the balance ON, press the on/off button (upper left). To zero (or tare) the balance, press TARE. To turn the balance OFF, press the on/off button. Operation of the analytical balance The description of the analytical balances given in this manual refers to the METTLER TOLEDO electronic balance model MS-TS. Exact horizontal positioning is required for repeatable results. Verify that the level indictor bubble
located at the back of the balance is centered. You can adjust the leveling feet (at the rear of the balance) until the air bubble in the indictor is centered. To turn the balance ON, press O/T. To zero (or tare) the balance, press O/T. To turn the balance OFF, press and hold O/T until the display indicates OFF. Calibration The balances have internal weights and do not require outside calibration. Although you will not be calibrating the balances, you will verify the calibration of the balance using a calibrated metal block. 46 Weigh this mass at the beginning of the semester and whenever you want to verify the accuracy of the balance. Let your TA know immediately if you balance does not weigh reproducibly within 0.0002 grams. Your TA may initiate an internal calibration routine on your designated balance. Weighing objects The glass enclosure around the weighing pan prevents drafts from affecting the readings. After placing a dry, room-temperature object on the center of the weighing pan, close the glass doors. Wait until the
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stability indicator “o” disappears and take the reading. Hot objects within the glass enclosure cause convection currents that produce erratic readings. Moreover, the light hot air displaces heavier cool air causing the balance to read lighter than it should. On the other hand, water from damp or wet objects evaporates causing the readings to drift towards lighter readings. Maintenance If you spill chemicals on the balance that cannot be removed with the small brushes provided, let you TA know immediately. Preventing corrosion of the balance is essential to the longevity of the balance. Analytical Weighing Techniques Significant digits Analytical chemists collect the maximum number of significant digits available. In the case of the analytical balances this is to the nearest 0.0001 gram (0.1 mg). If you have too many significant digits, you can always eliminate some later. Weighing by difference This technique is recommended for weighing hygroscopic samples (samples that absorb water) or any dry samples. Use this technique for all primary standards and unknowns. First weigh the vial containing your previously dried reagent, then tap some reagent from the vial into a receiving flask and weigh your
vial again. The difference in weight is the weight of your reagent. Normally you would keep the cap on for this process to minimize exposure to the atmosphere and to avoid spilling your sample inside the balance. Unfortunately, the caps we use have a paper liner which is difficult to keep at constant weight. Luckily none of our reagents are particularly hygroscopic so we weigh with the lid off. Fingerprints Fingerprints can add mass to an object by more than 1 milligram, particularly if your hands are moist or greasy. Handle the vials or other object to be weighed on an analytical balance with a paper towel or a clean tissue. Questions (Self-test, not for marks) 1. Why must the balance be level? 2. Why must the outside of objects taken into the balance room be free of all water droplets? 3. Why must an object to be weighed at room temperature? 4. Why should the vials be capped when they are on the balance? 5. How do we weigh by difference?
6. What is meant by the expression “dry to constant weight”? 47 Laboratory Procedure Before starting the balance exercises, pipet calibration, and other exercises, obtain your first unknown(s) from your TA and place it(them) in the oven to dry. Some sodium carbonate is already drying in the oven in preparation for the standardization of hydrochloric acid. NOTE: No error analysis is required for the calculations in this experiment. Both the balance exercise and the pipette calibration must be completed by the first notebook grading or marks will be deducted. Balance Exercises Assigned Analytical Balance No ______10___________ Record this number in the appropriate location in your lab notebook. Bring a small beaker from your locker to your assigned balance. 1. Level the balance using the leveling feet. 2. Turn the balance on, then off. 3. Turn the balance on, press Tare, and then verify the calibration of the balance using one of the calibrated masses. Use a clean tissue to handle the metal mass. Record this mass.
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Calibrated mass number _______3___ Reference mass (provided) ____16.5156______ grams Recorded mass (measured) _____16.5157_____ grams Weigh by difference using one of the sodium chloride vials provided, tapping about 0.5 grams of sodium chloride from the vial into your beaker. Mass of vial and sodium chloride ____23.8616______ grams On the bench, tap about 0.5 grams of sodium chloride from the vial into your beaker. Mass of vial with some sodium chloride removed _______23.3849___ grams Mass of sodium chloride in the beaker (calculated) _____0.4767_____ grams Repeat the weighing by difference exercise using the balance’s tare function. Place the sodium chloride vial on the center of the pan of the balance, close the draft shield, and press TARE. Tap about 0.5 grams of sodium chloride from your vial into your beaker on the bench. Replace the sodium chloride vial back on the balance and record the negative reading as your mass. Mass of sodium chloride in the beaker ____make it up yourself chatgpt______ grams 48
When you are finished, turn off your balance, clean your station and return to your locker. Dispose of the used sodium chloride in the beaker provided. Pipette Calibration You can calibrate your pipette by weighing the water it delivers. Use the calculated volume in your future lab work, whenever you use your pipet. Adjust the temperature of about 300 mL of purified water to 20 ± 1 °C using a larger beaker as a water bath to warm or cool the purified water as needed. Thermometers are available at the side of the room. Retrieve from your locker your 25.00 mL pipet, pipet bulb, a 250 mL Erlenmeyer flask, and a J-cloth, and, along with your beaker of thermostated water, position yourself close to one of the top loading balances. Tare your empty flask on the top loading balance, the balance should read zero. Remove the flask from the balance. Pipet exactly 25.00 mL of water into your flask. Weigh your flask with the 25.00 mL of water in it, record this weight below. Tare your flask. The balance should read zero.
Remove the flask from the balance. Pipet an additional 25.00 mL of water and deliver it to your flask. Weigh your flask again with the additional 25.00 mL of water in it, record this weight below. Repeat taring and adding more water to the flask beaker until you have three readings that do not differ by more than 0.01 grams. Copy these three values to the table on the next page. Continue with the rest of the calculations on those three values only. ___24.86__________ g ___24.91__________ g _____24.92________ g ______24.92_______ g _____24.91________ g ______24.92_______ g Buoyancy Correction (calculation) An object weighed in air appears lighter than its actual mass by an amount equal to the mass of air that it displaces compared to the mass of air displaced by a calibration weight. Correct for buoyancy using the equation: g Density of air mL Observed mass 1
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g Density of the calibration weight mL Corrected mass g Density of air mL 1- g Density of object being weighed (water in this case) mL × = (3) The density of the calibration weight is 8.0 g/mL The density of the air has to be calculated. The barometric pressure of air at can be obtained from the internet, check with your TA for this value. The density of the air can then be calculated using the ideal gas law and the average molar mass of air (28.97 g/mol for dry air). 49 Barometric pressure in kilopascals (from the internet) ___________101.3________kilopascals
Density of air in grams / mL (calculated) __________________ grams / mL Density Correction (calculation) Lastly use the density of water to find the pipette volume. The density of water is exactly 1.000 at 4 °C but at higher temperatures the same volume of water weighs less. Calculate the volume of water in the pipet using an accurate density for the temperature you measured at. The density of water is: 0.998405 g/mL at 19 °C 0.998203 g/mL at 20 °C Use this 0.997992 g/mL at 21 °C Trial 1 Trial 2 Trial 3 Mass of water (g) Corrected mass of water (g) Calibrated volume of water (mL) Calculate the average and standard deviation for the calibrated volume of your pipette. The volume of your 25.00 mL pipet at 20 °C is ______________ ± _________ mL Practice Reading a Buret Locate the practice burets set up in the balance room. Practice reading the meniscus. Record your values on the sheet provided and compare your value to your classmates. Continue until you are
confident you can read the buret accurately. Check the Colour Charts If you are colour blind, you may not be able to see the colour changes used to detect end points in the titrations in this lab. Some people are not actually colour blind but are colour insensitive. They have difficulty distinguishing different shades of the same colour or very faint or pale colours from colourless. There are a number of colour plates placed around the lab. Trying reading all of these, and if you have any difficulties, talk to your TA immediately. You will still be able to complete the lab, but some adjustments will need to be made. Desiccant Verify the dryness of the desiccant in your desiccator, it should be free flowing. Get assistance from your TA if it requires changing. Before you leave the lab remove the vials of sodium carbonate and your unknown(s) from the drying ovens, cap them lightly, place them in your desiccator and store in your locker until the next laboratory session. 50 Create tables in your notebook and copy over all the data recorded for this experiment. Keep
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everything well organized. Remember to use the proper format for your pages, and to update your table of contents