This experiment is designed to give you an opportunity to analyze gravimetric (mass) and volumetric (volume) data. The data will be analyzed to discuss the precision and accuracy of laboratory glassware. The first part of this exercise involves the use of the top-loading (bench-top) analytical balance and a 25-mL graduated cylinder to reproducibly measure the mass of an assigned aliquot (sample or portion) of water. The second part of the experiment is to use a 10-mL volumetric pipet to measure the aliquots of water while still using the top-loading balance to measure the mass. The third part of this exercise involves the use of a graduated pipet to measure varying amounts of water and correlating the volume to mass (Density??).
Reading Liquid Volume: LOOK STRAIGHT ON AT THE BOTTOM OF THE LIQUID MENISCUS
When water is placed in a glass cylinder, a concave surface forms; this curve is called the meniscus. Calibrated glassware used in the lab is manufactured so that the volume is read where the bottom of the meniscus lines up with the markings on the equipment.
Your eye must be perpendicular to the bottom of the meniscus. Viewing the meniscus from above or below causes parallax, the deceptive displacement of the meniscus to be below or above the correct position, as shown below
25 mL graduated cylinder
10 mL Volumetric pipet
Use of 10 mL Volumetric Pipet Video:The Volumetric Pipet and Pipetting TechniqueLinks to an external site.
Use of Graduated Pipet Video:Using A Graduated PipetteLinks to an external site.
BEFORE THE EXPERIMENT
DURING THE EXPERIMENT
AFTER THE EXPERIMENT
Calculations and Questions (Complete the things in BLUE in your Notebook)
Where xavg is the mean (average) value, xi are the individual values and n is the number of measurements taken.
Calculate, using the formula above, the Standard Deviation of the 4 masses of water from the 25-mL graduated cylinder. Show a sample set up for the calculation with your numbers. Your answer should have the same number of decimal places, and units, as your original data. Also calculate the Standard Deviation for the 4 masses of water from the 10-mL Volumetric pipet, but you do not need to show the sample calculation.
Standard Deviation of the masses of water from the 25-mL grad. cyl.: ________________
Standard Deviation of the masses of water from the 10-mL Vol. Pipet.: _______________
Excel can also be used to do calculations. In the Excel spreadsheet in the Experiment 1 module “Excel for Expt 1 Measurements” notice that the calculations are now complete. If you click on a cell, you can see the calculation formula in the Formula Bar. Notice the Mass of Water was calculated using a subtraction “=D10-C10”, whereas the average was calculated using “=AVERAGE(E10:E13)”. You can display different number of decimal places by formatting the cell. Also notice the Standard Deviation was calculated using “=STDEV.S(E10:E13)” which is the Sample Standard Deviation of the four aliquots of water and should match your answer to 1a. above.
Which device (cylinder or pipet) is more precise in the given data? _____________________.
Generally, the Standard Deviation will start to show uncertainty (imprecision) in the same decimal place as the least precise measurement linked to the data. Even the best chemists would typically have Standard Deviations of 0.10 to 0.50 g using the graduated cylinder and 0.01g to 0.05g using the Volumetric pipet.
Since the balance is very precise, what is the major difference between the two volumetric devices that makes the pipet approximately ten times more precise? Briefly explain why.
Average volume of water using 25-mL Grad.Cyl.:_______________
Average volume of water using 10-mL Vol.Pipet:_______________
(Note: These are truly the volumes you measured!!! These are “Accepted” values)
% Error for 25-mL Grad.Cyl. =___________________________ High or Low?
(High means the Experimental value is Higher than the Accepted)
% Error for 10-mL Vol.Pipet =___________________________ High or Low?
Which Device is more Accurate for the data given dispensing 10.00 mL of water? _______________
What does “TC” on a graduated cylinder mean? ___________________________
What does “TD” on a Volumetric pipet mean? _____________________________
Knowing the Graduated Cylinder is a TC device, would you expect a Percent Error for the graduated cylinder to be High or Low? Briefly explain. (High/Low does not mean Big/Small)
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