EXPERIMENT 7 Accuracy and Precision of Glassware and common Lab techniques Citrus College Chemistry Objectives Differentiate between accuracy and precision Quantify the precision of laboratory glassware Graph experimental data PreLab questions 1. Lab. Why might the mass of the paper towel in part two decrease during the lab? 2. Computation. A 50 mL beaker is found to have a mass of 47.2 g. When 21.0 mL of water is added to the beaker the new combined mass is found to be 48.1 g. What mass of water was added? 3. Context. Describe how parallax can cause problem in today's lab. You will look at the accuracy and precision of several pieces of laboratory glassware by measuring the volumes (20 mL aliquots) with the glassware, then comparing your volume to the accurately determined mass of the samples. Separately you will determine the rate of water evaporation from a damp sample. PROCEDURE Part I. Glassware. Obtain a 100 mL beaker, two 50 mL beakers, a 50 mL graduated cylinder and a 20 mL volumetric pipette. Label the 50 mL beakers "A" and "B". Wet and briskly shake dry beaker A, then record its mass. (Fully dry the outside of the beaker with paper towels as necessary) Note that all mass measurements for Part I will be taken with Beaker A. Do not take any other glassware into the balance room. Before heading to the balance room be absolutely sure that the outside of the beaker is completely dry. Keeping the beaker on a dry paper towel will help ensure this. After each measurement, pour the contents of the beaker into the sink, and briskly shake the beaker dry. Do not towel dry the inside of the beaker. Measure 20 mL of water in your 100 mL beaker. If your beaker is one without the stamped on lines, get different beaker that does have the lines. Try to look straight through the beaker to avoid parallax. Pour the contents of the 100 mL beaker into Beaker A and weigh. Repeat this for 3 trials. Measure out 20 mL of water with Beaker A and weigh as above. Repeat this for 3 trials There is no need to transfer the water in this step as you are using "Beaker A". But be sure to dump the water and start fresh with each trial. Repeat the above with 50 mL beaker "B". Do transfer the water to beaker "A" for weighing. Repeat the measuring process with your 50 mL graduated cylinder. Again, transfer the liquid to beaker A for weighing Repeat the process with the 20 mL volumetric pipette. The contents of the pipette should be drained directly into beaker "A" Part II. Moisture. Weigh a 50 mL beaker and record the mass. Cut or tear a piece of paper towel approximately 4 inches square, add it to the beaker and reweigh. Add about 1 ml (20 DROPS) of water to the paper towel and squeeze it a few times to make the moisture somewhat uniformly distributed. Using a timer (watch, clock, phone), record the mass of the beaker & paper towel every 5 minutes. You don't have to be exact with the time, but try to be within 30 seconds of the 5 minute interval for each measurement. Record the masses for 25 minutes