Lab Activity #1
Measurement Techniques, Precision & Accuracy

This experiment is all about precision and accuracy. The precision of a measurement is designated by the number of significan figures in your reported data. The correct way of reporting data for each measuring device is determined by the markings and/or digits on the particular device. For example, if the finest markings on your ruler are one centimeter apart, then you'd report to the nearest millimeter (estimating the last decimal place, e.g., 12 mm or 1.2 cm). If the ruler is marked in millimeters, you report the data to the nearest tenth of a millimeter (again, estimating the last decimal place, e.g., 12.2 mm).

Part A: Mass Measurement

In this part of the lab, and throughout the semester, you will use the electronic balance to measure mass. This balance must first be set to read zero when there is nothing on the plate. Since the balance is a digital readout, the number of significant figures that you read on the balance is what you will report on your data sheet. So if the balance reads 29.500 grams, you report 29.500 grams, not 29.5 grams.

A.1. Direct Measurement

In this part of the experiment, you'll be finding the mass of a rubber stopper several times. If you're working in a group, have a different person do one or two weighings each. Make sure you write the names of your partners on the front page of your report (just to the left of where it says "Locker #").

The formula for calculating the "estimated precision" of a set of measurements is on Page 1 of your Lab Manual.

A.2. Weighing by Difference

In this part, you measure the mass of a liquid (water). You're basically using the beaker to measure how much water is transferred out of a 50-ml graduated cylinder which had been filled to the 50.0 milliliter mark. Again, if you're in a group, each person in the group should do the procedure once or twice. The objective is to get familiar with handling glassware and the balance, and to learn something about the precision of measuring devices. Calculate the mass of water by subtraction for each trial, then do the average.

Part B: Length Measurement

This is an exercise in using a ruler, which most of you have probably done many times before. Be sure to record the length to the correct number of significant figures (which is determined by the markings on the ruler or meter stick that you are using). I'll do the measurement myself (from my own lab manual) to get a number you can use for the "true" value. The formulas for "estimated precision" and "systematic error" are on pages 1 and 2, respectively, in your lab manual. The systematic error is an estimate of your accuracy. Use your best judgement to answer the question on page 7; there is no wrong answer, as long as your data support it.

Part C: Volume Measurement

C.1. Beakers & Flasks vs. Graduated Cylinders

Here, you are using a graduated cylinder to determine which is more accurate, the markings on a beaker or the markings on an Erlenmeyer flask. Make sure you record the data obtained from the graduated cylinder to the correct number of significant figures (which is determined by the markings on the cylinder). When you're thinking about how to improve the experiment, think about how you could improve the precision of the cylinder. If you're working in a group, each person should do his/her own measurements in this part. Don't just copy someone else's data.

C.2. Volumetric Flasks vs. Graduated Cylinders

In this part, you are using a 500-ml graduated cylinder to figure out exactly how much liquid is in a volumetric flask that is filled exactly to the etched white line on the neck of the flask. Make sure you record the volume to the correct number of significant figures (which is determined by the markings on the flask). If you're working in a group, each person should do his/her own measurements in this part. Don't just copy someone else's data.

C.3. Pipettes vs. Graduated Cylinders

In this part, you'll be using a 10-ml graduated cylinder to determine how much liquid is dispensed from a 5-ml pipette. Make sure you record the volume to the correct number of significant figures (which is determined by the markings on the cylinder). If you're working in a group, each person should do his/her own measurements in this part. Don't just copy someone else's data.

Lab Report

The Lab Report for Activity #1 is Pages 4-9 of the Lab Manual. Please put your name on each page, take them out of your lab manual, and staple them together. If you worked with partners, put their names on the first page (p. 4) of your report, just to the left of where it says "Locker #". If you ripped the pages out of your Lab Manual, please cut off the fringed edges. Thanks.

As always, you must write your own report and do your own calculations for each lab report, even if you're working in a group or with a partner. Sometimes, you will advised to work in groups of 2-6 students (the number will vary depending on the activity), and data will be shared among group members. If you've been instructed to work individually (no partners or groups), then copying of data is not allowed. Copying calculations and answers to questions is NEVER allowed. You must do your own work. Discussing it is okay, but if you copy without thinking, you're cheating. Try at least to reword an answer, or set up a problem in your own way, than to copy directly from someone else.

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