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Laboratory Techniques I, CHY 114
Chemistry Department, University of Southern Maine

Measurement

Assignment

It is essential to read and carry out all Assignments before coming to lab. Pre-lab quiz questions will be based on the Assignments and the background material on this page. In some cases (like this one!), the Assignment includes work to do and then bring to lab. Also see Preparing for Lab, below.

• Read sections 1.7 through 1.9 in Chang ("Measurement", "Handling Numbers", and "Dimensional Analysis in Solving Problems").
• Read Sections I-IV and Section XIII in the Laboratory Handbook. Pay particular attention in Sections III and IV to instructions for the use of balances, graduated cylinders, volumetric pipets, and burets.
• TO DO AND BRING TO LAB (REQUIRED): Study Section XIII of the Laboratory Handbook, and carry out a) the Manual Graphing Exercise and b) the Excel Graphing Exercise on page 100. For the Excel graphing, you will find a recent version of Excel on all computers (Windows and Macintosh) in the USM Computer Labs (Luther Bonney Hall in Portland, Bailey Hall in Gorham).
• Follow these instructions:
  1. For the Manual Graphing Exercise, use the piece of graph paper that is included as the last page of your Laboratory Handbook.
  2. For both the manual and Excel graphs, add all features needed to make the graphs conform to the Chem Lab One Guidelines for Graphs.
  3. On the Excel graph, add these features by computer, NOT by hand; there should be no handwriting on the Excel graphs. Delete any default features that you are not using (like legends that say "Series 1".
  4. Get to know "Excel Help", which can guide you in modifying and beautifying your graphs.
  4. Bring these completed graphs to your first lab meeting. They count as one lab quiz, so this gives you a chance to start off the course with a good quiz score.

Goals

• To become familiar with devices for measuring mass and liquid volume
• To compare the precision of various measuring devices
• To learn basic graphing methods with Excel

Overview

Preparing for Lab

The following problems require calculations similar to those called for in the report on this experiment. Learn how to work these problems, showing your calculations with all units, and giving final answers with correct units and significant figures. Similar questions may appear on your pre-laboratory quiz. For guidance, look at the Report Form for this experiment. Answers are provided.

1. A student tries to measure exactly 20 mL of water using an erlenmeyer flask. She then pours this water into a 25-mL graduated cylinder and measures the volume, finding it to be 21.7 mL. Calculate the percent difference between the two measurements, treating the 20 mL value as exact, and using this formula:
   % difference = [(V_flask - V_cylinder)/V_cylinder] x 100%. If we assume that the cylinder is the more accurate of the two devices, we could call this percent difference a percent error. ANSWER: -7.83% In this calculation, we treat the 20 mL value as exact because we were trying to measure exactly 20 mL, and we were using the percent difference to see how close we came.
2. What is the meaning of the negative sign in the answer to question 1? ANSWER: It means that the volume measured by the flask is lower than the volume of the same sample as measured by the cylinder, which is the standard of comparision. Another way to put it is that the flask makes a negative error in the measurement (assuming that the cylinder is more accurate).
3. How many significant figures are there in these quantities: 50.0 g? 0.0004 mL? 100001 g? 100000 g? ANSWERS: 3,1,6,1
4. Your first quiz will include questions about lab safety. Click on Lab Safety Information on the main syllabus page to prepare. Here is a reminder of the location of safety devices in the lab:

Procedures

Download and print the Procedure for this experiment. Bring it with you to lab.

Report

Complete the Report Form for this experiment. Bring it with you to lab. You will write data and observations on the form during lab, as you carry out the Procedure. After lab, you will complete the Report Form by carrying out calculations to give your final lab results. These calculations include graphing your data with Excel, so if you need to use USM computers for this purpose, be sure to allow time to visit the USM computer lab for this work.

CHY114 Lab Manual