How is Lab Equipment Used?

How is Lab Equipment Used? CHM 113 Prelab Presentation

How is Lab Equipment Used? How to prepare for lab: • Read the experiment (Investigation 4) before viewing this presentation. • Read this presentation and review textbook Sections 1.4 (Units of Measurement) and 1.5 (Uncertainty in Measurement). • Click on the links in the presentation to see web-based material that will help you with the lab. • Take the online Prelab Quiz 2, available on the Lab Blackboard site. • In your lab notebook, write a header, purpose, and a short proposal describing for determining the precision and accuracy of various volumetric measuring devices BEFORE LAB.

How is Lab Equipment Used? • Quantitative measuring techniques are very important in science laboratories, and will be important in many investigations you will carry out this semester. • The main goal of this lab is to introduce volume and mass laboratory measuring equipment, and to show the varies levels of precision and accuracy of different types of volumetric glassware. • You will also explore ways of measuring specific masses or volumes.

Quantitative Measurements involve: • Accuracy • A measure of how close an experimental value (or values) is to the true (published, known) value. • Precision • A measure of how close measurements are to one another (reproducibility of results). • A more precise measuring devise allows for measurements to be made with more significant figures. • The precision of an experimental result is no more precise than the measuring devise used with the least precision.

Precision vs. Accuracy

Accuracy and Precision • In this investigation you will: • Determine the precision of volumetric measuring equipment by making multiple measurements. • Measuring devices that allow for more significant figures in the measurement tend to be more precise. • Determine the accuracy of volumetric measuring equipment by comparing the experimental values (or average) to the known value. • You will compare the known density of water to your calculated density (from mass and volume measurements).

Types of Volumetric GlasswareAll measurements are in units of milliliters (mL) Burette Volumetric Pipette Graduated cylinder Volumetric flask

Duplicate Measurements • To reduce percent error (and increase accuracy), it is good practice to make at least three measurements (or run at least three trials). • If measurements have poor precision, make a 4th or 5th measurement. • If there are any measurements that are far from the others (outliers), ask you lab instructor if you should omit those from your average.

Making Precise Measurements • When recording measurements such as volume or temperature, record enough decimal places so that the last digit is estimated – between the tick marks.

Making Precise Measurements • When measuring out a specific volume using graduated glassware, be sure to line up the desired volume line with the bottom of the meniscus.

Making Precise Measurements • Read the liquid volume in this graduated cylinder. • To accurately read a meniscus, be sure to have your eye at the same level as the meniscus. • Watch this very short video on how to read a meniscus: • http://www.veoh.com/watch/v342780hbGCAgQf

Making Precise Measurements • Because there are four tick marks between each mL line, each tick mark represents 0.20 mL. • A good volume reading for this expamle is 6.61 mL. Because the last digit is estimated, you may have gotten something close like 6.60 or 6.62 mL. • Your measurement should have two decimal places (three significant figures).

Making Precise Measurements • When recording mass measurements on an electronic balance, record all digits that appear in the readout. • See this short video on using an electronic balance: • http://www.youtube.com/watch?v=QtnPiKSKKtI • Alwaysclean up any chemicals you spill on or near the balance. • How might you determine the mass of only the liquid in the flask?

Significant Figures • Significant figures are important when making measurements in the lab. • It is important to know the number of significant figures in your measurements and in results of your calculations. • Make sure your calculated density values have the correct number of significant figures. • Review significant figures: • Section 1.5 in your Textbook • Useful web links for practicing with Significant figures: • Tutorial:http://www.chem.sc.edu/faculty/morgan/resources/sigfigs/index.html • Practice http://science.widener.edu/svb/tutorial/sigfigures.html

Density Measurements • In this investigation you will calculate the density of your water samples from your mass and volume measurements, and compare calculated experimental values to the known density values • Appendix J in your lab manual contains a table of the densities of water at various temperature. • Review Density and Density calculations in Chapter Section 1.4 of your textbook.

Percent Error • A percent error calculation tells us about the accuracy of an experimental measurement (or an experimental result). • In this experiment, your calculated volume (from density) will serve as the “accepted value”.

In Your Notebook BEFORE Lab • In your lab notebook, write a header, purpose, and a short proposal describing a general procedure for determining the precision and accuracy of various volumetric measuring devices. Bring your proposal to lab.

How is Lab Equipment Used?