EXPERIMENT 1 Laboratory Safety and Precision and Accuracy: Measuring the Density of Coke®

Introduction

Laboratory safety is the single most important lesson you can learn from your experience in these general chemistry labs. Good lab safety is essential to ensure your well-being, as well as that of the people working around you. There are a few simple safety principles that you will learn in this first experiment. You will learn about important safety protection devices (eye goggles, gloves, long pants, closed-toe shoes, fume hoods) as well as about the many hazards in a chemistry laboratory (toxic and caustic chemicals, fires, poisonous gases, very hot or cold apparatus, and broken glassware). In addition, your TF will show you the location of important safety equipment for use in case of an emergency (fire extinguishers, eyewash fountains, fire blankets, and safety showers). The first part of this lab will consist of an introduction and safety training in the laboratory. This is the most important laboratory “experiment” of the term, and no student will be admitted to subsequent labs without full participation in this safety training.

Before coming to this first lab, be sure to read the material on “Laboratory Safety” that is included at the beginning of your lab manual, and complete all of the questions in the “Lab Safety Assessment”.

Once you have been acquainted with the basics of lab safety, you will learn about some of the experimental apparatus that will be commonly used throughout the term. You will see two instruments for measuring mass (the top-loading digital balance and the digital analytical balance) and four techniques for measuring volume (buret, pipet, graduated cylinder, and bottle-top dispenser). Using several different methods you will measure the density of the soft drink Coca-Cola®. At the end of the experiment all students in the lab will compare their data and examine the accuracy and precision of each technique.

Experiment 1 1 Experimental Error

Experimentally determined values for physical quantities (the mass of an object, the heat produced by a reaction, the molar mass of a compound) always differ from their true values. The difference between the true value and a value determined by experiment is called the experimental error. Experimental error is a natural consequence of the limitations in the measurements and analysis used to obtain a result. Although experimental error can never be eliminated, a thorough understanding of an experiment enables you to estimate experimental error, and take steps to reduce it. For example, suppose you want to measure the density (the ratio of the mass to volume) of liquid methanol, CH3OH. To measure its density, you weigh an empty volumetric flask calibrated to hold 50.0 mL, fill it to the calibration mark with methanol, and weigh it again. The density of methanol in g/mL is given by the difference in the two masses divided by the sample volume. This calculation will propagate experimental error: any error in one of the three measurements will generate error in the density value. For example, if the sample and flask together actually weigh 62.12 g, but the balance reads 62.39 g, the density computed from the weights will be erroneously high. Other sources of error arise from the way measurements are interpreted. For example, a contaminant in the methanol might alter its density, so that even an accurate measurement would not be measuring pure methanol. Error also arises from failing to control experimental conditions affecting the result. Since density depends strongly on temperature, a density measurement is less useful if the temperature is unspecified, or if the temperature varied during the experiment. Finally, the theory used to make an indirect measurement of some quantity is rarely exact, and this inexactness generates error. In our example, the theory that density can be measured by the difference in the two balance readings divided by the volume of the liquid seems so obvious that we expect it to be exact. It is not, because the increase in the balance reading is actually the mass of the liquid minus the mass of the displaced air. With these potential sources of error in mind, you could design a more accurate experiment. For example, you can check for a contaminated sample by confirming that its density is unaffected by an added purification step. The temperature of the sample could be maintained at a specified value during the experiment, and you could correct for the mass of the displaced air. Of course, all potential sources of error are not always significant. If you use a balance that can only measure the mass of the sample to within an uncertainty of 1 gram, other sources of error would be insignificant by comparison. The only useful improvement would be to use a better balance.

Systematic and Random Errors

Systematic errors remain unchanged each time an experiment is repeated. In our density measurement, the repeated use of a contaminated sample or of the same improperly calibrated flask would lead to systematic errors. Random errors affect a measurement in positive and negative directions with equal probability. One example is error arising from the vibration of a table supporting an analytical balance. Unlike systematic errors, random errors can be reduced by averaging many measurements. This averaging cancels the positive and negative fluctuations with each other.

Experiment 1 2 Many sources of error have both systematic and random characteristics. For example, suppose the “empty” flask used in the density measurement is weighed while it is still wet. A systematic error in the weight of the flask will result, but the exact size of the error will vary randomly between successive measurements, depending on just how wet the flask is when it is weighed “empty.”

Measurement Errors: Uncertainty, Accuracy, and Precision

All experimental measurements inherently involve some uncertainty. The uncertainty is usually expressed as the variation from the true measurement. For instance, if you weigh a standard 10.0000-gram mass on an analytical balance and get three measurements of 9.9997, 10.0002, and 9.9995 grams, then you would report that the balance has an uncertainty of ±0.0005 grams. The uncertainty of most laboratory equipment has already been determined by the manufacturer. Some uncertainties for the equipment used in this laboratory are listed below: digital analytical balance ±0.0005 g top-loading digital balance ±0.1 g 50 mL graduated cylinder ±1 mL 50 mL class A buret ±0.1 mL (for two readings) 50 mL class A volumetric flask ±0.05 mL The uncertainty of a pipet depends on its size, and is usually written on the pipet.

Our balance has an uncertainty of 0.0005 g, which is the maximum deviation of any particular measurement from the true mass. The precision of a balance measures the extent to which individual measurements of the same quantity by the same apparatus differ from each other, irrespective of their differences from the true value. You can also think of precision as the maximum variation a particular measurement can have from the average. The accuracy of an instrument is the deviation of the average of many repeated measurements of the same quantity from the true value.

Precision Accuracy Precision & Accuracy

In a perfect world, all scientific instruments would be both accurate and precise. However, in most cases one of the qualities is more important than the other. For example, the average of many weighings of an object will be close to correct if the balance used is accurate, even if it is imprecise and successive weighings differ. A precise balance can detect small mass differences between two successive measurements, even if the balance is inaccurate. Experimental error can also cause problems with precision and accuracy. In general, systematic errors cause results to be inaccurate while random errors tend to create imprecise measurements. Think about these connections next time you weigh yourself on a scale or measure your height!

• • •

Experiment 1 3 Measuring the Density of Coke

Most of us are familiar with the refreshing soft drink Coca-Cola, commonly known as Coke. The formula for Coke was devised in the 1880’s by Dr. John Pemberton of Atlanta, and remained a secret for many years. Not long ago, the secret formula (“7X”) was revealed by a researcher searching through Coca-Cola Company archives. Of course, the main ingredients are water and sugar, and indeed the density of Coke is primarily due to its sugar content. (Because of this, Diet Coke® is significantly less dense than regular Coke. Indeed, cans of Diet Coke will float in a tub of water while the same cans of Coke will sink.) Density is defined as mass per unit volume—in our case, in g/mL—and you will thus have to measure both the mass and volume of a given sample of Coke. You will use several different methods to do so. Then, the data from all the students in the lab will be “pooled” on the computer so you can determine the best possible experimental value for the density of Coke. In addition, you can evaluate the several experimental techniques in terms of their precision and accuracy.

Using the Balances

Before you weigh an object, there are certain considerations you must follow:

• Any object to be weighed must be at room temperature; air currents from hot or cold objects will affect the weighings. Likewise, on the analytical balance, close the doors before weighing.

• Chemicals must be held in a weighing boat, on weighing paper, or in a beaker. Never place any chemicals directly on the balance. Don’t forget to weigh the empty container as well so you can find the weight of the chemicals alone.

• Clean up any spills carefully and promptly using a small brush.

Both the top-loading digital balance and the digital analytical balance are similar in operation. The top- loading balance is less precise (only to ±0.1 g) but can weigh heavy objects and is very sturdy. The analytical balance is much more precise (to ±0.00005 g) but is more delicate and can only weigh relatively light objects. The lab manual will usually instruct you as to which balance to use, but you should use your own judgment in each case. Both balances have a large button labeled “O/T” which is used to turn on and “tare” the balance. The balance is “tared” when it reads zero grams. You can tare the balance with an empty pan (this is called “zeroing” the balance) or you can tare it with an object on the pan (such as an empty beaker that you intend to fill with a chemical). This is the only button that you will have to use.

Experiment 1 4 TF: ______Name: ______Experiment 1: Data Collection, Lab Report and Prelab

Before You Come to Lab: • Read the “Introduction to Laboratory Safety” and answer all questions on the “Laboratory Safety Assessment”. Have this safety assessment ready to turn in as you arrive in the lab. • Read the entire lab report, including the previous introduction and discussion, and the entire procedure. • Complete the prelab, which is the last page of the lab report, and turn in the prelab to your TF as you enter the lab.

Safety in the Laboratory • Safety glasses or safety goggles must be worn at all times while in the laboratory. • Lab Coats should be worn at all times in the lab. • Nitrile gloves should be worn while performing experiments or handling chemicals. • Use caution when using glass pipets with the pipet plungers to be sure that you do not break the glass pipet. When inserting the pipet into the plunger, hold the pipet and plunger at the ends so your hands are touching each other as you carefully slide the pipet into the plunger.

Waste Disposal and Cleanup • Place all used equipment back into bins located on the center bench. • All of the substances in this experiment are non-toxic and may be rinsed down the drain.

Before You Leave the Lab • Have your TF check your lab bench for cleanup. • Submit your data and lab report to your TF. This page and all subsequent pages must be stapled and turned in. • Wash your hands before leaving the lab.

Grading: Prelab: _____ / 10 Lab Report: _____ / 20 Safety: _____ / 3 Cleanup: _____ / 2

Total: _____ / 35

Experiment 1 5 TF: ______Name: ______Measuring the Density of Coke: Data, Observations, and Notes A Rough Estimate Using a Beaker

Obtain an empty 50-mL beaker, and make sure the beaker is clean and dry. Go over to the analytical balance. There will be a glass test tube at the balance. With the balance pan empty, close the doors and press the “O/T” button to zero the balance. Place the test tube on the balance pan and After zeroing the balance: close the doors. Record the mass of the test tube. Mass of test tube: Remove the test tube, place the beaker on the balance pan and close the doors. Record the mass of Mass of beaker: the beaker. Now press the “O/T” button again while the beaker is on the pan. You should see the display return to zero. Remove the beaker from the balance pan and record the mass of the “empty pan” after being After taring the balance for the beaker: tared for the beaker. (What should this be?) Mass of “empty pan”: Put the test tube inside the beaker and place both onto the balance pan. Record the mass displayed. Mass of beaker plus test tube: Re-zero the balance. It is good etiquette to reset the balance to zero (with an empty pan) before leaving it. Leave the glass test tube at the balance for the next student. Obtain a large beaker of Coke from the center lab bench. Add 25 mL of Coke to your small beaker, simply using the marks on the side of the beaker to make your best estimate of this volume. Bring the After zeroing the balance: beaker with Coke to the analytical balance, zero the balance if necessary, and determine the mass of the Mass of beaker plus Coke: beaker with the Coke. Subtract the mass of the beaker from this in Calculations: order to determine the mass of the Coke. Since you Mass of Coke: know that the volume is 25 mL, calculate the density of the Coke in g/mL. Would you expect this result to Density of Coke: be very accurate? What is the most significant source of uncertainty in this measurement?

Experiment 1 6 TF: ______Name: ______Using a Buret

The buret (shown on the following page) is generally used to dispense precisely any volume of solution. As shown in the diagram, the buret is clamped using the special buret clamps with the stopcock and tip pointing down. The stopcock at the end is used to control the flow of liquid from the buret; when the handle is in the horizontal position the stopcock is closed. Place a beaker under the buret tip and turn the stopcock swiftly in order to fill the tip with liquid. There must be no air bubbles in the buret tip. Do not try to align the liquid meniscus with the zero on the buret; rather, record the starting volume and subtract it from the final volume to obtain your net volume. Note that the numbers increase going down the buret. To read the buret, look at the bottom of the liquid meniscus and then estimate the last place to obtain two places after the decimal. Some people find it best to close one eye while reading the buret.

Using a Pipet

The pipet is used when exact volumes are required. Gently fit the pipet piston over the pipet as shown on the following page. Be careful to hold the pipet at the “top” end when fitting it with the piston; we have sent several students to the hospital who broke a pipet in their hands and were cut badly. Turn the thumbwheel to raise the piston and draw liquid into the pipet. Adjust the thumbwheel until the liquid meniscus is aligned with the mark on the pipet stem. Hold the pipet over the destination flask, and depress the release lever to transfer the liquid. As with the buret, the pipet should first be rinsed with a small volume of the solution to be measured. Some pipets are labeled "TD", meaning "to deliver". They are calibrated to deliver exactly the volume indicated, so the last drop is not part of the measured volume. Do not "blow out" the TD pipet; it is designed to have some liquid left over in its tip after using it. Other pipets are labeled "TC", meaning "to contain". In this case the pipet will contain the indicated volume, and it is important to transfer even the very last droplet.

Using a Graduated Cylinder

A graduated cylinder is the simplest general tool for measuring volume; simply fill it with the desired quantity of liquid and pour to dispense. The cylinders we use are made of plastic and thus should not exhibit a meniscus; they also should deliver all the solution they contain. Do not worry about getting the last drop out of the cylinder.

Experiment 1 7 TF: ______Name: ______Buret and Pipet Setup

Correct eye position for reading a buret or pipet. Piston Thumbwheel

Release lever

Pipet Setup Buret Setup

Experiment 1 8 TF: ______Name: ______Determining the Density of Coke Data, Observations, and Notes

Take your 50-mL beaker, rinse it, and dry it thoroughly. Determine its mass. Then, using a buret, Using a buret: transfer 25.0 mL of Coke to the beaker and measure Mass of empty beaker: the mass of the beaker with the Coke. Calculate the density of Coke in g/mL. Mass of beaker plus Coke:

Mass of 25.0 mL of Coke:

Density of Coke: Repeat the density determination using a pipet to measure 25.0 mL of Coke. Using a pipet:

Mass of empty beaker: Mass of beaker plus Coke:

Mass of 25.0 mL of Coke: Density of Coke: Repeat the density determination once more using a graduated cylinder to measure 25.0 mL of Using a graduated cylinder: Coke. Mass of empty beaker:

Mass of beaker plus Coke:

Mass of 25.0 mL of Coke:

Using Bottle-Top Dispensers Density of Coke:

On the center lab bench are several bottles of water with pump-style dispensers. These dispensers deliver very accurate volumes of liquid, and will be Using the bottle-top dispenser: used for many of the experiments in this course. You Mass of empty beaker: select the volume of liquid you want to dispense, and then simply raise and lower the piston once to deliver Mass of beaker plus Coke: that volume of liquid. Use the dispenser on the Coke Mass of 25.0 mL of Coke: bottle in order to measure 25.0 mL of Coke and repeat the density determination one final time. Density of Coke:

Experiment 1 9 TF: ______Name: ______Lab Report

Results

1. Report the density of Coke that you measured by each of the 5 different techniques:

a) Using a beaker (rough measurement):

b) Using a buret:

c) Using a graduated cylinder:

d) Using a pipet:

e) Using the bottle-top dispensers:

2. Add your density measurements to the class data set.

3. Looking at all the class data, give the best possible value for the density of Coke.

Density of Coke:

Interpretation

1. Looking at the class data, which technique seems the most precise? Explain.

the least precise? Explain.

Is this what you expected given the details of the measurement techniques? Explain.

2. You are asked to determine the density of 10 different solutions rapidly and accurately. Which technique would you use and why?

Experiment 1 10 TF: ______Name: ______Prelab Report This report must be filled out before you come to lab.

1. Read the “Introduction to Laboratory Safety” and complete all of the questions in the “Laboratory Safety Assessment”.

2. You take some solution from a bottle labeled “Boric Acid” and, in the process of heating it, accidentally splash some into your eyes. Your TF says that boric acid is a very mild acid and is, in fact, used as an eyewash (this is true!), so you should not worry. What should you do in this situation and why?

3. Your eye itches, so you remove your gloves, lift up your goggles, and rub your eye. What have you done wrong?

4. With our analytical balances we have special calibration weights with very precisely known masses. If you measure a 10.000-gram calibration mass three times and get values of 10.010, 10.009, and 10.011 grams, is the balance more accurate or more precise? Why? Is this an example of systematic or random error?

5. You will use 5 different techniques to measure the density of Coke. Which do you expect to be

the most precise?

the least precise?

Explain.

Experiment 1 11 TF: ______Name: ______

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Experiment 1 12