What personal protective equipment should you wear when preparing a chemistry lab?

Wear a lab coat, chemical splash goggles, and gloves to protect yourself during laboratory preparation. These items shield you from potential chemical splashes and hazards. Always put on all required personal protective equipment before beginning any lab work or handling chemicals at your station.

How do you prepare sodium chloride for a multi-student lab experiment?

Weigh 50 grams of sodium chloride directly into small plastic bottles using a top-loading balance. Label each bottle 'NaCl' and place them at individual student workstations. Prepare enough bottles for all students plus excess material to account for potential spillage or mistakes during the experiment.

What glassware and equipment should be set up at each student lab station?

Each station requires a stirring rod, 20-mL volumetric flask, 10-mL volumetric pipette, pipetter, 600-mL beaker, 50-mL graduated cylinder, 12-inch ruler, egg, and eyedropper. This equipment enables students to measure, mix, and observe solutions accurately. Arrange all items before students arrive to ensure smooth lab workflow and efficient use of time.

How should deionized water be distributed to students in lab preparation?

Fill 250-mL wash bottles with deionized water, providing one bottle per student at their workstation. Deionized water is essential for preparing solutions and rinsing glassware without introducing contaminants that could compromise experimental results. Place filled wash bottles before the experiment begins to ensure immediate student access.

Why is a top-loading balance used for weighing sodium chloride in lab preparation?

A top-loading balance provides quick, accurate measurements of bulk chemicals like sodium chloride for multiple students. It allows direct weighing into containers without transfer, reducing measurement errors and saving time during lab setup. This efficiency is critical when preparing materials for large student groups working simultaneously.

What is the purpose of labeling chemical bottles during lab preparation?

Labeling bottles clearly identifies the contents and prevents dangerous mix-ups during the experiment. Proper labeling ensures students use the correct chemical and maintains safety throughout the lab. Always label containers immediately after filling to avoid confusion at student workstations and support organized, safe lab practices.

How should you organize materials at a student workstation before an experiment begins?

Arrange all required glassware, equipment, and chemicals in a logical order at each student workstation to minimize confusion and maximize efficiency. Place wash bottles with deionized water, labeled NaCl bottles, and measuring instruments within easy reach. Proper organization reduces setup time and allows students to focus on the experimental procedure rather than searching for materials.

Lab Techniques

Concepts Instructor Prep Student Protocol

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In scientific research, the reproducibility of an experiment is extremely important. Thus, keeping a lab notebook with detailed procedural recordings along with proper lab techniques helps reproduce experimental findings.

For example, simple measurements, like liquid volume, must be performed using the proper glassware to ensure accuracy. Always measure volume using volumetric glassware, like a volumetric flask, graduated cylinder, or volumetric pipette. The volume is measured at the bottom of the meniscus.

Volume markings on beakers and Erlenmeyer flasks are not accurate and serve as guidelines only. When selecting the volumetric glassware, select the smallest container possible for the volume needed. Volumetric glassware is calibrated either to deliver or to contain.

Containers calibrated to deliver are designed to provide the volume stated with the understanding that a small amount of liquid will remain in the glassware after it is emptied. In this case, there is no need to remove the remaining liquid or more than the desired volume will be emptied. Glassware calibrated to contain will hold and deliver the volume stated but require that the remaining liquid will be poured out so that the full volume is received.

We measure to obtain a true value. However, there will always be some level of uncertainty and error. The measured value is our best estimate of the actual value, which is often unknown to us. Error is the difference between the measured and actual values. Measurement uncertainty describes the range in which we think it is likely that the actual value lies. When recording measurements, it is important to maintain the appropriate number of significant figures.

Significant figures are the digits in a measurement that carry meaning. The last digit recorded defines the level of uncertainty. All numbers other than leading and trailing zeros are significant. And trailing zeros are significant when there is a decimal point preceding them.

For example, in a measurement of length using a ruler, we see that the length is at least one inch, but certainly not 2 inches. So, the first significant digit is one. The next tick mark represents 0.1 inches and is also significant.

A recording of 1.1 inches has two significant digits and implies that the uncertainty lies in the tenths place. However, the true width lies between two tick marks. So, the uncertainty lies here in the hundredths place as the length is reported as 1.15 inches.

When conducting calculations using measured values, remember not to carry out calculations to a higher resolution than the original measurement. Those additional digits are not significant and should not be included. For example, when calculating the area of the square with a side length of 1.15, we see that the length has three significant figures. So, the answer should also have three significant figures.

The calculated area of 1.3225 inches squared has five figures and introduces certainty into the calculation that was not there in the original measurement. Thus, the correct area is 1.32 inches squared.

In this lab, you'll practice proper lab skills by measuring the density of an egg and utilizing significant figures in your calculations and recordings. In addition, you will practice record keeping in your lab notebook and examine the accuracy of measurements with volumetric glassware.

At the end of this lab, students should know...

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Why are lab measurement techniques important?

Proper measurement techniques ensure that you accurately measure the materials that you will use in your experiments. This is essential for reproducibility.

How can you measure precise liquid volumes?

Liquid volumes should be measured with graduated or volumetric glassware, such as a volumetric flask, a graduated cylinder, or a volumetric pipette. Standard beakers and Erlenmeyer flasks should not be used for precise measurements.

What is the difference between 'to contain' and 'to deliver'?

Glassware marked ‘to contain’ will contain the specified volume of liquid when it is filled to the mark, but it may deliver slightly less than that volume. Glassware marked ‘to deliver’ will contain some extra liquid when it is filled to the mark, but it will deliver the precise volume specified.

What is measurement error?

Measurement error is the difference between the measured value and the true value. This is expressed as ‘uncertainty’, which describes the expected amount of variation between the measured and true values. Remember to write down the uncertainty of your instruments and glassware when you record your measurements.

What are significant figures?

Significant figures represent the precision of a value. All numbers other than leading or trailing zeroes are significant, including zeroes between non-zero digits. Trailing zeroes are significant only if they either follow a decimal point or are marked as significant using an overline, underline, or decimal point with no tenths place. Leading zeroes are never significant.