Lab manual, experiment 1

Lab 1 manual

Chemistry is the study of matter. Matter has mass and takes up space, so measuring mass and volumes is something we do all the time in chemistry. This lab introduces balances (to measure mass) and lab glassware (to handle and measure the volume of liquids). At the end of the lab, we hope you can answer questions such as:

• How do I use a balance?
• What are the items in my drawer called, and what are they used for?
• How can I mix a liquid, and why would I?
• How do we measure small volumes (less than a teaspoon)?
• When would I use a graduated cylinder, and when would I use a burette instead?

Safety and hazardous waste

This lab has no specific safety concerns. The substances used are water and food coloring. Review general safety concerns of working in a chemical lab (tripping hazards, broken glass, attire and PPE). We are not generating any hazardous waste today.

Organization

Use your lab notebook to record measurements and observations, and to show your work for calculations. There is a summary sheet you will hand in. This lab has 4 parts that can be done in any order. For part 3, you will receive a time slot to avoid waiting time at the analytical balances (we have four, and there are up to 18 students in the lab). Part 1 uses a burette which is not part of your drawer equipment. You will work in pairs for part 1 and 4, and individually on parts 2 and 3.

Part 1: Volumetric glassware

For this part, you will work in pairs, but write your own words into the summary sheet.

We use volumetric glassware such as graduated cylinders and burettes to measure the volume of liquids. There are other glass containers in your drawer (beaker, Erlenmeyer flask, filter flask) that are not designed to measure volume.

We will use a burette today as our most accurate and precise way of measuring volumes. We will practice to read volumes to 0.01 mL, so please always quote measurements to two decimal places. Note that the scale goes from 0.00 mL at the top (filled) to 50.00 mL at the bottom (empty). If you are outside of this range before or after dispensing, you do not know how much you dispensed. For instructions how to read off the burette, watch this 4-minute video.

We can figure out how much liquid we dispensed with a burette by writing down the liquid level before and after dispensing, and taking the difference. For example, if the liquid level was at 0.15 mL before dispensing and at 36.72 mL after, you dispensed V = 36.72 mL - 0.15 mL = 36.57 mL.

\(\ \ \ V_{\mathrm{before}}= 0.15\  \mathrm{mL}\)

\(\ \ \ V_{\mathrm{after}}= 36.72\  \mathrm{mL}\)

\(\ \ \ V_{\mathrm{dispensed}}= V_{\mathrm{after}} - V_{\mathrm{before}} =36.57\  \mathrm{mL}\)

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Before you can start, you have to set up the burette and fill it with water so that the water level is in between the 0 mL and the 1 mL mark and there is no air in the spout. Ask your instructor to check the setup before you start.

1a) Accuracy and precision of measuring 10 mL

We will compare and contrast two graduated cylinder sizes (10 mL and 100 mL) as well as a 50 mL beaker to explore which is best to measure 10 mL of water. Starting with a dry empty container and a burette with known starting volume, dispense water from the burette into the container to the 10 mL line. As you get closer to the line, you should adjust the dispensing speed to a slow drip. Once you reached the line, read off the burette, calculate the dispensed volume and record it in your notebook. For each item of glassware, do this in quadruplicate and calculate the average and the standard deviation. With your partner, you can do two measurements before you have to empty and dry your containers for the third and fourth measurement. Copy your results into the lab summary sheet.

For this set of experiments, you used the burette “to deliver” and the graduate cylinder “to contain”, the way it is intended. Measurements “to contain” are useful in making a solution or reaction mixture where the final volume in the container is critical. Measurements “to deliver” are useful in titrations, where you add solution until you observe a color change or other “endpoint”, and you need to know how much you added. We will encounter burettes again in future labs.

Just to experience using the burette and the graduate cylinder in the unintended way, take a graduated cylinder filled with 10 mL water and try to deliver the liquid to the burette, recording the beginning and the final volume. Make sure your first burette reading is larger than 10 mL, otherwise you will overfill the burette. How much water did you add to the burette according to the two readings? Is it more or is it less than you expected? What are the problems you encounter trying this?

1b) Testing your burette skills

Now that you have some experience with the burette, we will try to measure super-small volumes and practice delivering solution drop by drop. Drain the burette so that it reads between 39.00 mL and 39.10 mL and record the exact volume (#0). Then, taking turns, dispense a single drop and record the volume, repeating until you dispensed 20 drops. Record your measurements on the summary sheet and plot them on the provided graph paper. From the graph, estimate the average volume of a drop and the standard deviation. The standard deviation reflects how consistent you read off the volumes or dispensed the drops.

Part 2: Drops as volume measurement

There are expensive instruments to dispense or measure tiny volumes, such as a Hamilton syringe, an adjustable micropipette or the head of an inkjet printer. The cheapest way to dispense a small volume is to count drops (like you would do for eye drops, for example). Here, we will estimate the volume of drops made in different ways, and explore how consistent they are.

2a) Volume of a drop of water

Fill a 10 mL graduated cylinder to the 2 mL mark with water. Then, using three different ways to make drops (transfer pipette, glass dropper, beaker), count how many drops it takes to add 1 mL. Then, calculate the average drop size in µL (1 mL = 1000 µL) and record it on the summary sheet.

2b) Volume of a drop of liquids other than water

Using the reagents and the transfer pipettes provided at the station, measure the average drop size of different liquids and solutions in the same way as you did in 2a, and record it on the summary sheet.

2c) How consistent are drops?

To explore how consistent the drop volumes are, you will make a green solution from a yellow and a blue solution. In the first recipe, take two drops of yellow and one drop of blue, dilute to 5 mL with water, mix and transfer to a medium test tube. Do this in triplicate. Now, take ten drops of yellow and five drops of blue, dilute to 25 mL with water, mix, and transfer some of it to a medium test tube to the same height as the 5 mL samples for good visual comparison (discarding the remainder). Do this in triplicate as well. Compare the color of the solutions. How much variation is there within the triplicates, and between the two different recipes? Look around for another set of green solution, and compare yours with theirs.

Part 3: Using an analytical balance

An analytical balance is pretty amazing. It can measure masses down to a milligram, i.e. 0.001 g. To get the full power of the analytical balance, you have to learn how to use it and to take good care of it.

3a) Precision: mass of a penny

The mass of a penny is easy to measure. Set the balance to zero while all doors are closed. Open one of the doors, place the penny in the middle of the weighing pan, close the doors, and read off the mass once it has stabilized. There might be some fluctuations in the last digit even after waiting quite some time. The mass shown will be in grams. Test the balance for consistency by measuring your penny on two other balances.

3b) Mass of a powdered solid

To measure a powdered solid, you have to use weighing paper, a weighing dish or some other container. Your sample has a mass (“net”) and your container has a mass (“tare”), but the balance records the sum (“gross”). To get the net mass of your sample, either measure the mass of the empty container and write it down, or press the zero button while the empty container is on the balance (“taring the balance”). For the latter, you have to be sure than you can complete the measurement before someone else uses the balance.

When you use a weighing paper, it is good practice to make a crease in it before use. This makes it easier to transfer the sample off the paper later. Measure the mass of a “spatula tip” of baking soda using weighing paper and record it. Then, tare the balance with a fresh weighing paper, transfer the baking soda on the fresh weighing paper, measure the mass and record it. How much do the two measurements differ? Why might they be slightly off?

3c) Mass of a liquid

The maximal mass for an analytical balance is often 110 g. Make sure the container you choose for measuring the mass of a liquid is not too heavy for the balance. In this experiment, we will test how consistent the mass of 10 mL in a graduated cylinder is, and how close to the expected mass (at room temperature, the mass of 10 mL of distilled water is about 9.9820 grams). Measure the mass of a dry empty 10 mL graduated cylinder. Then, fill it to 10 mL, measure the gross mass, calculate the net mass and record it. Remove some water with a transfer pipette, fill to 10 mL again, and record the mass. Record a third mass the same way. Then, calculate the average and the standard deviation. Compare your result with the claim on the graduate cylinder. The easiest way to do that is the convert the average mass and the standard deviation into volumes by dividing by the density of 0.99820 g/mL.

3d) Evaporation of a drop of water

When determining the mass of liquids in an open container, the is a possibility of evaporation. Place one drop of water on the provided weighing dish. Place into the analytical balance and record the reading, then wait a minute and record the reading again. What happened?

For some solids (called hygroscopic), water from the air is absorbed by the solid, making it impure and increasing its mass. When handling a hygroscopic solid, it is important to close the container right after removing sample, and to work quickly to minimize the amount of water added to the sample while determining its mass.

3e) Cleaning the balance

Check on the weighing pan, the bottom of the balance and the surrounding of the balance whether there is any powder or liquid you spilled. It is important that you clean up after using the balance because you are the one that knows which substances you used. If there is a major spill, inform your instructor.

Part 4: Mixing liquids and dissolving solids

For this part, you will work in pairs, but write your own words into the summary sheet.

How will samples mix depends on their scale. In a small volume of a liquid (such as the cytosol of a bacterial cell), everything mixes rapidly because of diffusion, i.e. particles moving all the time. The larger the volume, the more effort you have to put into mixing if you want a homogenous mixture (a solution). Quoting from Wikipedia, “modern industrial processing almost always involves some form of mixing”. We will explore mixing of liquid volumes typically used in the GenChem lab.

4a) Order of addition

Take two beakers of the same size, one from each of your drawers. Fill one to the highest line with water. Then, add one drop of red solution to each. Fill the other to the highest line with water. Observe them for 20 seconds and describe the difference.

4b) Mixing by swirling

Fill a beaker, an Erlenmeyer flask and a graduated cylinder with water. Then, add a drop of red solution to each. Try to mix each by swirling. Compare your results, in terms of efficiency of mixing and in terms of splashes, if any.

4c) Mixing with a stir bar, a stirring rod or inverting

Repeat the setup from 4b) but without swirling. Use a stirring rod to stir the contents of the beaker. Use a magnetic stir bar and a magnetic stirrer for the Erlenmeyer flask. Use parafilm to cover the top of the graduated cylinder and invert to mix. Which of the methods did you like the best? Which do you think resulted in the largest loss of solution (clinging to the stir bar, to the stirring rod, or to the parafilm)?

4d) Dissolving some salt (thought experiment)

Using what you just learned, make a plan to dissolve a table spoon of salt in water, giving a total volume of 50 mL. Which container would you use to dissolve the salt in water? How would you mix it? Which container would you use to make the solution up to 50 mL with water? How would you minimize losses of transfer?

Cleanup

Before you clean up, show your summary sheet to your instructor to get an exit stamp on it. Once you received permission to wrap things up, empty your burette (no need to rinse because it held water only) and clamp it in an inverted orientation to dry. Rinse the containers you used, let them drip off at your bench and put them back into your drawer. Leave you lab notebook in your drawer. Wipe off your bench area and hand in your summary sheet before you leave.