Project #1a–Basic Concepts–Measurements

McMurry/Castellion 2.1 – 2.6, 2.11

Introduction

We make measurements and do “calculations” with measurements all of the time, not just if we are doing something scientific, but even in our normal daily lives. Whether we are doing something scientific, or not, measurements involve units. If we measure a wall in our apartment to determine the width of bookshelf that will fit into a space, the unit of measurement might be inches or feet. If we are cooking, the unit of measurement might be teaspoons, cups or ounces.

Scientists tend to use units that are part of the metric system or the International System of Units (SI). These are units that are recognized around the world and in many cases are easier to use than the English units of measure that are so commonly used in our daily lives.

Whenever we measure something, there is some uncertainty in the measurement. If five cooks measure out a cup of flour, the actual amount of flour each measured would likely be somewhat different–some amount of error is expected and acceptable. It would be reasonable to assume that we would still call it a cup of flour if it had a teaspoon too much or too little. Likewise, when we measure the distance from one city to another, some uncertainty is involved. Is the distance from Indianapolis to Fort Wayne 121 miles exactly? Could the correct answer really be 120 miles? How about 120.8 miles, or even 100 miles? Which answer we believe to be correct would depend on the tool used to measure the distance and the care with which it was used.

In science, the concept of significant figures (or significant digits) is used to tell us how certain we are of a measurement. That is, the uncertainty in a number that we report is known to us from the way the number is reported.

In this first experiment, you will start to make measurements using some of the measurement tools of a chemist and learn to report your measurements with the proper units and the proper number of significant figures. You will also learn some lab terminology (words in italics) and prepare a solution that will be saved for use at your next lab meeting.

Procedure

Part A. Measuring Mass (PRS 1a: Part A on page 4)

Weigh a #2 rubber stopper using a top loading digital balance. Record the mass in grams to 2 decimal places (X.XX g). Repeat using 4 different #2 stoppers so that you have a total of 5 weights recorded.

Weigh the same 5 stoppers again, this time using an analytical balance. Record the mass in grams to 4 decimal places (X.XXXX g).

Part B. Measuring Volume (PRS 1a: Part B on page 5)

Fill a small graduated cylinder with about 5 milliliters of de-ionized water. Use a dropper and bulb to get as close to 5.0 mL as possible. Make sure to line up the meniscus properly.

Determine the mass of 5.0 mL of water using a top loading balance. (There are various ways that you could do this. For example: Weigh a clean, dry graduated cylinder. Add water. Reweigh the cylinder and contents. Alternatively, could you use the “tare” function of the balance?)

Part C. Determining the Density of Rubber (PRS 1a: Part C on page 5)

Fill a 100 mL graduated cylinder with about 50 mL of water. Record the volume to one decimal place (XX.X mL). Gently slide a #2 rubber stopper into the graduated cylinder. Record the liquid volume (XX.X mL) in the graduated cylinder after the stopper is added. The volume of the stopper (determined by the volume displaced) is the difference between the original and final volumes. Keep adding stoppers to the cylinder and recording the volumes until you have at least 4 data points.

Part D. Preparing a Simple Chemical Solution ( PRS 1a: Part D on page 6)

When working on experiments, scientists normally take notes so that they can document exactly what they have done. On page 3 you will find a few diagrams and some notes that would be typical for a chemist who had performed Part D of this experiment. Look this information over. Then, in consultation with your lab instructor, carry out the following procedure.

Weigh out a sample of “hydrate” in a weigh boat. Transfer the sample to an Erlenmeyer flask, then add water. Add a few boiling chips and heat the sample to boiling on a hot plate. Continue heating until the sample has completely dissolved. Preheat your filtration apparatus with some hot water and allow the hot water to drain completely from the funnel. Gravity filter the dissolved hydrate, collecting the filtrate in a clean, dry flask. Use label tape to label your flask with your name(s) and section number. Suspend a string in the flask, add a stopper, and place the flask in the location indicated by your instructor.

Before leaving lab today (and every day) make sure:

• your bench area is clean

• all “shared materials” are returned

• all items are returned to the lab drawer (clean!)

• and that you have washed your hands.

Also make sure that your laboratory instructor has “signed off” on your work from today.