Goals

Measure and record length, volume and mass accurately.

Write the measurement to the correct number of significant figures.

Use significant figures correctly in calculations.

Making Measurements

Scientists and medical professionals carry out experiments and laboratory procedures regularly. They need to make measurements and be able to report results accurately and precisely. How well these experiments are carried out and reported is critical. You need to learn how to measure accurately and how to report your result in such a manner that the reader knows what you have measured and how well the result is known (significant figures).

Metric Units of Length

We will be using a ruler and a meter stick to measure length. The unit of length in the metric system is the meter (m). This is the unit that we will use. A meter is divided into 100 centimeters (cm). Each centimeter is divided into 10 millimeters (mm). On the reverse of most meter sticks is a yard stick.

Metric Units of Volume

Volume is a measure of the space a substance occupies. A cube that measures 1 meter in length, height and width has a volume of (1m)³ = 1m³. Volume is a derived unit based on length. The unit of volume most commonly used in the lab is the liter (L). 1 liter is equal to 1 cubic decimeter (dm)³. 1 liter = (1dm)³. There are 10 decimeters in 1 meter.

1 liter = (1dm)³ = (10 cm)³ = 1000 cm³

So 1 liter contains 1000 cubic centimeters (cm³ or cc) or 1000 milliliters.

Metric Units of Mass

The metric unit for mass is the gram (g).

Significant Figures in Measurements

When using any measuring device, the measurement should be reported as precisely as possible. The number of significant figures in the number reported will depend on the measuring device used. For example, when you use a 50mL graduated cylinder, the smallest lines marked on the cylinder represent 1mL. The last digit of the number is estimated. If a liquid has a volume halfway between 35 and 36mL, its volume would be reported as 35.5mL. If the volume lies exactly on the 35mL line then the volume would be reported as 35.0mL. The number of digits in the reported result, tells us how well we know the volume. 35.0 or 35.5 has three significant figures. Only three significant figures can be obtained using a 50mL graduated cylinder. Make sure that you keep all of the significant figures that you obtain in a measurement. Never round off any number obtained from a measurement.

Materials Needed

Student: 1 wood block, a ruler, a meter stick, string, rubber stoppers, and beaker.

Set up by the instructor: 1 each 10mL, 50mL, and 250mL graduated cylinders containing some water. Top loading balances.

Laboratory Activities

A. Measuring mass

1. Find a wood block and measure its mass. Report the mass using all of the possible significant figures and remember to use the correct units. Example: 25.0789 g. You will notice the last digit displayed on the electronic balances fluctuates. Don’t wait for this value to stabilize, try to estimate an average value for this digit.
2. Find a few rubber stoppers put them in a beaker. The purpose of this experiment is to determine the percent by mass of the rubber stoppers in a breaker as well as the percent by mass of the beakers. You will need the mass of the rubber stoppers, the beaker and the total mass of the rubber stoppers and the beaker. Note: If you are measuring more than one object and only want the mass of one of them, the balance can be set to "zero" or "tare" out the mass of an object. If you want the mass of just the rubber stoppers, put the beaker on the balance and press the tare button. This will subtract out the mass of the beaker, and the displayed mass on the balance is 0.000. Place the rubber stoppers in the beaker. The mass displayed on the balance is just the mass of the rubber stoppers.

B. Measuring Length.

1. Obtain a meter stick and determine your height in meters.
2. Obtain a wood block and a ruler. Measure the length (l), width (w) and height (h) of the wood block in cm. Calculate the volume (cm³) of the wood block. V = l• w • h.
3. Obtain some string and use it to measure the circumference of (the distance around) your head in cm.

3. Measuring Volume.

1. Measure the volume of the liquid in the 10mL graduated cylinder.
2. Measure the volume of the liquid in the 50mL graduated cylinder.
3. Measure the volume of the liquid in the 250mL graduated cylinder.