Measure and record
length, volume and mass accurately.
Write the
measurement to the correct number of significant figures.
Use significant
figures correctly in calculations.
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. Correct measurement and reporting is
critical to achieving successful experiments. You need to learn how to measure
accurately and how to report your result with the correct precision
(significant figures).
We
will be using a ruler to measure length.
The standard unit of length in the metric system is the meter (m). A
meter is divided into 100 centimeters (cm).
Each centimeter is divided into 10 millimeters (mm).
We
will be using a ruler and graduated cylinders to measure 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.
We
will use a balance to measure mass.
Mass is the measure of the amount of material or matter. This is different than weight, which is the
measure of the pull of gravity on a specific mass. Mass remains constant for a particular amount of matter while the
weight can change if you change gravity.
For example, you have the same amount of mass on the moon as on the
earth but on the moon you weigh less because the moon has less gravity than the
earth. The metric unit for mass is the
gram (g).
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.
|
1 metal block |
|
|
Ruler |
|
|
Graduated
cylinders |
|
|
Top loading
balances |
|
|
Deionized water
(in larger white containers on the lab benches) |
|
1. Find
a metal block, record its number or letter and measure its mass by using a
top-loading balance. 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.
1.
Obtain a ruler and measure the length
(l), width (w) and height (h) of your metal block in cm. Calculate
the volume (cm3) of the metal block. Make sure to determine the correct significant
figures for your calculated value based on the significant figures of your
measurement.
V
= l• w • h.
1.
Pour at least 8 mL of deionized water
in a 10mL graduated cylinder and record the volume to the correct number of
significant figures.
2.
Pour the water from the 10 mL graduated
cylinder into a 50 mL cylinder. Record the Volume to the correct number of
significant figures.
3.
Pour the water from the 50 mL graduated
cylinder into a 250 mL cylinder. Record the Volume to the correct number of
significant figures.
4.
What trend do you see as the relative
fullness in the cylinders goes down?
D.
Density
1. Using the mass and volume of the metal block
in parts A and B, calculate the
density of the block to the correct
number of significant figures.
2. Obtain a relatively dry 10 or 25 mL
graduated cylinder. Weigh the
cylinder
and record the mass to the correct
number of sig figs. Fill the cylinder
about
75% full with deionized water, record
the volume and reweigh. Use this
information to calculate the density of
water to the correct number of sig figs.