Chemistry Reference
In-Depth Information
9.
Which of the following represents the greatest volume of water?
a) 1 ml
c) 100 cm
3
d) 0.5 L
10. Which of the following represents the greatest mass of water?
a) 2.6 cg
b) 1 dm
3
b) 124 mg
c) 0.07 kg
d) 450 g
Lesson 2-3: Uncertainty in Measurements
There is no such thing as a “perfect” measurement. Each measure-
ment contains a degree of uncertainty, as a result of the limitations of the
instrument used to make the measurement and the care of the person tak-
ing the measurement. An instrument, such as a graduated cylinder, only
measures to a certain degree of accuracy, and no amount of care on your
part will allow you to improve measurements taken from that graduate
beyond that limitation. On the other hand, you could use a very sensitive
instrument, and the accuracy of your measurements might be hindered by
your carelessness. A chemistry student must work with the instruments that
he or she has in order to take the most precise measurements possible.
Before we continue talking about measurements, we should discuss two
important terms, as they apply to measurements in chemistry. The accu-
racy of a measurement refers to how close it is to the “true” or accepted
value. The term precision, which has several meanings, is used in chemistry
to indicate how close together a group of measurements are to each other.
The analogy of a dartboard is often used to illustrate accuracy and preci-
sion. Precise measurements are likened to a group of darts that are close
together on a dartboard. Regardless of how close the darts fall to the bull's-
eye, a group of darts that are close together would represent a high degree
of precision. A dart striking the bull's-eye would be considered accurate.
Not all instruments are created equal. Some are designed for relatively
crude measurements, whereas others are designed for careful, accurate
measurements. An easy example is to compare the scale that you find in
your bathroom to the scale that is used to measure fruits and vegetables at
your supermarket. Many bathroom scales only have lines or marks for the
whole pound. This is because even the most weight-conscious person is not
likely to want to check if he or she has lost 1/10th or 1/100th of a pound.
Such small changes in weight would probably be regarded as insignificant,
and so we tend to round our weights off to the whole pound, or even tens
of pounds, if we are not overly concerned with our weight.
In the supermarket, however, where you pay for fruit and vegetables by
the pound, we are interested in the 10th or even 100th of a pound, because if
