Chapter 5: Mastering Laboratory Skills 69

This chapter describes and illustrates the laboratory skills that you need to master to

complete the laboratory sessions in this book.

5

Mastering Laboratory Skills

The degree of uncertainty depends on the resolution of the
measuring instrument and the degree of skill used to make
the measurement. For example, if you use a ruler graduated in
centimeters (cm) to measure the length of a piece of wire, you
might find that the wire is between 2 cm and 3 cm long. Because
the ruler is graduated in centimeters, the most you can say with
certainty is that the wire is longer than 2 cm and shorter than 3
cm, because these are the two known values between which the
length of the wire falls.

However, you can estimate (interpolate) the length of the wire
to some value between the known values. For example, if the end
of the wire extends almost but not quite halfway from the 2 cm
marking to the 3 cm marking, you might estimate the length of
the wire as 2.4 cm. That value has two significant figures (usually
spoken as sig figs), which comprise the one known value and the

mEASUREmENT RESoLUTIoN ANd SIGNIfICANT fIGURES

Measurements are a key part of chemistry, so it’s important to understand them and their

limitations. Some measurements, called counts, are exact. For example, if you have a bag of

apples, you can count the apples and state definitively that the bag contains exactly eight apples.

But most measurements are inexact. For example, although the label on a bag of rice may state

that it contains two kilograms of rice, this is only an approximation of its mass. Depending on the

resolution of the scale used to weigh the rice (and on truth-in-labeling laws), a particular bag may

contain as little as 1.999 kg, 1.99 kg, or even 1.9 kg of rice, or it may contain as much 2.001 kg,

2.01 kg, or even 2.1 kg of rice. That variability (or uncertainty) is common to most measurements

made in a chemistry lab.

interpolated value you added. For example, Figure 5-10 (later in

this chapter) shows a burette that is graduated to (has resolution

of) 0.1 mL, but allows values to be interpolated to about 0.01 mL.

vARIABLE SIG fIGS

It’s important to understand that the number of

significant figures yielded by a particular measuring

instrument is not fixed. For example, if the segment

of wire that we measured with the ruler graduated in

centimeters was between 15 cm and 16 cm in length and

we interpolated a value of 15.8 cm, that value has three

sig figs rather than just two. The number of significant

figures varies not just with the resolution of the measuring

instrument, but also with the quantity being measured.