Section 1: Significant Figures

Chapter 2: Measurement and Problem Solving

Learning ObjectivesDetermine which digits in a number

are significant. Round numbers to the correct

number of significant figures.Determine the correct number of

significant figures in calculations.

UncertaintiesMeasurements are only as reliable as the instrument used, and the care with which the measurement is made.

UncertaintiesAccuracy:

How close a measurement is to the correct or accepted value

Precision: How close a set of measurements are

to each other

UncertaintiesThe percent error is how far off your

own measurement or calculation is from the correct value.

Question:What value

should you record as the length of the beetle? Explain your answer. (The ruler is showing cm)

Significant FiguresSignificant figures are the digits in a

measurement that are known for certain, plus one uncertain digit.

As scientists, we do not care about any digits that are not significant. When we take measurements in lab we

only report significant figures.

Significant Figures In the example, we are certain that the beetle

is at least 1.5 cm in length, but the hundredths place is uncertain. We would therefore report this value as 1.55▪ The one and five are certain digits

▪ The second five is uncertain (depending on your opinion, you might report this as 1.54 or 1.56 – these would also be correct)

▪ Therefore, this measurement has three total significant figures (two certain digits and one uncertain digit)

Significant Figures

1.55

Significant Figures How to determine significant figures:

1.All nonzero digits are significant.

2.Interior zeros (zeros between two nonzero numbers) are significant.

3.Trailing zeros (zeros to the right of a nonzero number) that fall after a decimal point are significant.

Significant Figures4.Trailing zeros that fall before a decimal point

are significant.

5.Leading zeros (zeros to the left of the first nonzero number) are NOT significant. They serve only to locate the decimal point.

6.Trailing zeros at the end of a number with no decimal point are NOT significant.

Significant Figures Exact numbers are not counted as

significant figures. Exact counting of discrete objects

Numbers that are part of an equation

Defined quantities (e.g. a dozen, a mole, etc)

Some conversion factors are defined quantities, while others are not.

▪ 1 in. = 2.54 cm exact

Significant Figures How many significant figures are in each

number?a)0.0035

b)1.080

c)2371

d)2.9 × 105

e)1 dozen = 12

f) 100.00

g)100,000

Significant FiguresRules for Rounding:

When numbers are used in a calculation, the result is rounded to reflect the significant figures of the data.

For calculations involving multiple steps, round only the final answer—do not round off between steps.

▪ This practice prevents small rounding errors from affecting the final answer.

Significant Figures Round down if the last digit dropped is

4 or less; round up if the last digit dropped is 5 or more.

Significant Figures Round to 3 sig-figs: 4.287 4.29

Round to 3 sig-figs: 4.284 4.28

Round to 3 sig-figs: 4.285 4.29

Significant Figures

Multiplication and Division Rule: The result of multiplication or division carries the same number of significant figures as the factor with the fewest significant figures.

Significant Figures

The intermediate result (in blue) is rounded to two significant figures to reflect the least precisely known factor (0.10), which has two significant figures.

Significant Figures

The intermediate result (in blue) is rounded to three significant figures to reflect the least precisely known factor (6.10), which has three significant figures.

Significant FiguresAddition and Subtraction Rule: In addition or subtraction calculations, the result carries the same number of decimal places as the quantity carrying the fewest decimal places.

Significant Figures We round the

intermediate answer (in blue) to two decimal places because the quantity with the fewest decimal places (5.74) has two decimal places.

Significant Figures We round the

intermediate answer (in blue) to one decimal place because the quantity with the fewest decimal places (4.8) has one decimal place.

Significant Figures In calculations involving both

multiplication /division and addition/subtraction: do the steps in parentheses first

determine the correct number of significant figures in the intermediate answer without rounding

then do the remaining steps.

Significant Figures 3.489 × (5.67 – 2.3)

do the step in parentheses first. 5.67 – 2.3 = 3.37 Use the subtraction rule to determine that the intermediate

answer has only one significant decimal place. To avoid small errors, it is best not to round at this point; instead, underline the least significant figure as a reminder.

3.489 × 3.37 = 11.758 = 12 Use the multiplication rule to determine that the intermediate

answer (11.758) rounds to two significant figures (12) because it is limited by the two significant figures in 3.37.

Significant FiguresPerform each calculation to the

correct number of significant figures.

a) 56.55 × 0.920 ÷ 34.2585

Significant FiguresPerform each calculation to the

correct number of significant figures.

a) 1.10 × 0.512 × 1.301 × 0.005 ÷ 3.4

Significant Figures Perform each

calculation to the correct number of significant figures.

Significant Figures Perform each

calculation to the correct number of significant figures.

Significant FiguresPerform each calculation to the

correct number of significant figures.

a) 6.78 × 5.903 × (5.489 – 5.01)

Significant FiguresPerform each calculation to the

correct number of significant figures.

a) 3.897 × (782.3 – 451.88)

Significant Figures Which calculation would have its

result reported to the greater number of significant figures?

a) 3 + (15/12)

b) (3 + 15)/12