Significant Figures (digits) = reliable figures obtained by measurement = all digits known with certainty plus one estimated digit.

Significant Figures (digits)= reliable figures obtained by

measurement= all digits known with certainty plus

one estimated digit

Taking the measurement

• Is always some uncertainty

• Because of the limits of the instrument you are using

EXAMPLE: mm ruler

Is the length of the line between 4 and 5 cm? Yes, definitely.Is the length between 4.0 and 4.5 cm? Yes, it looks that way.

But is the length 4.3 cm? Is it 4.4 cm?

Let’s say we are certain that it is 4.3 cm or 43mm, but not at long as 4.4cm. So – we need to add one more digit to ensure the measurement is more accurate.Since we’ve decided that it’s closer to 4.3 than 4.4 it may be recorded at 4.33 cm.

• It is important to be honest when reporting a measurement, so that it does not appear to be more accurate than the equipment used to make the measurement allows.

• We can achieve this by controlling the number of digits, or significant figures, used to report the measurement.

As we improve the sensitivity of the equipment used to make a measurement,

the number of significant figures increases. Postage Scale

3 g

1 g 1 significant figure

Two-pan balance

2.53 g

0.01 g 3 significant figures

Analytical balance

2.531 g

0.001g 4 significant figures

Which numbers are Significant?

How to count them!

55.00 mm

0.003g

9000 L

5,551,213

Non-Zero integers

• Always count as significant figures

1235 has 4 significant digits

Zeros – there are 3 types

Leading zeros (place holders)

The first significant figure in a measurement is the first digit other than zero counting from left to right

0.0045g

(4 is the 1st sig. fig.)

“0.00” are place holders.

The zeros are not significant

Captive zeros

Zeros within a number at always significant – 30.0809 g

All digits are significant

Trailing zeros – at the end of numbers but to the right of the decimal point

2.00 g - has 3 sig. digits (what this means is that the measuring instrument can measure exactly to two decimal places.

100 m has 1 sig. digit

Zeros are significant if a number contains decimals

Exact Numbers

Are numbers that are not obtained by measuring

Referred to as counting numbers

EX : 12 apples, 100 people

Exact Numbers

Also arise by definition

1” = 2.54 cm or 12 in. = 1 foot

Are referred to as conversion factors that allow for the expression of a value using two different units

Rules for sig figs.:•Count the number of digits in a measurement from left to right:

•Start with the first nonzero digit•Do not count place-holder zeros.

•The rules for significant digits apply only to measurements and not to exact numbers

Sig figs is short for significant figures.

Significant Figures

Determining Significant Figures

State the number of significant figures in the following measurements:

2005 cm

25,000 g

25.0 ml

0.25 s

0.00250 mol

4

2

3

2

3

0.050 cm

0.0280 g

50.00 ml

1000 s

1000. mol

2

3

4

1

4

Rounding Numbers

• To express answer in correctly

• Only use the first number to the right of the last significant digit

Rounding

• Always carry the extra digits through to the final result

• Then round

EX:

Answer is 1.331 rounds to 1.3

OR

1.356 rounds to 1.4

Rounding off sig figs (significant figures):

Rule 1: If the first non-sig fig is less than 5, drop all non-sig fig.Rule 2: If the first sig fig is 5, or greater that 5, increase the last sig fig by 1 and drop all non-sig figs.

Round off each of the following to 3 significant figures:

12.514748 12.5 0.6015261 0.602

192.49032 192 14652.832 14,700

Significant Figures

Math Problems w/Sig Figs

When combining measurements with different degrees of accuracy and precision, the accuracy of the final

answer can be no greater than the least accurate measurement.

Adding and Subtracting Sig. Figures

This principle can be translated into a simple rule for addition and subtraction:

When measurements are added or subtracted, the answer can contain no more decimal places than the least accurate measurement.

Adding and subtracting sig figs - your answer must be limited to the value

with the greatest uncertainty.

Significant Figures

Line up decimals and Add

150.0 g H2O (using significant figures)

0.507 g salt 150.5 g solution

150.5 g solution

150.0 is the least precise so the answer will have no more than one place to the right of the decimal.

ExampleAnswer will have the same number of decimal

places as the least precise measurement used.

12.11 cm

18.0 cm

1.013 cm

31.132 cm

9.62 cm

71.875 cmCorrect answer would be 71.9 cm – the last sig fig is “8”, so you will round using only the first number to the right of the last significant digit which is “7”.

Multiplication and division of sig figs - your answer must be limited to the measurement with the least number of

sig figs.

5.15X 2.3 11.845

3 sig figs2 sig figsonly allowed 2 sig figs

so 11.845is rounded to 12

5 sig fig2 sig figs

Significant Figures

Multiplication and Division

Answer will be rounded to the same number of significant figures as the component with the fewest number of significant figures.

4.56 cm x 1.4 cm = 6.38 cm2

= 6.4 cm2

28.0 inches 2.54 cm

1 inch

Computed measurement is 71.12 cm

Answer is 71.1 cm

x = 71.12 cm

When both addition/subtraction and multiplication/division appear in the same

problem

• In addition/subtraction the number of significant digits is limited by the value of greatest uncertainty.

• In multiplication/division, the number of significant digits is limited by the value with the fewest significant digits.

• Since the rules are different for each type of operation, when they both occur in the same problem,

– complete the first operation and establish the correct number of significant digits.

– Then proceed with the second and set the final answer according to the correct number of significant digits based on that operation

(1.245 + 6.34 + 8.179)/7.5

• Add

1.245 + 6.34 + 8.179 =

Then divide by 7.5 =