Significant Figures

1.Explain what significant figures are. 2.Use Significant figures in measurements and calculations. 3.Understand how significant figures impact precision.

Why do we need to know significant figures? We as scientists need to measure things as we perform experiments. Instruments have different degrees of precision We measure to the last known calibration, and estimate the unknown.

Measurements in Experiments Chapter 1 Significant Figures Even though this ruler is marked in only centimeters and half-centimeters, if you estimate, you can use it to report measurements to a precision of a millimeter.

The Rules

Significant Figures – The Rules 1. Nonzero numbers 1 – 9 are always significant. Examples: 1 meter 1 sig fig 92 liters 2 sig figs grams 5 sig figs

Significant Figures – The Rules 2. Imbedded zeros (zeros between nonzero numbers) are always significant. Examples: 202 cm3 sig figs mL5 sig figs 2039 kg4 sig figs g5 sig figs

Significant Figures – The Rules 3. Leading zeros are never significant. 4. Trailing zeros after a nonzero number after the decimal are significant. Examples: g3 sig figs mm4 sig figs L3 sig figs

Significant Figures – The Rules 5. Trailing zeros before the decimal are significant only if the decimal point is specified. Examples: 100. dg3 sig figs 100 dg1 sig fig 8900 km 2 sig figs km4 sig figs

Exact Numbers An exact number is a number that cannot be changed. (Cannot be halved or split up) Ex. 2 atoms, 1 proton, a hundred dollar bill We include most conversion factors as exact numbers Ex. 1m = 100 cm When you work with exact numbers, you consider them to have infinite sig figs. (You don’t have to worry about them!)

RECAP #1 Leading Zeros Imbedded Zero Nonzero numbers Trailing Zeros after the decimal

6 significant figures

RECAP #2 Leading Zeros Imbedded Zero (none) Nonzero numbers Trailing zero with no decimal

4 significant figures

Lets Practice!

56 meters 2 sig figs Rule 1

20 grams 1 sig fig Rule 1, 5

303.0 mL 4 sig figs Rule 1, 2, 4

200 kilograms 1 sig fig Rule 1, 5

207 kilometers 3 sig figs Rule 1,2

grams 4 sig figs Rule 1,3,4

m 5 sig figs Rule 1,2,3,4

km 3 sig figs Rule 1,2,5

1.10 x 10 2 hm 3 sig figs Rule 1, 4

2.2 x atoms infinite sig figs

Rounding Numbers If you have to round and the number you are looking to round is less than 5, don’t round. Example: 214 round to 2 s.f. Answer = 210

Rounding Numbers If you have to round and the number you are looking to round is 5 or greater, round up. Example: 215 round to 2 s.f. Answer = 220

Adding and subtracting with significant figures. When adding or subtracting significant figures, you round your answer to the least number of places after the decimal that are contained in your problem.

YOU ARE LOOKING AT PLACES AFTER THE DECIMAL NOT SIGNIFICANT FIGURES!

Example: = 6.0 You look for the least number of PLACES after the decimal = 2 places after the decimal 4.0 = 1 place after the decimal Your answer can only have one place after the decimal.

Example: = = 1 place after the decimal 4 = no places after the decimal Your answer can not have any places after the decimal.

Example: – = = = 5 places after the decimal = 3 places after the decimal Your answer can only have 3 places after the decimal.

Let’s Practice 17.0 – = Answer 16.5

= Answer 56

100.0 – = Answer 28.5

= Answer 20

Multiplying and Dividing with Significant Figures When multiplying or dividing with significant figures, your answer must be rounded to the least number of significant figures in the problem.

YOU ARE LOOKING AT SIGNIFICANT FIGURES NOT PLACES AFTER THE DECIMAL!

Example 20.0 x = Answer 284

430 x = 1.29 Answer 1

2020 x = Answer 1.60 x 10 6

50.0 / = 2500 Answer 2500

50.0 / = 2500 Answer 2.50 x 10 3