Significant Figures, Part 2

Significant Figures

Part 2

Rounding Rules after a calculation

Warm Up

Practice Identifying Number of Significant Figures

Atlantic-Pacific Method

Multiple Choice

How many significant figures in 890.0

• 1

• 2

• 3

• 4

890.0 has 4 significant figures

• P or decimal point Present

• Begin on Pacific side of number (left)

• First non-zero is the 8

• Count the 8 and all digits after it still scanning left to right

• 8, 9, 0, 0 (all of them) for four digits

Multiple Choice

How many significant figures in 8900

• 1

• 2

• 3

• 4

8900 has 2 significant figures

• A or decimal point Absent

• Begin on Atlantic side of number (right side)

• First non-zero is the 9

• Count the 9 and all digits after it still scanning right to left

• 9, 8 for two digits

Multiple Choice

How many significant figures in 0.08900

• 1

• 2

• 3

• 4

0.08900 has 4 significant figures

• P or decimal point Present

• Begin on Pacific side of number (left side)

• First non-zero is the 8

• Count the 8 and all digits after it still scanning left to right

• 8, 9, 0, 0 for four digits

Chemistry requires calculations

Next up... rules for rounding for the different types of calculations

Rule for Multiplication or Division

the "weakest link" rule

Read Page 3 of Notes Sheet

come back in 2 minutes

When multiplying or dividing

The answer should be rounded to have the same number of significant figures as the starting value with the least significant figures.     You could think of it as the “weakest link” rule.

Example 1:        502 x 3.6 = 1800     (do not report as 1807.2)

Multiple Choice

650.5 x 45.2094 = ?

Round your answer based on rules of significant figures

• 29,408.7

• 29,409

• 29,410

• 2,941

• 29,000

29,410

29,408.7147 is rounded to 29,410 because 650.5 was the "weakest link". It had 4 significant figures , compared to 45.2094 which had 6.

(The 0 in 29410 is not significant)

Multiple Choice

7600 / 18.57 = ?

Round your answer based on rules of significant figures

• 409.26

• 409.3

• 409

• 410

• 400

410

409.262251 should be rounded to 410. The "weakest" link was 7600 because it only had 2 significant figures (the 7 and the 6)

Rule For Addition or Subtraction

(entirely different from multiplication/division)

Read page 4 of the Notes Sheet

come back in 2 minutes

Addition or Subtraction

Focus on the decimal places beyond the decimal point (tenths, hundredths, etc.) . Your answer is rounded to have the same as the starting number with the least. 

You could think of it as the “shortest tail” rule.

Example 1:    425.6 + 934.75 = 1360.4

Multiple Choice

3707.5 + 2.425 = ?

Round to the correct number of decimal places based on sig fig rules.

• 3709.925

• 3709.93

• 3709.9

• 3709

• 3710

3709.9

3,709.925 is rounded to the tenths place because 3707.5 only goes to the tenths place (it has the shortest tail)

Multiple Choice

3500 - 2.425 = ?

Round to the correct number of decimal places based on sig fig rules.

• 3497.565

• 3497.57

• 3497.6

• 3498

• 3500

3498

3,497.575 is rounded to the ones place because 3500 does not have any decimals beyond the decimal point (It has no tail)

Poll

How would you rate your current comfort level with the concept of Significant Figures?

• great

• okay

• a bit shaky

• no confidence at all

It gets better

Keep trying and you will get it.

Complete assignments by end of Tuesday, Sept 22nd