2.4 Measurement and Significant Figures Every experimental measurement has a degree of uncertainty. The volume, V, at right is certain in the 10’s place, 10mL<V<20mL The 1’s digit is also certain, 17mL<V<18mL A best guess is needed for the tenths place. Chapter Two 1

What is the Length? We can see the markings between cm We can’t see the markings between the.6-.7 We must guess between.6 &.7 We record 1.67 cm as our measurement The last digit an 7 was our guess...stop there 2

Learning Check What is the length of the wooden stick? 1) 4.5 cm 2) 4.54 cm 3) cm

cm or 3 (2.2/8) ?

Measured Numbers Do you see why Measured Numbers have error…you have to make that Guess! All but one of the significant figures are known with certainty. The last significant figure is only the best possible estimate. To indicate the precision of a measurement, the value recorded should use all the digits known with certainty. 5

Chapter Two 6 Below are two measurements of the mass of the same object. The same quantity is being described at two different levels of precision or certainty.

Note the 4 rules When reading a measured value, all nonzero digits should be counted as significant. There is a set of rules for determining if a zero in a measurement is significant or not. RULE 1. Zeros in the middle of a number are like any other digit; they are always significant. Thus, g has five significant figures. RULE 2. Zeros at the beginning of a number are not significant; they act only to locate the decimal point. Thus, cm has three significant figures, and mL has four. Chapter Two 7

RULE 3. Zeros at the end of a number and after the decimal point are significant. It is assumed that these zeros would not be shown unless they were significant m has six significant figures. If the value were known to only four significant figures, we would write m. RULE 4. Zeros at the end of a number and before an implied decimal point may or may not be significant. We cannot tell whether they are part of the measurement or whether they act only to locate the unwritten but implied decimal point. Chapter Two 8

Practice Rule #1 Zeros  All digits count Leading 0’s don’t Trailing 0’s do 0’s count in decimal form 0’s don’t count w/o decimal All digits count 0’s between digits count as well as trailing in decimal form

2.5 Scientific Notation Scientific notation is a convenient way to write a very small or a very large number. Numbers are written as a product of a number between 1 and 10, times the number 10 raised to power. 215 is written in scientific notation as: 215 = 2.15 x 100 = 2.15 x (10 x 10) = 2.15 x 10 2 Chapter Two 10

Chapter Two 11 Two examples of converting standard notation to scientific notation are shown below.

Chapter Two 12 Two examples of converting scientific notation back to standard notation are shown below.

Scientific notation is helpful for indicating how many significant figures are present in a number that has zeros at the end but to the left of a decimal point. The distance from the Earth to the Sun is 150,000,000 km. Written in standard notation this number could have anywhere from 2 to 9 significant figures. Scientific notation can indicate how many digits are significant. Writing 150,000,000 as 1.5 x 10 8 indicates 2 and writing it as x 10 8 indicates 4. Scientific notation can make doing arithmetic easier. Rules for doing arithmetic with numbers written in scientific notation are reviewed in Appendix A. Chapter Two 13

2.6 Rounding Off Numbers Often when doing arithmetic on a pocket calculator, the answer is displayed with more significant figures than are really justified. How do you decide how many digits to keep? Simple rules exist to tell you how. Chapter Two 14

Once you decide how many digits to retain, the rules for rounding off numbers are straightforward: RULE 1. If the first digit you remove is 4 or less, drop it and all following digits becomes 2.4 when rounded off to two significant figures because the first dropped digit (a 2) is 4 or less. RULE 2. If the first digit removed is 5 or greater, round up by adding 1 to the last digit kept is 4.6 when rounded off to 2 significant figures since the first dropped digit (an 8) is 5 or greater. If a calculation has several steps, it is best to round off at the end. Chapter Two 15

Practice Rule #2 Rounding Make the following into a 3 Sig Fig number ,  ,  10 6 Your Final number must be of the same value as the number you started with, 129,000 and not 129

Examples of Rounding For example you want a 4 Sig Fig number , is dropped, it is <5 8 is dropped, it is >5; Note you must include the 0’s 5 is dropped it is = 5; note you need a 4 Sig Fig ,

RULE 1. In carrying out a multiplication or division, the answer cannot have more significant figures than either of the original numbers. Chapter Two 18

RULE 2. In carrying out an addition or subtraction, the answer cannot have more digits after the decimal point than either of the original numbers. Chapter Two 19

Multiplication and division  1.54 =  =  =  10 6  =     = 

Addition/Subtraction ‑

__ ___ __ Addition and Subtraction = = = – = Look for the last important digit

Mixed Order of Operation  = ( )  ( ) = = = = =  = =

What is Percent Error Percent Error is used to determine the inaccuracy, in percentage, of a measured or estimated value, compared to an accepted value.

The Components of the Formula Estimated Value (Measured Value)- The value that has been derived from an experiment, or an estimated value. Actual Value (What you should have gotten!)- This value is the exact value excepted throughout the scientific community, or the value which is determined exact, at a later point in time.

The formula The Formula for Percent Error is a follows: (EV – AV) AV Remember: EV= Estimated Value (measured value!) AV= Actual Value PS. We multiply by 100 to make to make the decimal a percent. X 100 = Percent Error

Practice Problems Johnny calculates from an experiment that he has 22.7 grams of carbon. However, he should have gotten 21.8 grams of carbon, which was the accepted value for this experiment. What was his percent error? Remember: (EV-AV) X 100 = PE AV

Answer (EV-AV) X 100 = PE AV ( ) X 100 = PE X 100 = PE X 100 = PE 4.13 % = PE

More Practice The actual value for the absorption of carbon is.135. Steven calculated that absorption was.235. What is the percent error in Steven’s experiment?

Answer (EV-AV) X 100 = PE AV ( ) X 100 =PE X 100 = PE X 100 = PE 74.1 % = PE