CHEM 20 DAY 4

MEASURING Math and Measurements: Chemistry math is different from regular math

in that in chemistry we use measurements and in math we use exact numbers. Because a measurement is never 100% accurate (because of sources of error), there is error to account for. In order to make sure that the answers to our mathematical problems are not more or less accurate than the measurement involved, we use sig figs.

MEASURING Precision vs. Accuracy Precision: Ability to reproduce the same

result. Error in precision (Random Error) occurs with poor technique

Accuracy: Ability to get the correct result. Error in accuracy (Systematic Error) occurs with poor instrument (calibration).

MEASURING

MEASURING Ex. A piece of metal is known to have a

mass of 18.01 g. When a student made three attempts to determine the mass of the metal, the following results were obtained:

a) 18.00 gb) 18.03 gc) 18.02 g Are these results accurate, precise, or

both?

MEASURING SI Units (Système International): Remember that Units are important! SI Units are

the units that are the most widely used units. SI Units are base units. This means all other units can be derived from the base SI Units. Length- metre : m Mass- kilogram: kg Time- second: s electric current- ampere: A thermodynamic temperature- kelvin: K amount of substance- mole: mol luminous intensity- candela: cd

MEASURING How to Measure: When measuring you always

estimate one digit. Example 1:

Example 2:

SIGNIFICANT FIGURES Significant figures (sig figs) are the

number of digits in a number or measurement that are meaningful. Sig figs are important in order to calculate answers to problems using measurements. Sig figs are used so that when you are performing calculations using measured values the answer is only as accurate as the measurements used.

SIGNIFICANT FIGURES Sig figs rules:1. All non-zero digits are significant.Ex. 384 has _______ sig figs2. All zeros between non-zero digits are significant.Ex. 1,002 has ______ sig figs3. Zeros before the first non-zero digit are not

significantEx. 0.0020901 has ______ sig figs4. Zeros after the last non-zero digit may be significant

(they are significant if the number has a decimal).Ex. 0.00200 has _______ sig figs Ex. 200 has ______ sig figs

SIGNIFICANT FIGURES Note: If you want the zeros after the

last non-zero digit to be significant, you should write the number in scientific notation. For example if 200 should have 3sig figs instead of 1, it should be written as 2.00 x 102

SIGNIFICANT FIGURES Examples:

315= 305= 300= 300.=0.135= 0.1350= 0.01350=0.013050=

SIGNIFICANT FIGURES Calculations with sig figs: Adding and Subtracting with Sig Figs When adding and subtracting, the

answer will have the same number of decimals or places as the digit with the least number of decimals/places.

Examples: 0.135 + 0.01 = 350 + 315 =

SIGNIFICANT FIGURES Multiplying and Dividing with Sig Figs When multiplying and dividing with

sig figs, the answer will have the same number of sig figs as the number in the question with the least number of sig figs.

Example:0.43986 x 0.10 =

SIGNIFICANT FIGURES When combining multiplying/dividing

with adding/subtracting Use BEDMAS and do not round until

the end (just keep track of the number of sig figs each number has using underlining of subscripts)

Example:0.31 + 4.00 x 3.6498

SIGNIFICANT FIGURES Why don’t you round until the end?

[(1.35+4.36) x (0.970x4.31)] x [(6.71x 5.98) / (0.10+3.31)]

[5.710 x 4.1807] x [40.1258 / 3.410]23.8718 x 11.767097

=280.9018

=281

[(1.35+4.36) x (0.970x4.31)] x [(6.71x 5.98) / (0.10+3.31)]

[5.71 x 4.18] x [40.1 / 3.41]23.9 x 11.8

=282

ASSIGNMENT Work on the Assignment at the back of

your booklet (pg 21/22)