Chapter 1 Measurements

1.1 Units of Measurement In chemistry we measure quantities. do experiments. calculate results. use numbers to report measurements. compare results to standards. In a measurement a measuring tool is used to compare some dimension of an object to a standard.

1.1 Units of Measurement The metric system or SI (international system) is a decimal system based on 10. used in most of the world. used everywhere by scientists.

Volume Measurement Volume is the space occupied by a substance. uses the unit liter (L) in the metric system. uses the unit m 3 (cubic meter) in the SI system. is measured using a graduated cylinder. 4

Mass: Amount of matter in an object Weight: Measures the force with which gravity pulls on an object. is measured on a balance. uses the unit gram (g) in the metric system. uses the unit kilogram (kg) in the SI system. Measuring Mass

Temperature Measurement The temperature of a substance indicates how hot or cold it is. is measured on the Celsius (  C) scale in the metric system. on this thermometer is 18 ºC or 64 ºF. in the SI system uses the Kelvin (K) scale. 6

1.1 Units in the Metric System In the metric and SI systems, one unit is used for each type of measurement: MeasurementMetricSI Lengthmeter (m)meter (m) Volumeliter (L)cubic meter (m 3 ) Massgram (g)kilogram (kg) Timesecond (s)second (s) TemperatureCelsius (  C)Kelvin (K) 7 All other units are derived from these fundamental units

Scientific Notation Scientific Notation Scientific Notation is used to write very large or very small numbers. for the width of a human hair of m is written 8 x m. of a large number such as s is written 4.5 x 10 6 s. 8

Numbers in Scientific Notation A number written in scientific notation contains a Coefficient (between 1 and 9) power of 10. Examples: coefficient power of ten coefficient power of ten 1.5 x x

Writing Numbers in Scientific Notation To write a number in scientific notation, move the decimal point to give a number 1-9. show the spaces moved as a power of 10. Examples: = 5.2 x = 1.78 x spaces left 3 spaces right 10

Comparing Numbers in Standard and Scientific Notation Here are some numbers written in standard format and in scientific notation. Number in Standard Format Scientific Notation Diameter of the Earth m 1.28 x 10 7 m Mass of a typical human 68 kg 6.8 x 10 1 kg Length of a pox virus cm 3 x cm 11

1.4 Accuracy, Precision, and Significant Figures Significant figures: The number of meaningful digits in a measured or calculated quantity. They come from uncertainty in any measurement. Generally the last digit in a reported measurement is uncertain (estimated). Exact numbers and relationships (7 days in a week, 30 students in a class, etc.) effectively have an infinite number of significant figures.

Known & Estimated Digits If the length is reported as 3.26 cm, the digits 3 and 2 are certain (known). the final digit, 6, is estimated (uncertain). all three digits (2, 7, and 6) are significant, including the estimated digit. 13

Examples. l l.... l l.... l 10.. cm What is the length of the line? 1) 9.2 cm 2) 9.13 cm 3) 9.19 cm 14

Examples Classify each of the following as (1) exact or (2) measured numbers. A.__Gold melts at 1064 °C. B.__1 yard = 3 feet C.__The diameter of a red blood cell is 6 x cm. D.__There are 6 hats on the shelf. E.__A can of soda contains 355 mL of soda. 15

Accuracy, Precision, and Significant Figures Rules for counting significant figures (left-to- right): 1.Zeros in the middle of a number are like any other digit; they are always significant. ◦ cm 4 sf 2.Rules for counting significant figures (left-to- right): ◦ Zero at the beginning of a number are not significant (placeholders) g 3 sfor 6.61 x g

Accuracy, Precision, and Significant Figures Rules for counting significant figures (left-to- right): 3.Zeros at the end of a number and after the decimal point are always significant K 5 sf 4.Zeros at the end of a number and after the decimal point may or may not be significant. 34,2000 ? SF

Rounding Numbers If the first digit you remove is 4 or less, it and all following digits are dropped from the number = (4 s.f) If the digit you remove is 5 or greater, the last digit of the number is increases by = (4 s.f)

Adding Significant Zeros Sometimes, a calculator displays a small whole number. To give an answer with the correct number of significant figures, significant zeros may need to be written after the calculator result. E.g 8.00 ÷ 2.00 = 4  s.f 3 s.f calculator 2 zeros are needed result to give 3 s.f

Multiplication and Division When multiplying or dividing the final answer must have the same number of significant figures as the measurement with the fewest significant figures. Example: x = = 5.3 (rounded) 4 SF 2 SF calculator 2 SF 20

Addition and Subtraction When adding or subtracting the final answer must have the same number of decimal places as the measurement with the fewest decimal places one decimal place two decimal places 26.54calculated answer 26.5 final answer with one decimal place 21

1.5Prefixes A prefix in front of a unit increases or decreases the size of that unit. makes units larger or smaller than the initial unit by one or more factors of 10. indicates a numerical value. prefix= value 1 kilometer= 1000 meters 1 kilogram= 1000 grams 1 microsecond = second 22

Metric and SI Prefixes 23