Measurement. Measurement in Chemistry. Measurement. Stating a Measurement. The Metric System (SI) Basic Chemistry. Chapter 2 Measurements

Transcription

1 Chapter 2 Lecture Chapter 2 Measurements 2.1 Units of Measurement Fifth Edition Measurement You make a measurement every time you measure your height read your watch take your temperature weigh a cantaloupe Learning Goal Write the names and abbreviations for the metric or SI units used in measurements of length, volume, mass, temperature, and time. 2 Measurement in Chemistry In chemistry we measure quantities do experiments calculate results use numbers to report measurements compare results to standards Measurement A measuring tool is used to compare some dimension of an object to a standard. An electronic balance is the measuring tool used to determine the mass of a nickel. 3 4 Stating a Measurement In every measurement, a number is followed by a unit. Observe the following examples of measurements: Number Unit 35 m 0.25 L 225 lb 3.4 h The Metric System (SI) The metric system and SI (international system) are decimal systems based on 10 used everywhere by scientists used in most of the world 5 6 1

2 Units in the Metric System In the metric and SI systems, one unit is used for each type of measurement: Measurement Metric SI Length meter (m) meter (m) Volume liter (L) cubic meter (m 3 ) Mass gram (g) kilogram (kg) Time second (s) second (s) Temperature degree Celsius ( C) Kelvin (K) Length Measurement Length is measured using a meterstick has the unit meter (m) in both the metric and SI systems 7 8 Inches and Centimeters The unit of an inch (in.) is equal to 2.54 cm in the metric and SI systems. 1 in. = 2.54 cm Volume Measurement Volume is the space occupied by a substance has the unit liter (L) in the metric system 1 L = qt has the unit m 3 (cubic meter) in the SI system is measured using a graduated cylinder 9 10 Mass Measurement The mass of an object is the quantity of material it contains is measured on a balance has the unit gram(g) in the metric system has the unit kilogram(kg) in the SI system Temperature Measurement The temperature indicates how hot or cold it is is measured on the Celsius ( C) scale in the metric system is measured on the Kelvin(K) scale in the SI system on this thermometer is 18 ºC or 64 ºF

3 Time Measurement Time measurement uses the unit second(s) in both the metric and SI systems is based on an atomic clock that uses a frequency emitted by cesium atoms For each of the following, indicate whether the unit describes 1) length 2) mass or 3) volume. A. B. C. D. A bag of onions has a mass of 2.6 kg. A person is 2.0 m tall. A medication contains 0.50 g of aspirin. A bottle contains 1.5 L of water Identify the measurement that has an SI unit. A. John s height is. 1) 1.5 yd 2) 6 ft 3) 2.1 m B. The race was won in. 1) 19.6 s 2) 14.2 min 3) 3.5 h Chapter 2 Lecture Chapter 2 Measurements 2.2 Measured Numbers and Fifth Edition C. The mass of a lemon is. 1) 12 oz 2) kg 3) 0.6 lb D. The temperature is. 1) 85 C 2) 255 K 3) 45 F 15 Learning Goal Identify a number as measured or exact; determine the number of significant figures in a measured number. Measured Numbers A measuring tool is used to determine a quantity such as the length or the mass of an object provides numbers for a measurement called measured numbers Reading a Meterstick. l l.... l l.... l 4.. cm The markings on the meterstick at the end of the orange line are read as The first digit 2 plus the second digit 2.7 The last digit is obtained by estimating. The end of the line might be estimated between as half-way (0.05) or a little more (0.06), which gives a reported length of 2.75 cm or 2.76 cm

4 Known + Estimated Digits In the length reported as 2.76 cm, the digits 2 and 7 are certain (known) the final digit 6 was estimated (uncertain) all three digits (2.76) are significant including the estimated digit. l l.... l l.... l 10.. cm What is the length of the red line? 1) 9.0 cm 2) 9.03 cm 3) 9.04 cm Zero as a Measured Number. l l.... l l.... l 5.. cm For this measurement, the first and second known digits are 4.5. Because the line ends on a mark, the estimated digit in the hundredths place is 0. This measurement is reported as 4.50 cm. in Measured Numbers Significant figures obtained from a measurement include all of the known digits plus the estimated digit. The number of significant figures reported in a measurement depends on the measuring tool Counting All nonzero numbers in a measured number are significant. Measurement Number of cm ft lb m

5 Sandwiched Zeros Sandwiched zeros occur between nonzero numbers are significant Measurement Number of Significant Figures 50.8 mm min lb m 5 Trailing Zeros Trailing zeros follow nonzero numbers in numbers without decimal points are placeholders are not significant Measurement cm kg ml g 5 Number of Significant Figures Leading Zeros Leading zeros precede nonzero digits in a decimal number are placeholders are not significant Measurement Number of mm oz lb ml 3 in Scientific Notation In scientific notation all digits including zeros in the coefficient are significant Scientific Notation Number of 8 x 10 4 m x 10 4 m x 10 4 m State the number of significant figures in each of the following measurements: A m B L C g D m A. Which answer(s) contain three significant figures? 1) ) ) 4.76 x 10 3 B. All the zeros are significant in 1) ) ) x 10 3 C. The number of significant figures in 5.80 x 10 2 is 1) one 3) two 3) three

6 Exact Numbers Examples of Exact Numbers An exact number is obtained when objects are counted Example: counting objects 2 baseballs 4 pizzas from numbers in a defined relationship Example: defined relationships 1 foot = 12 inches 1 meter = 100 cm A. Exact numbers are obtained by 1. using a measuring tool 2. counting 3. definition B. Measured numbers are obtained by 1. using a measuring tool 2. counting 3. definition Classify each of the following as exact (E) or measured numbers (M). Explain your answer. A. Gold melts at 1064 C. B. 1 yd = 3 ft C. The diameter of a red blood cell is 6 x 10 4 cm. D. There are 6 hats on the shelf. E. A can of soda contains 355 ml of soda