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Numbers in Science Chapter 2 2

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**Measurement What is measurement?**

Quantitative Observation Based on a comparison to an accepted scale. A measurement has 2 Parts – the Number and the Unit Number Tells Comparison Unit Tells Scale There are two common unit scales English Metric 2

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The Unit

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**The measurement System units**

English (US) Length – inches/feet Distance – mile Volume – gallon/quart Mass- pound Metric (rest of the world) Length – meter Distance – kilometer Volume – liter Mass - gram

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**Related Units in the Metric System**

All units in the metric system are related to the fundamental unit by a power of 10 The power of 10 is indicated by a prefix The prefixes are always the same, regardless of the fundamental unit 6

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Fundamental Unit 100

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Fundamental SI Units Physical Quantity Name of Unit Abbreviation Mass Kilogram kg Length Meter m Time Second s Temperature Kelvin K Energy Joules J Pressure Pascal Pa Volume Cubic meters m3 Established in 1960 by an international agreement to standardize science units These units are in the metric system

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**Length….. SI unit = meter (m) About 3½ inches longer than a yard**

1 meter = distance between marks on standard metal rod in a Paris vault or distance covered by a certain number of wavelengths of a special color of light Commonly use centimeters (cm) 1 inch (English Units) = 2.54 cm (exactly) 7

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**Figure 2.1: Comparison of English and metric units for length on a ruler.**

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Volume Measure of the amount of three-dimensional space occupied by a substance SI unit = cubic meter (m3) Commonly measure solid volume in cubic centimeters (cm3) Commonly measure liquid or gas volume in milliliters (mL) 1 L is slightly larger than 1 quart 1 mL = 1 cm3 8

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**Mass Measure of the amount of matter present in an object**

SI unit = kilogram (kg) Commonly measure mass in grams (g) or milligrams (mg) 1 kg = pounds, 1 lbs.. = g 9

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Temperature Scales Any idea what the three most common temperature scales are? Fahrenheit Scale, °F Water’s freezing point = 32°F, boiling point = 212°F Celsius Scale, °C Temperature unit larger than the Fahrenheit Water’s freezing point = 0°C, boiling point = 100°C Kelvin Scale, K (SI unit) Temperature unit same size as Celsius Water’s freezing point = 273 K, boiling point = 373 K 22

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**Thermometers based on the three temperature scales in (a) ice water and (b) boiling water.**

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The number

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Scientific Notation Technique Used to Express Very Large or Very Small Numbers 135,000,000,000,000,000,000 meters liters Based on Powers of 10 What is power of 10 Big? 0,10, 100, 1000, 10,000 100, 101, 102, 103, 104 What is the power of 10 Small? 0.1, 0.01, 0.001, 10-1, 10-2, 10-3, 10-4 3

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**Writing Numbers in Scientific Notation**

1. Locate the Decimal Point : 1, Move the decimal point to the right of the non-zero digit in the largest place - The new number is now between 1 and Now, multiply this number by a power of 10 (10n), where n is the number of places you moved the decimal point - In our case, we moved 3 spaces, so n = 3 (103) 4

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**The final step for the number……**

4. Determine the sign on the exponent n If the decimal point was moved left, n is + If the decimal point was moved right, n is – If the decimal point was not moved, n is 0 We moved left, so 3 is positive 1.438 x 103 5

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**Writing Numbers in Standard Form**

We reverse the process and go from a number in scientific notation to standard form….. Determine the sign of n of 10n If n is + the decimal point will move to the right If n is – the decimal point will move to the left Determine the value of the exponent of 10 Tells the number of places to move the decimal point Move the decimal point and rewrite the number Try it for these numbers: x 106 and 9.8 x 10-2

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**Let’s Practice….. Change these numbers to Scientific Notation:**

1,340,000,000,000 697, 000 Change these numbers to Standard Form: 3.76 x 10-5 8.2 x 108 1.0 x 101 1.34 x 1012 6.97 x 105 9.12 x 10-12 820,000,000 10

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**Are you sure about that number?**

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**Uncertainty in Measured Numbers**

cm A measurement always has some amount of uncertainty, you always seem to be guessing what the smallest division is… To indicate the uncertainty of a single measurement scientists use a system called significant figures The last digit written in a measurement is the number that is considered to be uncertain 10

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Rules, Rules, Rules…. We follow guidelines (i.e. rules) to determine what numbers are significant Nonzero integers are always significant 2753 89.659 .281 Zeros Captive zeros are always significant (zero sandwich) 1001.4

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**Significant Figures – Tricky Zeros**

Leading zeros never count as significant figures Trailing zeros are significant if the number has a decimal point 22,000 63,850. 100,000

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**Significant Figures Scientific Notation**

All numbers before the “x” are significant. Don’t worry about any other rules. 7.0 x 10-4 g has 2 significant figures 2.010 x 108 m has 4 significant figures How many significant figures are in these numbers? 102, ,017 1.0 x , 1,908, x 1014

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**Have a little fun remembering sig figs**

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**Exact Numbers Exact Numbers are numbers known with certainty**

Unlimited number of significant figures They are either counting numbers number of sides on a square or defined 100 cm = 1 m, 12 in = 1 ft, 1 in = 2.54 cm 1 kg = 1000 g, 1 LB = 16 oz 1000 mL = 1 L; 1 gal = 4 qts. 1 minute = 60 seconds 14

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**Calculations with Significant Figures**

Exact numbers do not affect the number of significant figures in an answer Answers to calculations must be rounded to the proper number of significant figures round at the end of the calculation For addition and subtraction, the last digit to the right is the uncertain digit. Use the least number of decimal places For multiplication, count the number of sig figs in each number in the calculation, then go with the smallest number of sig figs Use the least number of significant figures 15

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**Rules for Rounding Off If the digit to be removed**

is less than 5, the preceding digit stays the same Round to 4 sig figs. is equal to or greater than 5, the preceding digit is increased by 1 Round to 3 sig figs. In a series of calculations, carry the extra digits to the final result and then round off Don’t forget to add place-holding zeros if necessary to keep value the same!! Round 80,150,000 to 3 sig figs.

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**Examples of Sig Figs in Math**

Answers must be in the proper number of significant digits!!! 5.18 x 116.8 – 0.33

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**Solutions: 0.107744 round to proper # sig fig 165.47 116.47**

5.18 has 3 sig figs, has 3 sig figs so answer is 0.108 165.47 Limiting number of sig figs in addition is the smallest number of decimal places = 12 (no decimals) answer is 165 116.47 Same rule as above so answer is 116.5

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**Moving unit to unit: Conversion**

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**Exact Numbers Exact Numbers are numbers known with certainty**

They are either counting numbers number of sides on a square or defined 100 cm = 1 m, 12 in = 1 ft, 1 in = 2.54 cm 1 kg = 1000 g, 1 LB = 16 oz 1000 mL = 1 L; 1 gal = 4 qts. 1 minute = 60 seconds 14

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The Metric System Fundamental Unit 100

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**Movement in the Metric system**

In the metric system, it is easy it is to convert numbers to different units. Let’s convert 113 cm to meters Figure out what you have to begin with and where you need to go.. How many cm in 1 meter? 100 cm in 1 meter Set up the math sentence, and check that the units cancel properly. 113 cm [1 m/100 cm] = m

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**Let’s Practice converting metric units**

250 mL to Liters 0.250 mL 1.75 kg to grams 1,750 grams 88 µL to mL 0.088 mL 475 cg to kg 47,500,000 or 4.75 x 107 328 mm to dm 3.28 dm nL to µL 0.75 µL

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**Converting Between Metric and non-Metric (English) units**

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**Converting non-Metric Units**

Many problems involve using equivalence statements to convert one unit of measurement to another Conversion factors are relationships between two units Conversion factors are generated from equivalence statements e.g. 1 inch = 2.54 cm can give or 18

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**Converting non-Metric Units**

Arrange conversion factor so starting unit is on the bottom of the conversion factor Convert kilometers to miles You may string conversion factors together for problems that involve more than one conversion factor. Convert kilometers to inches Find the relationship(s) between the starting and final units. Write an equivalence statement and a conversion factor for each relationship. Arrange the conversion factor(s) to cancel starting unit and result in goal unit. 19

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**Practice Convert 1.89 km to miles Convert 5.6 lbs to grams**

Find equivalence statement 1mile = km 1.89 km (1 mile/1.609 km) 1.17 miles Convert 5.6 lbs to grams Find equivalence statement 454 grams = 1 lb 5.6 lbs(454 grams/1 lb) 2500 grams Convert 2.3 L to pints Find equivalence statements: 1L = 1.06 qts, 1 qt = 2 pints 2.3 L(1.06 qts/1L)(2 pints/1 qt) 4.9 pints

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**Temperature Conversions**

To find Celsius from Fahrenheit oC = (oF -32)/1.8 To find Fahrenheit from Celsius oF = 1.8(oC) +32 Celsius to Kelvin K = oC + 273 Kelvin to Celsius oC = K – 273

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**Temperature Conversion Examples**

180°C to Kelvin To convert Celsius to Kelvin add 273 = 453 K 23°C to Fahrenheit Use the conversion factor: F = (1.80)C + 32 F = (1.80) F=73.4 or 73°F 87°F to Celsius Use the conversion factor C=5/9(F-32) C = 5/9(87-32) C = … or 31°C 694 K to Celsius To convert K to C, subtract 273 = 421°C

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**Measurements and Calculations**

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Density Density is a physical property of matter representing the mass per unit volume For equal volumes, denser object has larger mass For equal masses, denser object has small volume Solids = g/cm3 Liquids = g/mL Gases = g/L Volume of a solid can be determined by water displacement Density : solids > liquids >>> gases In a heterogeneous mixture, denser object sinks 23

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**Using Density in Calculations**

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**Density Example Problems**

What is the density of a metal with a mass of g whose volume occupies 6.30 cm3? What volume of ethanol (density = g/mL) has a mass of 2.04 lbs? What is the mass (in mg) of a gas that has a density of g/L in a 500. mL container?

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**How could you find your density?**

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**Volume by displacement**

To determine the volume to insert into the density equation, you must find out the difference between the initial volume and the final volume. A student attempting to find the density of copper records a mass of 75.2 g. When the copper is inserted into a graduated cylinder, the volume of the cylinder increases from 50.0 mL to mL. What is the density of the copper in g/mL?

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A student masses a piece of unusually shaped metal and determines the mass to be grams. After placing the metal in a graduated cylinder, the water level rose from 50.0 mL to 60.2 mL. What is the density of the metal? A piece of lead (density = g/cm3) has a mass of g. If a student places the piece of lead in a graduated cylinder, what is the final volume of the graduated cylinder if the initial volume is 10.0 mL?

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Percent Error Percent error – absolute value of the error divided by the accepted value, multiplied by 100%. % error = measured value – accepted value x 100% accepted value Accepted value – correct value based on reliable sources. Experimental (measured) value – value physically measured in the lab.

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Percent Error Example In the lab, you determined the density of ethanol to be g/mL. The accepted density of ethanol is g/mL. What is the percent error? The accepted value for the density of lead is g/cm3. When you experimentally determined the density of a sample of lead, you found that a 85.2 gram sample of lead displaced 7.35 mL of water. What is the percent error in this experiment?

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**Joe measured the boiling point of hexane to be 66. 9 °C**

Joe measured the boiling point of hexane to be 66.9 °C. If the actual boiling point of hexane is 69 °C , what is the percent error? A student calculated the volume of a cube to be cm3. If the true volume is cm3, what is the student’s percent error? Tom used the density of copper and the volume of water displaced to measure the mass of a copper pipe to be g. When he actually weighed the sample, he found a mass of g. What was his percent error?

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