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1 Scientific Method & Measurement Chemistry

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Scientific Method Observation Hypothesis Experiment Theory Law If hypothesis is false, propose new hypothesis. Must repeat several times.

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Models, Laws & Theories model: visual, verbal and/or mathematical explanation of data; can be -tested -used to make predictions theory: explanation based on supported hypothesis -broad principle of nature supported over many years -can be modified -can lead to new conclusions 3

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Models, Laws & Theories law: describes something known to happen without error -doesnt explain why it happens -there are no exceptions -several scientists come to the same conclusion 4

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Data When making observations or gathering information, we separate the data into 2 types: 1. qualitative-uses the 5 senses -physical characteristics 2. quantitative-numerical data -measurable

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Variables There are 2 types of variables when doing an experiment: 1. independent: variable you change 2. dependent: variable that changes due to a change in the independent variable. It is also important to have controls, or standards for comparison.

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7 Significant Figures significant figures: number of all known digits in a measurement plus one estimated digit. -allows more precision in measurement -not all measuring devices show the same precision Example: In the following measurement, what are the known values and what is the estimated value? mL known = 162 estimated = 5

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8 Significant Figures-Rules The easiest way to determine significant figures of a given number is by using the Pacific/Atlantic rules. 1. Decimal point PRESENT, start from the PACIFIC. -Begin counting on the left hand (Pacific) side of the number. Move toward the right and start with the first nonzero number has 7 significant figures has 2 significant figures

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9 Significant Figures-Rules 2. Decimal point ABSENT, start from the ATLANTIC. -Begin counting on the right hand (Atlantic) side of the number. Move toward the left and start with the first nonzero digit has 2 significant figures 1207 has 4 significant figures Zeros that act as placeholders are not significant: and 1200

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10 Significant Figures Practice 1 Determine the number of significant figures in the following numbers. 1) ) ) ) ) 50008) ) 5016) Determine the number of significant figures in the following numbers. 9) ) 2.00x ) ) ) ) ) Copy the following questions and answer.

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11 Significant Figures and Rounding Suppose you are asked to find the density of an object with a m=of g and whose V=14.2 cm 3. Using a calculator, you get , which has 8 significant figures. Does this answer make sense? No. The mass only has 4 sig figs and the volume has 3. Your answer would be more precise than the starting information.

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12 Significant Figures and Rounding How would you correctly round this? By using the starting data with the fewest sig figs (when multiplying/dividing), which is 3: 1.58 g/cm 3 -when adding/subtracting, your answer will have the smallest number of decimal places based on the starting information m m = 6.32 m = 6.3 m

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13 Significant Figures Practice 2 Perform the following operations expressing the answer in the correct number of significant figures. 1) 1.35 m x m 2) 1035 m 2 ÷ 42 m 3) mL mL + 6 mL 4) g – 28.9 g 5) 1.278x10 3 m 2 ÷ x10 2 m Round all numbers to four significant figures. 6) kg8) g10) m 7) g9) mL

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SI Units and Derived Units Unit QuantitySymbolUnitAbbrev. lengthlmeterm massmkilogramkg timetseconds temperatureTKelvinK amount of substance nmolemol electric current IampereA luminous intensity IvIv candelacd 14 The SI base unit is the unit in a system of measurements that is based on an object or event in the physical world.

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Temperature There are three possible temperature scales: 1.Celsius-based on metric system -based on temp when water freezes and boils 2.Kelvin-SI Unit -based on the idea of absolute zero, the lowest possible theoretical temperature -will discuss more in Ch 14 (Gas Laws) 3. Farenheit-what we are used to using 15

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Converting Temperature 1.Celsius to Kelvin / Kelvin to Celcius T K = T C T C = T K Celsius to Farenheit / Farenheit to Celsius T C = (T F -32 o F)5 o C 9 o F T F = T C 9 o F + 32 o F 5 o C 16

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17 SI Units and Derived Units Not all quantities can be measured with base units. derived unit: unit that is defined by a combination of base units -volume: space occupied by an object; unit is the liter, L, for liquids and gases, or cubic centimeter, cm 3, for solids V = l x l x l -density: ratio of the mass of an object to its volume; unit is g/mL or g/cm 3 since 1 mL = 1cm 3 D = m/V

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Prefixes PrefixSymbolMeaningMultiple of Base Unit10 n yotta-Yseptillion1,000,000,000,000,000,000,000, zetta-Zsextillion1,000,000,000,000,000,000, exa-Equintillion1,000,000,000,000,000, peta-Pquadrillion1,000,000,000,000, tera-Ttrillion1,000,000,000, giga-Gbillion1,000,000, mega-Mmillion1,000, kilo-kthousand hecto-hhundred deca-daten base deci-dtenth centi-chundredth milli-mthousandth micro- millionth nano-nbillionth pico-ptrillionth femto-fquadrillionth atto-aquintillionth zepto-zsextillionth yokto-yseptillionth

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Converting Between Prefixes Dimensional analysis is a method of problem-solving that focuses on the units used to describe matter. A conversion factor is a ratio of equivalent values used to express the same quantity in different units. -they change the units of a quantity without changing its value -ratio of units, such as 1 km 1000m -set up so the units you dont need cancel out 48 m x 1 km = km 1000 m 19

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Dimensional Analysis It is common in scientific problems to use dimensional analysis to convert more than one unit at a time. What is the speed of 550 m/s in km/min? 1.Convert m to km 2.Convert s to min 20

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Dimensional Analysis Sometimes we need to convert from metric to standard (and vice versa). -some of these common conversions you will need to know are: 1 cm 3 = 1 mL 60 s = 1 min 1 in = 2.54 cm 60 min = 1 hr 1 ft = 12 in Practice: cm = ____ m s = ____ hr in = ____ ft4. 15 mL = ____ cm 3 21

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22 Accuracy and Precision accuracy: how close a measured value is to an accepted value. precision: how close a series of measurements are to one another. -may not be accurate Example: For the following data, the actual density value is 1.59 g/cm 3. Density collected by Three Students. ABC T1 (g/cm 3 ) T2 (g/cm 3 ) T3 (g/cm 3 ) Avg. (g/cm 3 )

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23 Percent Error percent error: ratio of the difference in the measured value and accepted value divided by the accepted value multiplied by 100 % error = measured value – accepted value x 100 accepted value Ex: Calculate the % error of Student As Average Data. % error = 1.57 g/cm 3 – 1.59 g/cm 3 x g/cm 3 = g/cm 3 x g/cm 3 = 0.02 g/cm 3 x g/cm 3 = 1 %

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24 Accuracy & Precision Practice Density collected by Three Students. ABC T1 (g/cm 3 ) T2 (g/cm 3 ) T3 (g/cm 3 ) Avg. (g/cm 3 ) Calculate the percent error for each of the three students (A, B, C). The accepted value is 1.59 g/cm 3 )

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25 Graphing-You Try Graph the data set A for T1, T2, and T3 using the rules you know. Density collected by Three Students. ABC T1 (g/cm 3 ) T2 (g/cm 3 ) T3 (g/cm 3 ) Avg. (g/cm 3 )

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26 Graphing In chemistry, we mainly deal with line graphs. A graph is used to reveal patterns by giving a visual representation of data. a. must know the independent (x axis) and dependent variable (y axis) b. determine the range of data that needs to be plotted for each axis: try to take up at least ¾ of the paper -use a pencil and ruler c. number and label each axis: dont forget the units d. plot the points and draw a line of best fit -curved or straight e. title the graph

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27 Graphing Practice Complete the problem-solving lab at the bottom of page 44 in your textbook. Answer the Analysis and Thinking Critically questions on the back of the graph.

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29 Scientific Notation Values in science are often very large or very small, requiring a lot of zeros. -ex: the distance between Earth and Neptune is 4,600,000,000,000 m apart and the speed of light is 300,000,000 m/s. this is a lot of zeros to keep track of. Q: What do scientists do? A: they use scientific notation, a short hand method of writing extremely large or small numbers, to make their calculations easier.

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30 scientific notation is a value written as a simple number multiplied by a power of 10. Power of 10 equivalents: 10 4 = 10, = = = = = = = =

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31 Writing Scientific Notation 1. Write the first 2 or 3 digits as a simple number with only one digit to the left of the decimal point. 2. Count the number of decimal places you move the decimal. This will give you your power of 10. -If you move the decimal to the left the power of ten will be positive. -If you move the decimal to the right the power of ten will be negative. 3.If you must adjust the decimal: -if you move it to the left, you add to the exponent -if you move it to the right, you subtract the exponent

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32 Dividing with Scientific Nototation Example Lets calculate the time it takes for light to travel from Neptune to Earth. The speed of light is 3.0x10 8 m/s and the distance from Neptune to Earth is 4.6x10 12 m. -Use the formula v = d/t -Rearrange to solve for t: t = d/v -d = 4.6x10 12 m, v = 3.0x10 8 m/s, t = ? - t = 4.6x10 12 m = 1.5x10 4 s (no adjustment) 3.0x10 8 m/s

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33 Dividing with Scientific Notation Practice You may not use a calculator. Convert the following into scientific notation Convert the following into common form x x x x10 2 Solve the following: x10 5 ÷ 3.0x x10 4 ÷ 5x x10 6 ÷ 4.00x10 3

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34 Multiplying with Scientific Notation Example If it takes 2.7 x seconds for light to travel from one planet to another, how far apart are the planets? Remember light travels at a speed of 3.0 x 10 8 m/s. -Use the formula v = d/t. -Rearrange to solve for d: d = vt -d = ?, v = 3.0 x 10 8 m/s d = vt = (2.7 x10 23 s) (3.0 x 10 8 m/s) = 8.1 x m (no adjustment necessry)

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35 Multiplying with Scientific Notation Practice You may not use a calculator. Review with dividing: 1.1.2x10 3 ÷ 2.4x x10 -3 ÷ 2.3x x10 5 ÷ 2.0x10 2 Multiplying: 4. (1.2x10 3 )(2.4x10 4 ) 5. (4.6x10 -3 )(2.3x10 -5 ) 6. (6.02x10 5 )(2.0x10 2 ) 7. (2.70x10 5 )(3.0x10 -2 )

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36 Cumulative Scientific Notation Practice You may not use a calculator. 1. Write the following measurement in scientific notation. a. 37,500,000,000,000,000,000,000 m b kg 2. Write the following values in long (standard) form a. 4.5 x 10 3 grams b x km 3. Multiply. a. (3.5 x )(2.2 x 10 5 ) b. (7.5 x )(1.2 x ) 4. Divide a. 3.5 x b. 4.6 x x x 10 -7

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37 Combined Measurement Practice Show all work, including units!! Metrics: Convert the following: 1) 35 mL = ____ L2) kg = ____ g Dimensional Analysis: Convert the following: 3) 3500 s = ____ hr4) 4.2 L =_____ cm 3 Scientific Notation: Convert to scientific notation: 5) ) 5057) Scientific Notation: Convert to standard notation: 8) 1.5x10 3 9) 3.35x10 -6 Calculations: using Scientific Notation 10) (1.5 x 10 3 )(3.5x10 5 )11) (3.45x10 -3 )/(1.2x ) 12) (7.6x10 -3 )(8.2x10 7 )13) (6.8x10 7 )/(2.2x10 -5 )

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