measurements and calculations world of chemistry
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Measurements and Calculations
World of Chemistry
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• Quantitative observations like measurements must have units
• Measurements can be very small or very large• Example: – Distance from Earth to Sun = 93,000,000 miles– Average size of eukaryotic cell: 0.0000062 meters
• For very large and small measurements, use scientific notation
Numbers and measurements
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• In writing in scientific notation, you will1) Make the number between 1 and 10 (move the
decimal)2) Determine the power of 10 - if the regular number is >1, move decimal to the right, positive exponent- if the regular number is <1, move decimal to left, negative exponent
• Example1) 93,000,000 miles = 9.3 x 106 miles2) 0.0000062 metes = 6.2 x 10-6 meters
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Put the following numbers into standard scientific notation:
1) 238,000 =2) 1,500,000 = 3) 24.7 =4) 0.135 = 5) 0.0024 =
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Units
• All measurements must have units• USA – English system, most international
countries – metric system• Science community – International System (SI) • when it comes to scale, use prefixes
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SI Units for chemistryQuantity Unit Abbreviation Symbol
Mass Kilogram kg m
Time Second s t
Temperature Kelvin K T
Amount of substance
Mole mol n
Although not included, volume is measured in liters (L) and represented by V.
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Unit PrefixesPrefix Symbol Power of 10
Mega M 106
kilo k 103
centi c 10-2
milli m 10-3
micro υ 10-6
nano n 10-9
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Unit prefixes
Prefixes can be used to replace powers of 10 in scientific notationExamples:– 5000 m = 5 x 103 m = 5 km– 0.00315 L = 3.15 x 10-3 L = 3.15mL– 0.00000000465 s = 4.65 x 10-9 s = 4.65 ns – 33000 m = 33 km– 0.00000056 L = 0.56υL (microliters)
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Practice Problems
Convert the following1) 382 g = _____kg2) 0.0056g = _____mg3) 490 mL = _____L4) 6,560,000 m = _____Mm (Megameters)5) 99 Mg = ____________g6) 8.8υg = ______g
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Measurement Uncertainty
• Measuring anything (especially mass, volume, length):
certain and uncertain numbers
meter stick
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Uncertainty in measurements4.75 cm
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Measurement Uncertainty
• Determine the “certain” numbers of the measurement• Meter stick broken down into centimeters and millimeters
4.7 cm• Determine “uncertain” numbers (estimate)– value between millimeters– 4.75 cm
• Every measuring device (ruler, graduated cylinder, balance) has some degree of uncertainty…except for digital measuring devices
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Significant Figures
What counts as sig fig?1) All regular integers (1457 = 4
sig figs)2) Trapped zeros (12059 = 5 sig
figs)3) Trailing zeros (only if there is a
decimal involved)
100. 3 sig figs
78.0 = 3 sig figs
100 = 1 sig figWhat does not count as sig fig?Leading zeros (0.00025 = 2 sig figs)
In a measurement, specific numbers are considered significant figures
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Practice ProblemsDetermine the number of sig figs:
1) 0.000304g2) 1.270 x 102m.3) 125g 4) 10 A5) 0.09020L6) 6.5 x 103g7) 6.5mg8) 9.02 g9) 10.0 mL10) 21.40 s
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Calculating using Sig Figs1) Multiplication/Division: # of sig figs in answer is the same as the measurement with smallest # of sig figs
Example: 4.56 x 1.4 = 6.384 = 6.4
3 sig figs
2 sig figs
Round off 2 sig figs
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Calculating using Sig FigsExample: 8.315 ÷ 298 = 0.0279027 =
0.0279
4 sig figs
3 sig figs
Round off
3 sig figs
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Calculating using Sig FigsExamples5.4 / 3 =2.5 x 5.230 =(8.62 x 103) / (33) =(0.54 / 6.4) x 100.00 =
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Calculating using Sig Figs2) Addition/subtraction: sig figs in answer determined by measurement with smallest number of decimal places
Example: 12.11 + 18.0 + 1.013 = 31.123 31.1
1 decimal place
1 decimal place
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Calculating using Sig Figs
Example: 0.6875 – 0.1 = 0.5875 0.6
1 decimal place
1 decimal place
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Calculations Examples• Multiplication and division – count sig figs• Addition and subtraction – count decimal places1) 8.90 + 63.45 =
2) 103.4 – 34.94 =
3) (1945)(17.1) =
4) 32.789 / 270 =
5) (13.3 x 105)(45.45) =
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Dimensional Analysis• Dimensional analysis is used to convert units
Ex: g mol g L • Use conversion factors as bridges• Examples of conversion factors include..
1 lb = 453g1 in = 2.54cm1 mol carbon = 12.01g
• Use railroad method to cancel out units
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Dimensional Analysis
Convert the following
12 ft into cm
3.2 L into ounces
32.0 ft in km
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5.7 Temperature• Temperature is the measure of heat • Three scales:– Celsius (°C)– Fahrenheit (°F)– Kelvin (K)
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Celsius• Anders Celsius• Original 1742 “Centigrade” scale– 100°C water boils– 0°C water freezes
• 100 degree scale
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Fahrenheit• Daniel Gabriel Fahrenheit• Scale based on three fixed points:– “Brine” mixture: water, ice, ammonium chloride =
0°F– Water and ice mixture = 32°F– Human body = 96°F
• 180 degree scale– 32°F water freezes– 212°F water boils
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Kelvin• William Thomson (Lord Kelvin)• SI unit• 273 K water freezes• 373 K water boils• 100 degree scale• Absolute zero = 0 K– all molecular motion ceases– has never been reached
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Temperature Conversion Equations°C to °F T°F = (1.8)T°C + 32
°F to °C T°C = (T°F - 32) x (0.56)
K to °C TK =T°C + 273
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5.8 Density• Physical property, specific for a pure substance• Ratio of mass and volume• Density = mass
volume • Volume units– solids cm3
– liquid mL– gas L
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World of Chemistry, pg. 143
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Determining DensityExample:Mass = 55.64 gVolume = 10.0 mL
Density = 55.64g = 5.56 g/mL 10.0mL