Presentation on theme: "Scientific Measurement. Measurements Quantities with both a number and a unit – Used to measure a physical property –.5 grams Mass – 2 liters Volume –"— Presentation transcript:
Measurements Quantities with both a number and a unit – Used to measure a physical property –.5 grams Mass – 2 liters Volume – 1.6 meters Length
Why does this matter? In chemistry, we make lots of measurements – Examples? We need to have a common language of measurement so that we can understand each other’s results.
Scientific Notation In chemistry, we deal with numbers that are very big and very small – How many water molecules are in the beaker? 600,000,000,000,000,000,000,000 – What is the width of a water molecule? meters
Scientific Notation How can we express big and small numbers in a simpler way? – Powers of = = = = = = ,000 = = – Exponent = # of places you move the decimal point to get behind the first digit Left = positive Right = negative
Practice problem Express the following numbers in exponential form: – 100,000 – 100 – –.1 – 1
Scientific notation To put a number in scientific notation, express it as the product of – A number between 1 and 10 – A power of 10 Example: 5000 meters = 5 x 1000 meters = 5 x 10 3 meters.025 liters = 2.5 x.01 liters = 2.5 x liters
Practice problems Express the following numbers in scientific notation: – 5280 feet – grams – 602,000,000,000,000,000,000,000 atoms
Accuracy v. Precision Accuracy = how close your measurements are to the correct value – Ex: I can run a mile in 7 minutes If you measure my mile time and get 7 minutes, you are highly accurate If you measure my time and get 8 minutes you are inaccurate
Precision Precision = how close your measurements are to each other – If you measure my mile time and get 7 minutes three times, you are very precise – If you measure my mile time and get 8 minutes three times, you are still very precise But you are not very accurate.
Accuracy v. Precision
Error Error is the difference between your measurement and the accepted value Error = experimental value – accepted value
Practice problem Calculate the error: – Experimental value = 1 meter Accepted value = 1.02 meters – Experimental value = 10.7 seconds Accepted value = 10.5 seconds
Percent Error How much error is a lot of error? – If my measurement of the weight of a car is 10 lbs too much, do I care? – If my measurement of the weight of a baby is 10 lbs too much, do I care?
Percent error practice Mr. Tunney’s new car weighs 2795 lbs. The dealership weighs the car at 2805 lbs. – What is the error? What is the percent error? When Mr. Tunney was born, he weighed 9.5 lbs. The doctor weighed him at 19.5 lbs. – What is the error? What is the percent error?
Percent error practice Felipe sticks a thermometer in a pot of boiling water, and measures its temperature as 99.1°C. The true temperature of boiling water is 100°C. – What is Felipe’s error? What is his percent error?
Significant Figures The accuracy of our measurements is limited by the accuracy of our tools What’s the smallest increment marked on this scale?
Significant Figures When making measurements, you can record as many digits as your tool measures, and you can estimate one more digit. All of these digits have useful information, so they are called significant figures
Significant figures? Can I measure this length as cm? Why/why not? How many significant figures should this measurement have?
Significant figure rules How do I know how many significant figures a given measurement has? – Example: I tell you I am meters tall. How many significant figures are there? Rules: – All nonzero digits are significant – All zeroes between nonzero digits are significant – Leftmost zeroes are not significant – Rightmost zeroes are significant if they follow a decimal point or are followed by a decimal point
Significant Figures Rules All nonzero digits are significant – How many significant figures: – 220 – 1000 – 345
Significant figure rules All zeroes between nonzero digits are significant – How many sig. figs? – 202 – 1050 – 10,002
Significant Figures Rules Leftmost zeroes are not significant – How many sig. figs? – 100 – 0245 – – 1.01
Homework 9/30 13) Error = -1.6°C, Percent error = 1.3% 14) a. unlimited b. 5 c. 3 d. 3 15) 6.6 x 10 4 b. 4.0 x c d x e. 1.9 x 10 14
Significant Figures Rules Zeroes at the end of a number to the right of a decimal point are significant – How many sig. figs? – 1.00 – 0.01 –
Zeroes at the end of a number are also significant if they are followed by a decimal point – How many sig figs? – – –
Significant Figures Rules Countable or standard measurements have infinite sig. figs – How many sig. figs? – 4 apples – 60 seconds/minute
Significant Figures in Calculations In general, a calculated answer cannot be more precise than the least precise measurement from which it was calculated.
Addition and Subtraction Round your answer to the largest digit that is the least significant digit of one of your measurements – Example: Answer = 369.7
Multiplication and Division Round your answer to the same number of sig. figs as the term with the least number of sig. figs 175 m x 0.1 m = 17.5 m Answer = 20 m
The Metric System The metric system is an international system of units designed to make measurements simple and easy – The five metric units we’ll use in class are Meters Grams Kelvins Seconds Moles
Meters and Seconds Meters (m) are the metric unit of distance – 1 meter ≈ 3.3 feet Seconds (s) are the metric unit of time
Grams Grams (g) are the metric unit of mass – A paperclip weighs about 1 gram
Kelvins A kelvin (K) is the metric degree of temperature – 1 degree Kelvin = 1 degree Celsius – BUT the Kelvin scale starts at the lowest temperature possible, -273°C – To convert from Celsius to Kelvin, subtract 273.
Moles A mole is the metric unit of quantity – 1 mole = 6.02 x – Very large, usually used for talking about how many atoms or molecules are in an object.
Metric prefixes We can use a standard set of prefixes to make the scale of metric units more convenient – Mega = 1 million times the unit – Kilo = 1 thousand times the unit – Hecto = 100 times the unit – Deka = 10 times the unit – Deci = 1/10 of the unit – Centi = 1/100 of the unit – Milli = 1/1000 of the unit – Micro = of the unit – Nano = of the unit – Pico = of the unit
Volume Volume is the amount of space an object occupies. Volume = length x width x height The most common units we’ll use are a cubic centimeter (cm 3 ) and a liter (L) – A liter equals 1000 cm 3
Energy Energy is the capacity to do work or produce heat – The Joule (J) is the metric unit of energy. – The calorie is another (non-metric) unit of energy – 1 calorie = 4.18 Joules
Homework 10/3 21) a. 1/1000 or b c. 1/10 or d. 1/100 or ) m 3, L, dL, cL, mL, μL 23) 8.8 x 10 2 cm 3 25) C = K – ) 443 K 41) 1 hour/60 min b) 10 3 mg/1g c) 10 3 mL/1dm 3 42) 1.48 x 10 7 micrograms b) 3.72 g c) 6.63 x 10 4 cm 3
Practice problems How many: – Meters in a kilometer? – Centigrams in a gram? – Milliliters in a liter? – Microkelvins in a kelvin? – Joules in a Megajoule? – Nanometers in a meter? – Moles in a hectomole?
Conversion Problems A quantity can usually be expressed in several different ways – 1 dollar = 4 quarters = 10 dimes = 20 nickels The same is true of scientific quantities – 1 meter = 10 decimeters = 100 centimeters How do we convert from one unit to another?
Practice problems Convert using conversion factors: 1)750 milliliters to liters 2)15 centimeters to meters 3)1.5 decimeters to meters 4)750 kilojoules to Megajoules 5)5 nanograms to grams 6)4.5 kilokelvins to centikelvins
Dimensional Analysis Solve the following problems with conversion factors: How many seconds are in 8 hours? How many inches are in a mile? How many Joules are in 200 calories?
Density Use your intuition – What is more dense, a bucket of pennies or a bucket of feathers? – What is more dense, a block of wood or a block of iron? – What does density mean?
Practice problem You have four blocks of different elements. Use a ruler and a balance to calculate the density of each.
Buoyancy Density determines whether an object will sink or float in a fluid. – More dense sink – Less dense float Is wood more or less dense than water? Is a rock more or less dense than water? Is a person more or less dense than water?