The SI units Scientists all over the world use the SI units to express measurements. QuantityUnitSymbol Lengthmeterm Masskilogramkg Timeseconds TemperatureKelvinK Amount of Substancemolemol

Why SI ?  It is easy to use.  It is based on powers of ten. Example: megabytes = 10 6 bytes kilogram = 10 3 grams centimeter = meter milliliter = liters

SI Prefixes PrefixSymbolMeaningMultiplier gigaGbillion10 9 megaMmillion10 6 kilok 10 3 decidtenth10 -1 centichundredth10 -2 millimthousandth10 -3 microµmillionth10 -6 nanonbillionth10 -9

Check for Understanding Answer-Pair-Share Which SI unit will you use to express each measurement? 1) volume of water in a graduated cylinder 2.) mass of a spoonful of salt 3.) mass of a sack of rice 4.) temperature of cold water 5.) time it takes a marble to roll down a ramp 6.) density of a wooden cube

Factor Label Method of Converting Units What is 0.5 kg in grams? Step 1: Write the relationship between the two units. Step 2: Write the possible conversion factors. Step 3: Multiply the quantity by the correct conversion factor. 1kg 1000 g 1kg 1 kg = 1000 g 0.5 kg x 1 kg = doesn’t work! 1000 g 0.5 kg x 1000 g = 500 g 1kg

Factor Label Method of Converting Units What is 50 mL in L? Step 1: Write the relationship between the two units. Step 2: Write the possible conversion factors. Step 3: Multiply the quantity by the correct conversion factor. 1L 1000 mL 1L 1 L = 1000 mL 50 mL x 1 L = 0.05 L 1000mL 50 mL x 1000 mL = doesn’t work! 1 L

Check for Understanding Answer-Pair-Share Do the following conversions. Show your work. 1) 0.75 mL = ________L 2.) 2.0 m = ________ mm 3.) 2000 ms = ________ s 4.) 3.5 g = ________ cg 5.) 0.25 kg = _________ mg

Objective: Accuracy vs. Precision Calculate % error.

Read p. 34, paragraphs 1-3. Find out what accuracy and precision mean. Accuracy vs. Precision

Accuracy and Precision Precise but inaccurate Imprecise but accurate Precise and accurate Accuracy – refers to how close a measurement is to the true or literature value Precision – refers to how close measurements are to each other

Whose measurement is more accurate? True value = g/mL Student A:1.003 g/mLStudent B: g/mL Which set of measurements is more precise? A g, g, g B.2.32 g, 2.33g, 2.31 g

Check for Understanding Think-Pair-Share Accuracy or Precision? 1. May be determined by comparing a measured value to the true (literature) value. 2. May be determined by comparing several measurements.

Percent Error  expresses the accuracy of a measurement % error = /measured value – literature value/ x 100 literature value The boiling point of water was measured to be 98.6 o C. If the true (literature) value is 100 o C, what is the percent error? % error = /98.6 – 100/ x 100 = 1.4 % 100

Check for Understanding Answer-Pair-Share The melting point of gold was measured to be 1325 o C. What is the % error of this measurement if the literature value is 1338 o C?

Significant Digits  Significant digits are used to express how precise measurements are.  The number of significant digits depends on the kind of measuring device used.  Significant digits include all the certain digits and 1 uncertain digit in a measurement.

Counting Significant Digits  Non-zero digits are significant. 65 g – 2 significant digits  Zeros after a decimal point but before a non-zero digit are not significant g– 2 significant digits  Zeros between two non-zero digits are significant g – 3 significant digits  Zeros after a decimal point and a non-zero digit are significant 65.0 g – 3 significant digits 650 g – 2 significant digits

Check for Understanding Answer-Pair-Share Tell the number of significant digits: 1) mL 2.) L 3.) g 4.) 1200 cars 5. ) 500 kg 6.) o C

Counting Significant Digits  A calculated value cannot be more precise than the measurement from which it is based.  Example: 5.0 mL x 1.25 g/mL =  What is the best way to record the answer?  6.25 g or 6.3 g or 6 g?  Rules to remember: 1. When multiplying or dividing, the answer should have the least number of significant digits. 2. When adding or subtracting, the answer should have the least number of decimal places.

Check for Understanding Answer-Pair-Share Perform the following operations and express answer in correct significant digits. 1) 15.2 g – 3.50 g = 2.) 1.0 g/mL x 9.00 mL = 3.) 5.0 g / 2.50 cm 3 4.) 4.6 g g g = g / g/mL =

Objective: What is density?

Density  measure of the “compactness” of a material AB Which material is more dense?

Uses of Density Data  Identification of unknown substances  Calculation of molecular mass of substances  Explains floating/sinking of object

Calculating Density  amount of mass in a given space  D = m/V D = density m = mass V = Volume What is the density of ethanol if 10.0 mL of this liquid has a mass of 7.89 g? D = 7.89g /10.0mL = g/mL

Problem Solving  Identify the given information  Identify what is asked for  Develop possible solutions  Analyze the solutions and choose the correct one  Develop the steps to arrive at the answer  Solve the problem  Evaluate the result Where am I? What paths will I take? Which path is most likely the correct one? Plan the trip. Travel along the selected path. Did I reach the place I expected? Where do I want to be?

Density What is the mass of 5.0 mL of ethanol if its density is g/mL? Given: V = 5.0 mL D = g/mL m = ? D = m/V m = DV = g/mL x 5.0 mL = 3.9 g

Check for Understanding Answer-Pair-Share: 1. A block of wood has a mass of g and a volume of cm 3. What is its density? 2. The density of lead is 11.3 g/cm 3. What is the volume of 25.0 g of lead?

Objective: Human vs. Experimental Error Systematic vs. Random Error How can we eliminate/minimize experimental errors?

Experimental Errors “All experimental data is imperfect”. Types of Experimental Errors: 1. Random 2. Systematic

Random vs. Systematic Random - cause: unpredictable/uncontrollable factors - cannot be eliminated - can be minimized by averaging - Effect: data may be higher or lower Systematic - cause: faulty experimental design or measuring device - can be eliminated by changing the experimental design/measuring device - can not be minimized by averaging - Effect: data is consistently higher or lower

Determining the Mass of Alcohol Materials: digital electronic balance that reads up to 0.01 g 100 mL graduated cylinder, marked by 1 mL alcohol Procedure: 1. Find and record the mass of an empty graduated cylinder. 2. Fill the cylinder about ¾ full of alcohol. Record the volume. 3. Get the mass of the filled graduated cylinder Example

Random - Wind disturbs the balance causing the readings to fluctuate - Eye level of the experimenter moves a bit while reading the volume Systematic - Electronic balance is not working properly (not calibrated). - Some of the alcohol is lost (evaporates) as its mass is being read.

Check for Understanding Answer-Pair-Share: Random or Systematic? 1. may be minimized by averaging 2. may be eliminated by changing the experimental design

Human Errors - mistakes or blunders - may be avoided by careful experimentation - should not be included in a lab report - Examples: 1. Wrong calculations 2. Sloppiness/Spilling chemicals 3. Reading an instrument incorrectly 4. Not following procedures 5. Using wrong chemical

Check for Understanding Answer-Pair-Share: What should you do once you have realized you have made a “mistake” or human error in your measurement? A. Report the data anyway B. Discard the measurement and redo it C. Include the wrong measurement in calculating the average of several trials