The SI units

Scientists all over the world use the SI units to express measurements.

Quantity Unit Symbol

Length meter m

Mass kilogram kg

Time second s

Temperature Kelvin K

Amount of Substance

mole mol

Why SI ?

It is easy to use. It is based on powers of ten.Example:megabytes = 106 byteskilogram = 103 gramscentimeter = 10-2 metermilliliter = 10-3 liters

SI Prefixes

Prefix Symbol Meaning Multiplier

giga G billion 109

mega M million 106

kilo k kilo 103

deci d tenth 10-1

centi c hundredth 10-2

milli m thousandth 10-3

micro µ millionth 10-6

nano n billionth 10-9

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 ofConverting UnitsWhat 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.

1kg1000 g

1000 g1kg

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 ofConverting UnitsWhat 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.

1L1000 mL

1000 mL1L

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. PrecisionCalculate % 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 andaccurate

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 = 1.000 g/mL

Student A:1.003 g/mL Student B: 1.015 g/mL

Which set of measurements is more precise?

A. 2.315 g, 2.317 g, 2.318 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 98.6oC. If the true (literature) value is 100oC, 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 1325oC. What is the % error of this measurement if the literature value is 1338oC?

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.

0.065 g– 2 significant digits Zeros between two non-zero digits are significant.

6.05 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). 12.0 mL

2.) 0.007 L

3.) 15.05 g

4.) 1200 cars

5. ) 500 kg

6.) 0.0305oC

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 cm3

4.) 4.6 g + 11.2 g + 6.15 g =

5. 2.5 g / 0.789 g/mL =

Objective:

What is density?

Density

measure of the “compactness” of a material

A B

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 = densitym = massV = 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 = 0.789 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 0.789 g/mL?

Given: V = 5.0 mLD = 0.789 g/mLm = ?D = m/Vm = DV = 0.789 g/mL x 5.0 mL= 3.9 g

Check for Understanding

Answer-Pair-Share:1. A block of wood has a mass of 23.45 g and

a volume of 20.15 cm3. What is its density?

2. The density of lead is 11.3 g/cm3. What is the volume of 25.0 g of lead?

Objective:

Human vs. Experimental ErrorSystematic 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 calculations2. Sloppiness/Spilling chemicals3. Reading an instrument incorrectly4. Not following procedures5. 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 anywayB. Discard the measurement and redo itC. Include the wrong measurement in

calculating the average of several trials