Chapter 2Measurements in Chemistry

Chemistry 2A

Data

• Qualitative– Data obtained from

one’s opinion

– Does not involve numbers

• Quantitative– Data obtained from

measurements

– Involves numbers

U.S. Customary System

• Also called:– American System– English System

• Inch

• Gallon

• Pound

• Teaspoon

Metric System

• Système International (SI)

• International decimalized system of measurement

• First adopted by France in 1791

• Meter

• Gram

• Liter

Length

• How long something is, DUH!

• SI unit = meter (m)

Mass• Measure of the quantity of matter (stuff) in

an object

• SI unit = Kilogram (kg)

Volume

• The amount of space that an object or substance occupies.

• SI unit = Cubic meter (m3)

• 1 L = 0.001 m3

• 1 L = 1000 mL

• 1 mL = 1 cm3 = 1 cc

Time

• Duration of event

• SI unit = Second (s)

French Revolutionary Clock

System International (SI) Units

Prefix Symbol Multiple Example

Giga G 109 Gigabyte = Gbyte

Mega M 106

Kilo k 103 = 1000 Kilogram = kg

Hecto h 102 = 100

Deka da 101 = 10

No prefix

(Unity)

100 Meter, liter, gram = m, L, g

Deci d 10-1 = 0.1

Centi c 10-2 = 0.01

Milli m 10-3 = 0.001 Milliliter = mL

Micro μ 10-6

Nano n 10-9 Nanometer = nm

Pico p 10-12

Common Units and Their Equivalents

Length

1 kilometer (km) = 0.6214 mile (mi)

1 meter (m) = 39.37 inches (in.)

1 meter (m) = 1.094 yards (yd)

1 foot (ft) = 30.48 centimeters (cm)

1 inch (in.) = 2.54 centimeters (cm) exactly

Common Units and Their Equivalents

Volume

1 liter (L) = 1000 milliliters (mL)

1 liter (L) = 1000 cubic centimeters (cm3)

1 liter (L) = 1.057 quarts (qt)

1 U.S. gallon (gal) = 3.785 liters (L)

Mass

1 kilogram (km) = 2.205 pounds (lb)

1 pound (lb) = 453.59 grams (g)

1 ounce (oz) = 28.35 grams (g)

Problems

1) Green light has a wavelength of approximately 550 nm. What is this value in meters? Picometers? Kilometers?

2) Your neighbor lost 50 pounds after having a baby. How many kg did she lose? How many micrograms?

3) How many milliseconds in a year?

Dimensional Analysis• Using units as a guide to problem solving

is called dimensional analysis

• Figure out which unit you want to start with and which one you want to get to

• Use conversion factors to get there

– Relationship between two units

– May be exact or measured

– Generated from equivalence statements

• Always include units in your calculations!

12 eggs = 1 dozen

Temperature• A measure of the average kinetic

energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale.

• A physical property that determines the direction of heat flow in an object upon contact with another object.

• Fahrenheit (°F), Celsius (°C), Kelvin (K)

Fahrenheit (ºF), Celsius (ºC), Kelvin (K)

• ºF = ºC(1.8) + 32• ºC = (ºF – 32)/1.8• K = ºC + 273• ºC = K – 273

Lord William Thomas Kelvin

Problems

1) If it’s 35ºC in London, would you say that it’s probably winter or summer? What is this temperature in Kelvin?

2) You are feeling sick and decide to take your temperature. Your thermometer, which only reads temps in Kelvin, says that you are at approximately 312 K. Do you have a fever?

Density• The ratio of the mass of an object to its

volume

Mercury

13.6 g/cm13.6 g/cm33

8.94 g/cc8.94 g/cc

Water

1.0 g/mL

Problems

1) Calculate the density of the rock in the picture to the right. The rock has a mass of 29.5g.

2) What is the mass of 5.5mL of mercury if Hg has a density of 13.53 g/mL?

3) Calculate the height of the piece of wood to the right. Oregon Pine d = 0.53 g/mL

Scientific Notation1) Locate the decimal point2) Move the decimal so that there is only one

number to the left of it3) Write “x 10” behind you new number4) Count the number of places you’ve moved your

decimal point and make this number the exponent on your 10

5) Assign a + or – sign to your exponenta) If your original # is larger than your SN #, the

exponent is +b) If your original # is smaller than your SN #, the

exponent is –

Problems

1) 2522) 3428883) 0.00474) 0.0000085) 3.33 x 102

6) 4 x 10-4

7) 35000

Write the following standard numbers in scientific notation and write the numbers in scientific notation in standard form.

Significant Figures

• 3

• 3.0

• 3.00

• Scientific measurements are reported so that every digit is certain except the last, which is estimated

Certain Uncertain

Rules for Significant Figures

1) Numbers up to and including the “uncertain” number are significant

2) All non-zero numbers are significant

3) Zeros may or may not be significant

4) Zeros are significant ifa) They are between two non-zero digits

b) They are at the end of a decimal number

5) Zeros are not significant ifa) They are used as place holders in large

numbers without a decimal point

b) They are at the beginning of decimal numbers

6) All numbers displayed in a number written in scientific notation are significant

Problems

1) 45

2) 45.0

3) 405

4) 4050

5) 4050.

6) 0.0004057) 0.000405008) 900.009) 4.0 x 103 10) 3 x 108

Identify the correct number of significant digits in the figures below.

Mt. Everest29000 ft, 2.9000 x 104 ft., or 29002 ft?

Calculation With Significant Digits

• Multiplication and Division– The final answer has the same number of sig figs

as the measurement with the fewest sig figs

– Example 1:

• 22.2 cm x 3.4654 cm = ?

– Example 2:

• 0.0009 mm / 0.340 mm = ?

• Addition and Subtraction– The final answer is written so that it has the

same number of decimal places as the measurement having the fewest decimal places

– Example 1: • 44.4 L + 2.3967 L + 0.000002 L = ?

– Example 2:• 4107 in – 608.7 in = ?

Problems

1) 200 • 3.44 • 9.30

2) 0.00309 / 4.4 x 103 • 999

3) 24.464 – 10.63 – 2.2

4) 0.000800 + 909 + 3.6

Precision and Accuracy

• Precision: how well several determinations of the same measurement agree– Reproducibility/repeatability

• Accuracy: agreement of a measurement with the accepted value

Determine whether the following students exhibit good or poor accuracy and precision

Exam 1 Exam 2Accuracy &

Precision

Student A 99% 100%

Student B 100% 89%

Student C 59% 59%

Student D 25% 49%