A. Measurement

Measurements:Data that describe QUANTITATIVE and QUALITATIVE characteristics of matter

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QUANTITATIVE: How much?

• One subject

• 2 tusks

• Weight (or mass)

QUALITATIVE: What is it?

• Gray Elephant

• White tusks

• Wide and large

Measurement: Chemistry Example

Quantitative

Added 25.0 mL ofa solution containing1.0 g of potassium iodide (KI)to 100.0 mL of a test solutionthought to contain lead cations

The precipitate (PbI2) was filtered out, dried, and its mass was 3.7 g. Assuming an excess of KI, this means the test solution contains 1.7 g lead (Pb) cations

Qualitative

The addition of potassiumiodide (KI) to the test solutioncaused a YELLOW precipitateto form.

This suggests the presenceof lead (Pb) cations in the test solution

B.Units

• A UNIT tells what was measured

5.27 …….. WHAT?

meters…now we know what and how much

DERIVED UNITS….these come from multiplying or dividingbase units

Example: I drove 3.2 kilometers from home to work today. It took about 722 seconds to arrive.

3.2 km 722 s

= 0.0044 km s

Derived units

• Remember that base units combined to form derived units provide another property description or characteristic

• What I mean is…. Suppose I– Square a length unit Now I express area

– Cube a length unit Now I express volume

– Divide mass by volume Now I express density

http://www.seed.slb.com/en/scictr/watch/gashydrates/images/cube2.jpg

SI Units

Quantity Symbol Unit Name Unit Abbreviation

length l meter m

mass m kilogram kg

time t second s

temperature T Kelvin K

amount of substance n mole mol

current I ampere amp

luminous intensity Iv candela cd

International System of Units (SI)

Prefix Unit Abbreviati

on

Exponential Factor

Meaning

mega M 106 1,000,000 times

kilo k 103 1,000 times

centi c 10-2 1/100th

milli m 10-3 1/1,000th

micro 10-6 1/1,000,000th

Important Base and Derived units

Quantity Definition Common Unit(s) length

amount of linear space mm, cm, m, km

mass

amount of matter mg, g, kg

volume

amount of 3-dimensional space mL, cm3, L

density

mass/ volume g/ mL, g/ cm3, g/ L

temperature

average KE of particles C, K

amount

# of particles mol

pressure

force/ area mm Hg, atm, kPa

concentration

amount/ volume or mass/ volume %, M, m

C. Using Scientific Measurements

Precision and Accuracy

PreciseAccurate

PreciseNot Accurate

Not PreciseNot Accurate

Not PreciseAccurate

There are methods to quantify HOW accurate and HOW precise…

II. Measurements and the Characteristics of Numbers

• Significant Figures-digits with experimental meaning. All digits in a measurement are CERTAIN except the last which is understood to be UNCERTAIN or estimated

57.2574

CERTAIN UNCERTAIN

• Rules

0.00100050300

•Zeros that are place holders are NOTsignificant

• Zeros between non-zero digits AREsignificant

• Zeros at the END of a number are significantIF the number has a decimal point

ZEROS

100050300

Zeros NOTsignificant

Numbers: Rounding and Reporting

78,200.9834

Round to 3 significant digits: 78,200

Round to 5 significant digits: 78,201

Round to 1 significant digits: 80,000

Round to 7 significant digits: 78,200.98

Reporting:

This burette is marked in 0.1 mLincrements.

How many significant digits AFTERThe decimal point would you report?

Concave meniscus

Math Operations with Sig Figs

Multiplication and Division of numbers:The number of SIG FIGS in an answer should be reported with the least number of significant digits in any one of the numbers being multiplied, divided etc.

37.2872x 45.3________ 1690 (Ouch! Seems harsh but those 6 SIG FIGS in the first number were ”killed” by the 3 SIG FIGS in the second number)

Addition and Subtraction of numbers:The number of decimal places (not SIG FIGS ) in the answer should be thesame as the least number of decimal places in any of the numbers being added or subtracted. 23.3456+3.3_______26.6 (3 SIG FIGS total but only 1 behind the decimal place)

% ERROR

% error = | true value – expt value| x 100%

true value

AKA “accepted value”

Average experimental valueAbsolute value

% error is a method of expressing the accuracy of the measurement

By itself, it doesn’t say anything about the precision of multiple trials

Scientific Notation

• Why?– Would you rather write this: 6.023 x 1023

– Or THIS: 602,300,000,000,000,000,000,000

• FORMAT

M x 10n

1 ≤ M < 10AND with the propernumber of significantdigits; can be (-) or (+)

Base 10 number

Exponent is a whole numberinteger; can be (-) or (+)

Proportions-Relationship of Variables

A is directly proportional to B

A Bwhen

A = kB

A is inversely proportional to C

A Cwhen

A Bwhen

A Cwhen

A

B

A

C

A = k/C

Quotient of A andB is constant

Product of A andC is constant