Plan

• Dimensional Analysis ideas (continued)]– Online resource with examples:

http://www.alysion.org/dimensional/fun.htm

• Classification of Matter– Operational– Conceptual (Atomic Theory / Nanoscopic pics)– Chemical vs. Physical changes

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Try these

• If the price of apples is 25 cents each, how much will 5 apples cost?

• If DVD’s cost 20 dollars each, and you have 100 dollars, how many DVD’s can you buy (don’t worry about tax!)

• If you paid $31.04 to put 11.8 gallons of gas into your car, what was the price of gas? (Did you get a good deal?)

2

Try these

• If the density of Al is 2.7 g/mL, how much will a sample of 5.0 mL of Al “weigh”?

• If a substance has a molar mass of 20. g/mol, and you have 100. g of this substance, how many moles of it do you have?

• If 31.04 moles of NaCl is present in 11.8 L of solution, what is the concentration of NaCl in the solution (in units of moles/L)?

3

Assertion

• The two sets of problems above are fundamentally the SAME! – The second set is (much!) harder for you right now

because the quantities involved are not as familiar to you.

• Although we have discussed “dimensional analysis” (factor-label method) as a means of solving problems such as these, PLEASE recognize that you should keep striving for understanding – so that eventually, the second set of problems will be

as easy for you as the first set!

4

Dimensional Analysis Principles (review)

• Units are treated like a algebraic variables during calculations (use this to check your result!)

• You can do unit-conversion type calculations by starting with one quantity and then multiplying it by one or more “factors” (which I call “this-for-that”s) to get the units to “work out” (some units appear and other ones cancel out) Make absolutely sure that the “factors” you

construct are correct!5

See Chalkboard Presentation

• Make Conversion factors VISUAL

• Give them MEANING– 1 ft = 12 in– 1 km = 0.62137 mi– 1 J = 9.4781 x 10-4 BTU

• Which is bigger: a foot or an inch?

• a J or a BTU?

• a km or a mi?

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Conversion Factors

• If 12 in = 1 ft, then

– 12 inches needed to equal ONE foot, so a foot is larger

– How many times larger? 12 times larger– “feet” are the “big guys” and “inches” are the

little guys.

1 ft

1 in

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Dimensional Analysis can be used to do calculations that are not true “conversions”

• Use density (g/mL) with grams to calculate “mL”

• Use “mi/gal” (mpg) with “gallons” to calculate “miles” (how far you can go)

• These are not strictly “conversions” because the type of physical quantity (i.e., length, mass, etc.) is changing– But principle is the same

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Helpful Hints• If you are ASKED for an “amount”, then START with an

amount and multiply by conversion factors (“this for thats”) until you reach your goal

• If you are ASKED for a “this for that”, then START with one (and multiply by this for thats until you reach goal)

• Make absolutely certain that your “conversion factors” are correct! – You cannot make up “random” factors just because that what

appears to be needed to cancel things out!

• Check to see if your answer makes reasonable sense! (Who are the “big guys”/”little guys”?)

• Online resource with examples: http://www.alysion.org/dimensional/fun.htm

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Example—Price of Gas in Poland?

• What is the price in US dollars (USD) per gallon?

• Exchange rate:3.17 zł = 1 USD

(8/28/13)

• 1 gal = 3.785 L

• 1 dm3 = 1 L

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Classification of Matter

• NOTE: Tro discusses classification of matter in Ch. 1 blending theory with law/observation!– This assumes some info from Ch. 2

• I will take an approach that highlights the difference between the “what happens?” and “why it happens” realms.

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Summary of Classification of MatterNOTE: This figure is adapted from another text (it is similar to the chart on Tro, p. 7)

Monatomic Molecular Molecular Ionic

(later)

can be separated by physical means into

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Classification of Matter

• What is chemical change?– A process in which certain substances are

converted into NEW SUBSTANCES• If NEW substances are made, then chemistry

(chemical change) occurred

– What is a “substance”?• Not just any old thing!! In science, very specifically

defined

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Two kinds of definition

• “Operational”– Based on observations– Practical– in the real world (lab)– “I know it when I see

it”– (How things behave)

• “Conceptual”– Uses theory/model– It’s in your head– (Why things behave

the way they do)

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Example: Substance

• Operational– A type of matter which cannot be separated (turned)

into new substances physically (by physical method)*– A type of matter which has a fixed composition and

specific properties

• Physical method (separation): process that does not result in making new substances!e.g., Breaking, grinding, boiling, evaporating,

freezing, mixing (pictures later)

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Two Kinds of (Pure) Substance

• One kind can be separated chemically (energy added, away from other substances) into new substances– “Compound”

• One kind cannot be separated chemically into new substances– “Element”

• The “substances” that a compound can be “separated into” are “elements”!

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Law of Definite Proportion (same as “fixed composition”)

• See overhead (now next slide)

• Applies to any compound

• No matter where a sample of carbon dioxide comes from or how much you start with, when it is chemically “separated” into carbon and oxygen (elements), the ratio of mass of C to O is always the same.

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Reminder (Data illustrating the Law of Definite Proportion)

For a compound (red powder, here):

The (best estimate for the) mass ratio of Hg : O is 12.6 (i.e., 12.6 g Hg / 1 g O) in any sample of this compound.

Mass ratio of Hg : O 12.568… 12.6101… 12.575

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Why? Atomic Hypotheses (Postulates) (Dalton, 1808; http://en.wikipedia.org/wiki/Atomic_theory)

• Assume a sample of an element is composed of only one kind of “atom” (nanoscopic entity)– All C atoms are identical; All O atoms are identical

(But a C atom has a different mass than an O atom)

• Assume a sample of a compound is composed of more than one kind of atom (in a fixed ratio)

• Assume atoms remain unaltered during chemical change; Atoms merely REARRANGE

• If these are all so, a fixed mass ratio upon separation follows logically / can be “derived” mathematically (i.e., this model explains the Law of Definite Proportion) 19

Dalton, 1808; http://upload.wikimedia.org/wikipedia/commons/3/39/Daltons_symbols.gif

20

Example: Substance

• Operational– A type of matter which

cannot be separated (turned) into new substances by physical means

– A type of matter which has a fixed composition and specific properties

• Conceptual– A type of matter

composed of identical nanoscopic entities (or a fixed ratio of entities):

• Atoms• Molecules• Ions (fixed ratio of)

– Definition is meaningless without Atomic Theory!!

21

Example: Physical States

• Operational– Solid: (each piece)

has fixed shape and fixed volume

– Liquid: has fixed volume, but no fixed shape

– Gas: assumes the volume and shape of the container it is in (neither fixed)

• Conceptual– Solid: basic

nanoscopic units (entities) are “touching” and not free to move around

– Liquid: basic “entities” are “touching” but are free to move around

– Gas: basic ‘entities” are separate from one another (spread out)

22

Tro, p. 6. Physical States (of a substance made up only of atoms)

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See Handout For More Examples

• (handout has been incorporated into PowerPoint presentation [next slides]; you may wish to print out or spend some time studying the next slide)

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Nanoscopic Pics Example 1(NOTE: You have the next 5 slides on a handout)

• Which box is “most” different from the others?

• Describe in terms the type of matter found in the box as a whole

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Nanoscopic Pics Example 2

• Which box is “most” different from the others?

• Describe in terms the type of matter found in the box as a whole

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Nanoscopic Pics Example 3

• Which box is “most” different from the others?

• Describe in terms the type of matter found in the box as a whole

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Nanoscopic Pics Example 4

• Which box is “most” different from the others?

• Describe in terms the type of matter found in the box as a whole

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Nanoscopic Pics Example 5

• Which box is “most” different from the others?

• Describe in terms the type of matter found in the box as a whole

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Summary of Classification of MatterNOTE: This figure is adapted from another text (it is similar to the chart on Tro, p. 7)

Monatomic Molecular Molecular Ionic

(later)

can be separated by physical means into

31

Tro, p. 7

32

(a type of physical separation)

33

Boiling, another type of physical change

34

Rusting (a type of chemical change)

Oxygen molecules

(in air; other components of air not shown for simplicity)

Fe(s) + O2(g) Fe2O3(s)

35

Fig. 1.9 Physical and Chemical Changes

(a)

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Fig. 1.9 Physical and Chemical Changes

(b)

37

Fig. 1.9 Physical and Chemical Changes

(c)

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