Introductory Chemistry 101 Dr George Lessard Problem Solving

Introductory Chemistry 101 Dr. George Lessard

Problem Solving, The SI, Systeme Internationale Chapter 2 Chemistry 101 Lecture 2 b Dr. George Lessard

All intelligences use the SI system for measurement FAll measurement systems in the developed world use the SI (Metric)=measurement system FUS (English) measurements are defined by the SI system FFor that reason we will not try to learn how to convert between the ignorant (English) and the SI systems

The SI has one set of Standard Units * (learn them) * And all multipliers micro to mega, with abbreviations

This is a kilogram, one thousand grams F The basic unit of mass is a kilogram F Normally, the unit would be a gram, but a gram is small enough to get lost so they took a thousand of them to make one basic unit

The basic unit of length is the Meter FMultiple meters can be put end to end FPieces of a meter may be selected FWe use the appropriate sized multiple

The basic unit of volume is a liter F Well, a container 10 cm x 10 cm has a volume of 1000 cm 3 F 1000 cm 3 is called a liter F 1 milliliter (m. L) (ml) is a thousandth of a liter F It has the dimensions of 1 cubic centimeter = 1 cm 3 (important)

Each Standard Unit may be multiplied by exponential factors* (know all) F Ten of the standard units is a dekaunit F One hundred standard units are hectaunits F One thousand standard units are kilounits etc. F The commonly used multipliers are in this table

So, often we need to convert from one multiplier to another FIn Chemistry there are only two ways to solve problems! FAll other ways are wrong and will yield “zero” points on a test

A conversion factor problem makes equal by applying the correct conversion factor FHow many millimeters (mm) are there in 0. 34 meters (m)? FIn every problem we begin with a F“ Given”

Meters does not equal millimeters (mm) FMeters=Meters FSo how do we fix it? FIn algebra, we can multiply by an equality F 1 mm = 1 x 10 -3 m FThus multiplying by the equality changes nothing

But the units (dimensions) change! F 0. 34 m x 1 mm/1 x 10 -3 m Fm cancels m units change to mm F 0. 34 x 1/1 x 10 -3 = 0. 34 x 103 = 340 mm

How do we know how to write the conversion factor? FThere’s only one way, the way that does not change the actual value (that gives the right units) FWe call this approach”dimensional analysis”

Then in any conversion factor problem there is an answer FSo, F 0. 34 m x 1 mm/1 x 10 -3 m = 0. 34 mm x 1 mm/1 x 10 -3 m= F 0. 34 x l x 103 mm = 340 mm

Lets do a series of problems What is the mass in grams (g) of a 25 kg sack of potatoes? How many kilobytes are there in 33 megabytes? How many nanoseconds elapse in 1 sec? If the distance to San Francisco is 1000 km, How many meters is San Francisco from San Bernardino?

There is one and only one other way of solving problems F That is, based on a known relationship F This is called a “formula” problem or “definition” problem F Density is defined as the mass that is packed into a volume

In this case there are three variables F If you know 2, you can know the third F So First must know the definition or equation F We make a table of the values we know Mass = grams Volume= cm 3 Density= g/cm 3

Again, you enter the correct units (dimensions) the answer comes out right FWhat is the density of a 1. 00 cm 3 gold nugget which has a mass of 19. 2 g? M = 19. 2 g V = 1. 00 cm 3 D= ?

When the correct values are put into the equation! F The answer comes out right F 19. 2 g/1. 00 cm 3 = 19. 2 g/cm 3 F If you set the problem up right, you will get the correct answer

Lets solve some problems! What is the mass of 1000. 0 cm 3 of water if at 40 C, water has a density of 1. 00 g/cm 3? (remember this value=density of water@40 C) A block of Aluminum has a mass of 540 g and occupies a volume of 200 cm 3, what is its density?

Mastery Exam 1 in lab in two weeks gives me a chance to see if you are ready for chemistry! FNo tricks, no guessing! FAddition, Subtraction, Multiplication, Division FAlgebraic Manipulation FDecimals, Exponents and Scientific Notation FSignificant Figures FSI, Units, Multipliers FProblem Solving using two methods

Remember! FIt is assumed that you can solve all of the problems of this type and those on Exercise A and the skillbuilders FAbility to solve the assigned Problems (Ch 1 -2) FAbility to convert between SI units(worksheet) Remember, it takes lots of little amounts to make one big amount. 1000 mm/m 1 km/1000 m