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Chapter 3 Problem Solving in Chemistry
I. Introduction
- Chemistry is an experimental science aimed at solving problems
- Problem solving skills will improve with time and practice
II. 3.1 Word Problems
- Solving word problems and other problems become easier when good problem solving techniques
are used
- An orderly approach takes time and practice to develop but is the easiest and most consistent way
to get the correct answer
- ONE GALLON OF GAS IN A CAR’S ENGINE PRODUCES ON AVERAGE 7.95 KG OF CARBON
DIOXIDE. CALCULATE THE ANNUAL PRODUCTION OF CARBON DIOXIDE IN KG IF THERE ARE
47.8 MILLION CARS DRIVEN IN THE U.S. AND EACH CAR COVERS A DISTANCE OF 14,500
MILES AT A COMBUSTION RATE OF 22.5 MILES PER GALLON
III. 3.2 Five steps used in solving problems
1. Identify the unknown in the problem or what is the problem asking
2. Identify the known or given – decipher the important information
3. Plan a solution or a way to get from the known to the unknown – in complex problems many
simple solutions may have to be used and combined (moving a rock pile)
 Take and put your known over 1 or over its unit
 Set up conversion factors = determine which unit is the 1 and which is the exponent – larger
unit usually is 1 (unit writer not just numbers)
 Crisscross with conversion factor – top to bottom or bottom to top
 Always end with the unit answer you want in the final conversion factor – or combination of
the two if it is a double conversion
4. Do the calculations if you have done the first three correctly this is a very straight forward and
easy step (3 sig figs)
5. Finish up check your work!
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Careers in Chemistry
- Chemical technician carry out the chemist’s ideas in the development and use of new products
(start about $36000)
IV. 3.3 Conversion factors from equivalent measurements
- The same quantity can usually be measured or expressed in different ways
- Whenever two measurements are equal, or equivalent, a ratio of these 2 measurements will equal
unity, or one
- Ratios of equivalent measurements are called conversion factors
- In conversion factors the numerator is equal to the denominator but expressed in different units
- 1m/100cm = 100cm/1m or 1L/1000mL = 1000mL/1L and all are equal to one
- Conversion factors are defined or exact quantities, which means they have an unlimited sig figs –
what you start with determines your SF
V. 3.4 Apply the technique of dimensional analysis to solving conversion problems
- In dimensional analysis you use the units (dimensions) that are part of measurements to help solve
(analyze) the problem
- When using conversion factors in dimensional analysis always use the conversion factor that has
the unit of the known in the denominator – top to bottom or bottom to top
- This cancels the known unit and gives the answer in the unknown units Example page 65
-
In every problem make sure that the units cancel and that the numerator and denominator of each
conversion factor are equal to each other
There is more than one way to solve problems, such as simple
Algebra, or ratio and proportion, the more problem solving
Skills you develop the better you will be in Chemistry
Example 1
10. A 1.00 increase on the Celsius scale is equivalent to 1.80 increase on the Fahrenheit scale. If the temperature increases
48.00C, what is the corresponding increase on the Fahrenheit scale?
11. One of the first fillings used to fill cavities in teeth consisted of 26.0g of Ag, 10.8g of Sn, 2.40g of Cu, and .800g of Zn.
Determine how many grams of each component are in 25.0g of the filling.
Example 2
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VI. Convert measurements within the SI system
- In chemistry it is often necessary to convert from one unit to
another - conversion factors are used to do this
VII. 3.6 Apply problem-solving techniques to multi step problems
- Multi step problems must be broken down into simple steps
-
Develop a written plan for the solution – it should include the known, all the intermediate units, and
the unknowns
Once the steps are in place choose the correct conversion factors and put them in the proper order
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VIII. 3.7 Apply dimensional analysis to solving complex unit conversion problems.
- To be successful you need correct conversion factors and keep
track of the cancellation of units in the numerator and the
denominator
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