Transcript The Factor-Label Method

Lesson 1-5
Chemistry Problem Solving
Measurement and
Metric Unit Conversions
“The Chemist’s Toolkit”
What is unique about Solving
Chemistry Problems?
1. Metrics
– System accepted by most of the world
– Simple – Units are multiples of 10
– Scientific Notation
2. Use the Factor – Label Method
also called “dimensional analysis” or the
“railroad track” method
– A procedure for keeping track of and converting units
Changing Metric Units
• To most easily change from one prefix to
another in the metric system you move the
decimal.
The Factor-Label Method:
the most important
mathematical process in
chemistry!
The Factor-Label Process
•
As you know, numbers in science almost always have some measurement
units: 300 g, 0.62 Km, 0°C
The numbers are factors, and units are labels:
• 300 is a factor and grams is the label
• 0.62 is a factor and Kilometer is the label
• 0 is a factor and degrees Celsius is the label
•
With the factor-label process we consider both the “factor” and the “label”
when we calculate answers.
•
The factors and labels are equally important - they are a team - and when
multiplying a factor (number), we must also consider the label (units).
The general form of a factor-label problem is shown below.
The factor-label method is a tool that allows us to write down the
calculations that are in our brain, and to keep track of the relationship
between quantities with different “labels”.
The critical step in the factor-label method is using conversion factors
Quantity - Label 1
Quantity - Label 2
Answer - Label 2
Quantity - Label 1
A “Conversion Factor”
When we use factor label it’s important to understand that each space in
the chart represents a multiplication or a division when using our
calculator
Problem #1: Convert 1000 grams to pounds.
Sample Problem
Explanation
Write what is given.
Draw the horizontal and vertical lines.
1000 g
1454
pound
g
454,000
g2 / lb
2.2 pounds
g
1454
pound
It is known that 454 grams = 1 pound.
This becomes our conversion factor.
The problem is solved when the label
of “pounds” is in the numerator and all
other units are canceled.
Wrong Way – Units Don’t Cancel
Correct Way – Finish with Desired Units
The fraction is turned to put grams on
bottom so it will cancel the “grams”
labels.
Use your calculator to do the math 1000 times 1 divide by 454.
Problem #2:
Convert 65 miles/hour to meters/second.