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factor label method

Submitted by pink24 on Sun, 08/30/2009 - 18:42

??? 100L= ____moles

*22.4L= 1 mole

Set up a proportion:

22.4L    100L
---   =  ---
1 mole   x mole

You cross multiply and get 22.4x = 100, then get x = 4.46 moles

(are random people allowed to answer other people's questions? or only moderator/tutors?)

Most people help when they can.

What you've shown certainly works for single step conversions but may get difficult or cumbersome when more than 2 conversion steps are called for.

For example converting 12 oz. to grams knowing that 1 lb. equals 454 grams. The factor label method allows for stringing together the conversion steps in one problem. The important consideration is to place the conversion factors so the units cancel (factor out) until your left with your answer in the proper units.

To begin, always start with your given (a number and unit)

    12 oz

We need to cancel out the oz so we use 1 lb/16 oz (common knowledge, for us)

    12 oz  x    1 lb     
                  16 oz

see that the oz cancels and lb label remains
but we want our answer in grams so to cancel lb we use 454 g/1 lb

    12 oz  x    1 lb    x 454 g  
                  16 oz        1 lb

see that the lb cancels and g label remains
As you can see the labels have been factored (hence Factor-Label)

Calculate and the answer is expressed in grams.

Dimensional Analysis is another name for this method.

Hope this helps.