DONNA BOGNER Wichita State University Wichita. KS 67208
An Alternative to Using the PV = nRTEquation George Desmarals
since the unit K must appear in the numerator of the starting units
Biddetord High School Biddeford. ME 04005
While there has been much debate of the use of the factorlabel method in solving problems in chemistry, I find the method to he of use to many of my students who are weak in the application of algebra to chemistry. In this short note, I would like to share my application of the factor-label method to solving gas law problems, which involves using the ideal gas constant as the starting point in the relationship. Example 1. Finding the temperature needed for 5.1 mol of a gas to occupy avolume of 6.2 L under apressure of 9.5 atm. Algebraic solution: P V = nRT T = PV nR
Factor-label solution: mol K --K 1 atm
392
Journal of Chemical Education
Many students have problems remembering where the numerical portion of the ideal gas constant belongs among the units. I often point out that the 0.0821belongs with the L atm portion of the fraction. One may note that there is a t i n the eight of 0.0821 and a t in atm. The use of this method obviates the direct use of an equation and can be used for calculations of P, V, n, T, mass, density, and atomic and molecular mass. Example 2. Find the density of 02 in grams per liter if the pressure is 10.29 atm and the temperature is 782.6 K. Since there is a direct relationship between grams and moles, this is a problem of mol K/L atm g/L.
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I t is the opinion of the author that factor-lahel method includes less chance of error because of the number of steps involved and the increased possibility that the solver will consider which units should cancel to give the desired unit results.
