Determine the Molar Mass of an Unknown Gas, from the Gas Density

Part A5b:Using the graph that follows, you will also see, outlined in pink, a prediction of the density of ammonia. We used the molar mass of ammonia an incorporated it with the rest of the gasses on the graph. The density of SF6 is too large to fit on the graph that is given, but if you use the algebraic equation for slope (y-x) / (y-x), we can find out an approximate density for the gas. To determine the slope you just use the values for any two points that you know for sure on the graph. (We are omitting the possibility of propane as a choice for one of the points, see part A5c for explanation.) The slope of the main line on the graph, indicated in yellow, is 1/2. The slope is a nice proportional number so we can easily take the molar mass of SF6 and divide it by three, take that value and compare it to our graph to find a density, then multiply that density by three. This will give us an approximate density for SF6 of .00858.
Part A5c:As indicated in the lab manual, propane does not fit in with the other values when we look at them on the graph. Propane is sometimes sold with another lighter gas included in it. We apparently have got some of the "cheap" propane. If we assume that natural gas is the other component, we can then determine how much really is propane and how much is natural gas. By doing a proportional calculation. We determined that 36% of the propane was actually methane, and 64% was really propane. To calculate this we needed to use the molar mass of propane as well as methane to do the conversion.
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