Answer 4.3.1: Why we don't need density

Problem:

The reaction between propane and hydrogen chloride to form propyl chloride and hydrogen is carried out in a continuous reactor. The product stream is analyzed and found to contain 27.45 mole% C3H7Cl, 27.45 mole % H2 and 14.6% HCl. The feed to the reactor contains only propane and hydrogen chloride. Calculate the fractional conversion of the limiting reactant and the percentage by which the other reactant is in excess. If the molar flow rate of the feed stream is 290 mol/s, what is the extent of reaction? (Give its numerical value and its units.)

Explanation:

Density is a variable we often use when we need to get from masses to volumes for liquids or solids. For gases, we'd use the ideal gas law instead, which you'll see how to do later on in some other problems.

Remember that specific gravity (SG) is what we use to get the density of a substance and that:

ρ = mass/volume = m/V

In this problem, we aren't given any information about volume or asked to solve for it. And we never, ever, ever want to do a volume balance because volumes are not additive like masses or moles are! So we don't need density in this problem to solve for anything.







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