Answer 4.3.1: Why we may or may not need to choose a basis in this problem

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:

We've highlighted some of the problem information above to help you see where we want to choose our flowrate. Take a minute and look up at the statement again.

You see that we know a lot of information about the products, including all the percentages of each chemical in the stream. This means that we can find the number of moles of each chemical in this product stream if we assume a total flowrate for the stream. This could be done by using:

F_{products, HCl} = F_{products, total}*x_{products, HCl}

Or in word form: The flowrate of the hydrogen chloride in the product stream is equal to the total flowrate in the product stream times the mole fraction of hydrogen chloride in the product stream.

This means we should choose our basis for the product since that choice will instantly give us a lot of information about the process that we can use in equations.