Answer 4.3.1: Why we always need to check our answer

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 should always go back and check to make sure our answer makes sense when we think we are done with a problem. This is particularly important when you have time in non-exam situations so you can fix any problems that happen. If you have time, you should also check your answer during exams so you don't lose any points for a silly reason.

When we say "Check your answer" we don't mean to necessarily go back through every step to check what you did. Instead, we mean to question whether your answer makes sense or not in a "big-picture" kind of way. Let me give you an example:

When I was in physics, I remember an exam where I was asked to use gravity to figure out how fast water would be falling when it hit the ground after running off a two story house. My answer was something like 7*10⁹ m/s...Hmmm...Common sense should have told me I had a units mistake somewhere since this is quite a bit faster than the speed of light. So I could have easily caught my mistake and at least tried to fix it if I had been thinking when I put my box around an answer.

How can you check your answers quickly when you finish? There are a couple things you can quickly check when you finish a problem (and this information will become more useful to you as you solve more problems and get better at engineering):

If you've found an absolute temperature, is it greater than zero?
If you've found an absolute pressure, is it greater than zero?
If you've found mole or mass fractions, are they all greater than zeron and less than 1?
If you've found a speed, is it reasonable (like, slower than the speed of light?)?
Does your answer make sense with reality and science as you know it? A case of this one is solving for something like how long it would take to heat up 10 cups of water and finding that it should take 11 microseconds on your stove. You should think of when you've boiled water for coffee and that it took quite a while longer than 11 microseconds to heat up the water.

Let's go back now and find some other steps we can do now.