CEM131 Answers to Questions and the Obvious Thing I Forgot

Here is a copy of the questions from class with the last four answered.

Questions from Class

Now to the second issue, my huge brain fart in class.  I knew when I worked it out (which was about five minutes after the last person left) I would want to bang my head on the board for missing the obvious.  The problem was that we were solving the wrong problem.  The Dilution Equation (C1V1 = C2V2) works when you think about how you made each experiment, not comparing different experiments.  So looking at the second experiment you did, where you made the bleach solution by mixing 5 ml of bleach with 5 ml of water.  Instead of comparing this to experiment one, think about the dilution itself.  C1 = 1.7X10(-4) M, and the V1 = 5 ml (the amount of bleach added), C2 = x, and V2 = 10 ml (the amount of bleach plus the water added).  Solve for C2 = (C1 X V1)/V2 which gives you a concentration half the original.  in the third experiment, change V1 to 2.0 ml and you get 1/5 the original concentration.  I told you it was simple.

I apologize for the brain fart.  Sometimes when you are so used to doing the idea in your head, you forget how to translate it into the math… and this was one of those time.