I found a bottle in the stockroom of my lab about a year ago. When I opened it, I knew from the burning smell that it was a bottle of nitric acid. However, I had absolutely no idea what the concentration of the acid was. Without knowing the concentration of the acid, it would be difficult to find a use for it.
Fortunately, there's a way to solve this problem. As I remembered from way back in Chemical Equations, acid-base reactions occur when an acid and base combine by the following general equation:
The Mole Says
In the lab, it's common to determine the point at which a solution has become neutralby using an indicator. When the indicator turns from the color for an acid to the colorfor a base, the titration has taken place. The use of indicators, however, does cause error because they don't change color at a pH of exactly 7.00. As a result, the point at which an indicator changes color in a titration is the "endpoint" (distinguishing it from the "equivalence point").
This sparked the following line of reasoning in my brain:
When the solution is perfectly neutral (called the "equivalence point"), the number of moles of acid that I started with will be equal to the number of moles of base that I added to make them neutral. As a result, we get the very handy equation:
MaVa = MbVb
for neutralization reactions at a pH of exactly 7.00. Titration is the process in which neutralization reactions are used to determine the concentration of either an acidic or basic solution.
Here's how I did my experiment:
You've Got Problems
Problem 4: If it took 245 mL of 0.500 M HCl to neutralize 175 mL of a NaOH solution, what is the concentration of the NaOH solution?
I placed 175 mL of nitric acid into a beaker and added 1.00 M NaOH solution to it. I found that the solution was completely neutral after I had added 365 mL of sodium hydroxide to the acid.
Using the earlier equation, M1 = 175 mL, V1 is unknown, M2 = 1.00 M, V2 = 365 mL, we find that:
The nitric acid had a molarity of 2.09 M. Why anybody would make a solution with this molarity, I have no idea, but that's what it was!
Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.