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Hi, so I'm new here and wanting to contribute I decided to write about an area near and dear to me, chemistry. First a little about me, I hold a B.S. in Chemistry, my research was actually in computational methods in Chemistry but my current work is in Quality Control as an Analytical Chemist. Actually Analytical Chemistry has become one of my favorite areas of chemistry and one that I think people might enjoy.

Why? Well quite simply Analytical Chemistry is every where, it's used everywhere from food to law enforcement to soaps and detergents not to mention in research and pharmacy and much much more. Not only that but Analytical methods apply across all disciplines of chemistry and even into other areas.

Now then having thought long and hard about this I would like to introduce people to one of the oldest and most popular analytic methods (and yet at least in my opinion one of the easiest to understand), Stoichiometry.


I am trying to write this for all levels and ranges so this first part might be review for some as it is about what Stoichiometry is and it's use in strong acid/base determination

Stoichiometry comes from the Greek words stoicheion meaning "element" and metron meaning "measure" (likely this provides a hint as to what Stoichiometry is). The principles of Stoichiometry were first laid out by Jeremias Benjaim Richter in 1792 as

'Stoichiometry is the science of measuring the quantitative proportions or mass ratios in which chemical elements stand to one another.'
This has remained the basic concept of Stoichiometry, that by understanding a reaction a quantitative determination can be made about either the reactants (the chemicals used in the reaction) or the product (the chemicals created in the reaction). However mass ratios are not typically used as it is much easier to use moles by using the molar mass of each element (and for compounds you can simply up the molar mass of each element). Dividing by the molar mass gives the number of moles.

Now I'm simply going to skip over how exactly chemical equations are derived (if there's interest that's another diary or several in and of itself) and simply say they are derived. However let's say you have a chemical reaction, let's say hydrogen gas (H2) and oxygen gas (O2) yield water when mixed and exposed to a source of energy

H2+O2 -> H2O

This reaction while valid is not balanced which is to say that the number of hydrogens and oxygens on the reactant side does not equal the number of hydrogens and oxygens on the product side. To balance the equation coefficients must be added until the equation is balanced, because water is a simple reaction (and because most know the answer) one can simply guess and check to balance this reaction. More complicated reactions require a rigorous mathematical approach unless one is incredibly lucky.  Thus a few guesses should yield

2H2+O2 -> 2H2O

As the equation is now balanced, the law of proportionality can be seen. The law of proportionality allows us to say  for example one mole of hydrogen gas (so long as there is enough oxygen) yields one mole of water or for every one mole of oxygen gas (so long as there is enough hydrogen)  2 moles of water will be created. This ratio of proportionality is the foundation of Stoichiometry as an analytical method.

To explain why exactly this is true let's walk though an example.

Say you have a solution that is a solution of hydrochloric that one of your fellow chemists thoughtfully prepared for you. However said chemist didn't write down the concentration and thus you have no real idea how strong it is, so what do you do? Well not wanting to waste it you prepare to titrate the solution. Titration is simply using a solution with a known concentration added in a controlled manner to create a reaction that you can monitor until the reaction is completed. In this example sodium hydroxide will be used (NaOH). The balanced reaction of hydrochloric and sodium hydroxide is:

HCl + NaOH -> H2O + NaCl

Now two important things to notice from this reaction: first once completed the reaction results in the formation of water and sodium chloride (salt) and the second for every mole of hydrochloric acid one mole of sodium hydroxide is required for the reaction. The first is important because water has a set pH of almost exactly of 7 where as the pH of hydrochloric acid is much lower. It is also important because it gives us a way to monitor to the reaction using what are known as indicators. Indicators are an interesting class of chemicals that under go noticeable color change depending on the pH of a solution. One of the most common indicators for acid titration is phenol red. For now it is sufficient to know that around a pH of 7 or just above 7 the indicator turns to a fuscia/pink color. Thus we now have a method of monitoring the equation.

The second is important because it provides the means  of calculating the original concentration of the hydrochloric acid by tightly controlling and counting the amount of sodium hydroxide added. How does this work? Well here we have to get into some math (I know I know math is evil but this all multiplication and division so no worries no calculus) and I mostly will just breeze over it for now. But the basic calculation is by monitoring the volume of solution delivered the number of moles can be calculated once the number of moles calculated there is a 1:1 ratio of  moles of titrant  (sodium hydroxide) to moles of analyte (the hydrochloric acid) and a little more back calculation gives the concentration of the original solution.

Now this is just a simple introduction to Stoichiometry and in later diaries we'll discuss it's role in more complicated weak acid/base titration, metal ion determination, reduction-oxidation reactions and electronic pH determination; assuming there is interest in me continuing. However if people are interested in other things related to chemistry let me know, if I can answer in a comment I will or expand it into a diary or perhaps both.

So what do people think?

Originally posted to duhban on Tue Sep 11, 2012 at 03:06 AM PDT.

Also republished by SciTech.

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