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<img src="../../media/stoich/stoichtitle.gif" alt="Stoichiometry" width="287" height="41">
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<!--msthemelist--><tr><td valign="baseline" width="42"><img src="../../../../_themes/education/blbull1.gif" width="15" height="15" hspace="13"></td><td valign="top" width="100%"><!--mstheme--><a href="#stan">Standard</a>
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<h2><!--mstheme--><font color="#0033CC">Standard<!--mstheme--></font></h2>
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<p>Stoichiometry consists of the calculations in chemistry that involve how
much of each reactant is required to make the products of the reaction. The
coefficients in the balanced equation are used to determine the number of moles
of each element that are required in the reaction. Therefore, from the
equation,
<center>
<p>3H<sub>2</sub> + N<sub>2</sub> --> 2NH<sub>3</sub>
</center>
<p>you can surmise that to make 2 moles of ammonia you need three moles of
hydrogen gas and one mole of nitrogen gas. Yes, I know that almost everyone
reading this page is saying that hey that sounds really cool, but what good
is it to know how many moles you need to make something. I have the answer, if
you know how many moles you need you can convert that to grams. On the
periodic table there is an atomic weight. The atomic weight is equal to the
number of grams in one mole of the element. If you have more than one
element in a compound add the number of grams together according to the
subscripts.
<p>Examples:
<ol>
<li><p>How many moles of O<sub>2</sub> are produced when 7.5 moles of
KClO<sub>3</sub> decompose according to the following equation?
<center>
<p>KClO<sub>3</sub> --> KCl + O<sub>2</sub>
</center>
<p>Balance the Equation
<center>
<p>2KClO<sub>3</sub> --> 2KCl + 3O<sub>2</sub>
</center>
<p><img src="../../media/stoich/ex_1.gif" alt="example one" width="387" height="108">
<li>How many grams of Al will react with 2.3 moles of HCl according to
this equation?
<center>
<p>Al + HCl --> AlCl<sub>3</sub> + H<sub>2</sub>
</center>
<p>Balance the Equation
<center>
<p>2Al + 6HCl --> 2AlCl<sub>3</sub> + 3H<sub>2</sub>
</center>
<p><img src="../../media/stoich/ex_2.gif" alt="example 2" width="391" height="122">
<li>How many grams of ammonia are produced when 32g of nitrogen react with
excess hydrogen?
<center>
<p>N<sub>2</sub> + H<sub>2</sub> --> NH<sub>3</sub>
</center>
<p>Balance the Equation
<center>
<p>N<sub>2</sub> + 3H<sub>2</sub> --> 2NH<sub>3</sub>
</center>
<p><img src="../../media/stoich/ex_3.gif" alt="example 3">
</ol>
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<h2><!--mstheme--><font color="#0033CC">Limiting Reactant<!--mstheme--></font></h2>
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<p>Now you say that stoichiometry is great, but what if you don't have such pretty
numbers that work out evenly? That is where limiting reactant comes into play.
In most situations one reactant will run out before the other. The reactant
that is totally consumed is called the limiting reactant because it stops the
reaction. Any other reactant that does not run out is called the excess
reactant. Therefore the amount of product that can be produced is directly
related to the limiting reactant.
<p>The first step in a limiting reactant question is to determine the limiting
reactant. To do this, for all reactants calculate the amount of product that
would be produced if all of the reactant was used and there was excess of all
the other reactants. The reactant that has the least product is the limiting
reactant.
<p>The amount of products is due to the amount of the limiting reactant.
<p>An example:
<p>Using the following reaction
<center>
<p>Zn(s) + 2HCl(aq) --> ZnCl<sub>2</sub>(aq) + H<sub>2</sub>(g)
</center>
<p>Which is the limiting reactant and how much H<sub>2</sub> is produced if there
is 0.30 mol of Zn and 0.52 moles of HCl?
<p>Step one:
<p><img src="../../media/stoich/ex_4.gif" alt=".30 moles of Zn makes
.30 moles of H2" width="287" height="134">
<p><img src="../../media/stoich/ex_5.gif" alt=".52 moles of HCl makes
.26 moles of H2" width="302" height="126">
<p>Since HCl is the limiting reactant, the number of moles of
H<sub>2</sub> produced is .26 moles.
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