Moles Moles Moles Moles
- Stoichiometry describes the relationships between reactants and products in chemical equations.
- A chemical equation can be interpreted in several different ways. The most basic way to think about a balanced chemical equation is to think of it as the simplest recipe for a particular reaction. Take the following equation as an example: CH4 + 3 CuO ---> 3 Cu + 2 H2O + CO. The simplest way of interpreting this is in the following way: 1 molecule of methane reacts with 3 units of copper(II) oxide to form 3 atoms of copper, 2 molecules of water, and 1 molecule of carbon monoxide.
- But this is just the basic recipe. We can multiply the recipe many times
if we wish, which can lead to new interpretations of the equation:
CH4 + 3 CuO ---> 3 Cu + 2 H2O + CO basic 1 molecule 3 units ---> 3 atoms 2 molecules 1 molecule x2 2 molecules 6 units ---> 6 atoms 4 molecules 2 molecules x12 12 molecules 36 units ---> 36 atoms 24 molecules 12 molecules 1 dozen 3 dozen ---> 3 dozen 2 dozen 1 dozen x6.02e+23 6.02e+23 1.81e+24 ---> 1.81e+24 1.20e+24 6.02e+23 1 mole 3 moles ---> 3 moles 2 moles 1 mole - Chemical equations are, in fact, usually interpreted as a representation of the molar ratio of reactants and products in an equation as represented by the last line of the table above.
- As long as we keep the ratio of one substance to another we can perform a chemical reaction using any amount of materials that we want. We can use the chemical equation as a guide for adjusting our recipe to match any particular experiment that we might do.
- Moles - Moles calculations
- Consider the following problem: Given the reaction below, how
many moles of aluminum oxide will you produce if you start with
5.40 mol of aluminum?
4 Al + 3 O2 --->2 Al2O3 -
First think about what the chemical equation states. The question asks about the relationship between aluminum oxide and aluminum. We can see from the reaction that if 4 moles of Al are used then 2 moles of Al2O3 will be produced. This ratio is fixed, so if we start with 5.40 moles of Al instead, how do we predict the amount of Al2O3 produced?
- To do this we must set up a ratio between the two substances asked about
in the question.
5.40 x 4 Al + 3 O2 ---> 2 Al2O3
- By solving for x, you will determine the moles of aluminum oxide produced.
- So, the balanced chemical equation can be used to set up a MOLE RATIO between any two substances in the equation.
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It is important to remember that you can only set up a
- Consider the following problem: Given the reaction below, how
many moles of aluminum oxide will you produce if you start with
5.40 mol of aluminum?
MOLE RATIO!
- This is emphasized because you will try to do this with numbers containing other units, like grams. Do not try to set up a ratio using the mass of a substance! This will not work because the coefficients in a chemical equation represent molar quantities (numbers of things), not the mass of things.