March 19, 2008

Let's Try to Learn Stoichiometry

You may think it's funny for you when you check the tittle of this post. 'Is it a view? Why don't you leave this post and let other blogs to create this?' you may ask me. Let's me explain. As I'm a chemistry teacher and I've created this blog for my students especially so it's no wonder why I create an explanation of lessons as a view. I hope such posts will be used by other students of other senior high schools especially in Indonesia. Indonesia has a specific curriculum called national curriculum which adopts O-level curriculum and A-level curriculum and adds some new fields for X, XI, and XII grades.

As I've been teaching in a senior high school with plus curriculum for about 11 years, I'm experienced enough to know what kinda chemistry problems for Indonesian students. I hope they may find some solutions for their chemistry problems here. This post will explain the stoichiometry concept only because it's commonly the hardest part of chemistry for Indonesian students but it will relate with many parts of chemistry and it's the basic of chemistry calculations.

What's the basic of Stoichiometry?

The basic concept is mathematics. Once you understand math well you can learn stoichiometry easier for sure.
To make this field being narrower, here are the equations you need to get closer with.

General equations

Molar mass of an atom is equal to the molecular weight of the given atom.
Molar mass of a molecule is equal to the molecular weight of the given molecule.

For all conditions
  • mass = the number of moles x molar mass
m = n . mm
  • the number of molecules = the number of moles X Avogadro number
X = n . L
  • the number of moles = Molarity x Volume
n = M. V

For specific conditions
  • At standard temperature and pressure
Volume = the number of moles x Molar Volume
V = n . Vm
V = n x 22.4 liter/moles
  • At specific temperature and pressure
P V = n R T
where P = pressure (atm)
n = the number of moles
R = the ideal gas constant (0.082 liter.atm/mol.K)
V1 . n2 = V2 . n1
where V1 and V2 = volume of gases
n1 and n2 = the number of moles of gases

What are steps to solve a stoichiometry problem?

Here are the steps and do them in the manner:
  • Read and understand the problem and list the known stuffs. Don't mention the earlier word of a given number but the later one of the given number.
Here's an example of a part of an unfinished question:
What is the number of mass of 2.24 liters of ammonia....

So the known stuff is 'volume of ammonia or V' not 'the mass of ammonia or m'
  • Determine if the problem needed a chemical equation or not. Write a balanced chemical equation if it needed it.
    Go to the next point if it didn't.
200 mL of nitrogen gas is flown into a tube of 200 mL of hydrogen gas and they are

In this case, you need to write a chemical equation and then balance it. Just use a balanced
equation to solve any chemistry problems.
  • List the equation(s) which you need to solve by using the known stuffs. Circle the known numbers and leave the unknown ones. Solve equations as many as possible.
  • Add the new known number(s) into the equation(s) which you must solve.
  • Convert the equation into the asked stuff in the problem and run it.
  • You just solve a chemistry problem!

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