I’ve uploaded to the YouTube channel a set of problems relating to calculations from chemical equations. You can **find the set here**.

Calculations based on chemical equations are among the most important calculations in general chemistry because of the large amount of descriptive and quantitative knowledge condensed into the equations. The balanced chemical equation is an algebraic expression with all the reactants on the right side, all the products on the left.

The relative numbers of reacting and product molecules are indicated by the coefficients of the formulas representing the molecules. Thus

4NH_{3 }+ 3O_{2} → 2N_{2} + 6H_{2}O

indicates 4 molecules of ammonia reacts with 3 molecules of oxygen to produce two molecules of nitrogen and 6 molecules of water.

Some may think the equation* is not balanced* because there are 7 molecules on the right hand side, 8 molecules on the left. Look again and you’ll see there are 22 atoms on each side … 4 nitrogen, 12 hydrogen, 6 oxygen, to be specific … which has to be, ‘cause of the **law of conservation of matter**, right?

Now the fun stuff … all atoms and molecules have definite weights. These actual weights of atoms and molecules are proportional to their atomic and molecular weights. This means a* balanced equation shows the relative weights of reactants and products*. Most important!! In the above balances equation we have 4 moles of NH_{3} (4 x 17g NH_{3}) reacting with 3 moles O_{2} (3 x 32 g O_{2}) to produce 2 moles N_{2} (2x 28g N_{2}) and 6 moles of H_{2}O (6 x 18g H_{2}O). So 4 x 17 parts by weight of ammonia react with 3 x 32 parts by weight of oxygen to produce 2 x 28 parts by weight of nitrogen and 6 x 18 parts by weight of water …. And **any unit of weight may be used** … gram, microgram, pound, ton, you name it.

Remember! Whatever unit of mass you choose, the sum of the reactant weights must equal the sum of the product weights … the law of conservation of mass says so.

I hope you enjoy the video. Next week, some more physics … a look at equilibrium, perhaps.

See you at the whiteboard. Peace.

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