Chemistry Helper 

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Stoichiometric Calculations

    Stoichiometry is the solution of problems involving specific quantities of substances.

   When substances react, a chemical equation is used to describe the change. The symbols and formulas indicate what substances are involved in the change. The coefficients in the equation provide a mole ration. The mole ratio tells us how many moles of one particular substance are used up in the reaction.

Example: How many grams of oxygen would be needed to react with 60g of carbon to produce carbon dioxide?

Step 1
Write your equation.

O2 + C    --->    CO2

Step 2
Balance your equation.

O2 + C    --->    CO2
This is balanced!

Step 3
Decide your known and unknown quantities.

O2 + C    --->    CO2
Xg   60g                       

We know that we are looking for how many grams of Oxygen would be needed to react with 60g of Carbon, so we know that Oxygen is our unknown quantity., while Carbon is our known quantity.

Step 4
Convert your grams to mols.

60g of C = 1 mol
                                                --------- = 5 mols of C
                     12.0 g

Step 5
Do your mol-mol ratio.

answer from g to mol conversion = coefficient of unknown quantity
                                                                     coefficient of known quantity

5 mols of C = 1 mol of O2
                                                        ------------------  = 5 mols of O2
                           1 mol of C

(We simply multiplied 5 by 1, and then divided by 1)

Step 6
Convert mols to grams.

5 mols of O2 = 32.0 g of O2
                                                       ---------------------   = 160g of O2
                         1 mol