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Reaction Stoichiometry



Stoichiometry is a powerful tool that all chemist must utilize. It separates the scientists from everyone else. It allows a chemist to determine an amount of another compound that is needed or produced in a chemical reaction.


Question- How many grams of water can be produced from 10.grams of H2 with excess O2?

excess=you have more than enough O2 to complete this reaction, worry about H2

1. You need a balanced chemical reaction.

2H2 + O2 ==> 2H2O

(remember that the coefficients are really mole ratios)

2. FIND THE MOLES of your compound. Sometimes it is already given, sometimes you have to calculate it.

Given==> 10. grams of H2

Find the moles=> 10.g H2 (1mol / 2.0g H2)= 5.0 mol H2

3. DO THE RATIO-Now you can find the moles of any other compound in this reaction using the coefficients of the balanced reaction. Here are a few ways to do this.

Factor Label     5.0 mol H2 ( 2 mol H2O /2 mol H2) =5.0 mol H2O

or set up a proportion

5.0 mol H2 =  x mol H2O
________________________________ ___________________________________________________________________
2 H2 2 H2O

x= 5.0 mol H2O

or set up an equation proportion

(remember that the coefficients are really mole ratios)

5.0 mol H2  ? mol O2 ===>  ? mol H2O
________________________________ ________________________________ ________________________________
2H2 + O2 2H2O

Now you solved for O2 also(2.5 mol) and H2O (5.0 mol)

4. ANSWER THE QUESTION-Now find the mass from the number of moles

5.0 mol H2O ( 18.0g/mol)= 90. g H2O


If I wanted to know about the O2, I could just take the moles and find the mass.

2.5 mol O2 ( 32.0g/mol)= 80. g O2

FYI the mass in equals the mass out.

10.g +  80.g ===>  90.g
________________________________ ________________________________ ________________________________
2H2 + O2 2H2O

Conclusion- Find the moles, do the ratio, answer the question

On to Limiting Stoichiometry

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