ReactionStoichiometry

ApchemWiki | RecentChanges | Preferences

Showing revision 3
1. Write the balanced equation (reaction). 2. Convert the known information (e.g. mass) of the reactant to moles.

      A. If you need help on determining the apporpriate equation, look at the graphic organizer.
       B.Will modify this step when we get to the Limiting Reactant problems.

3. Use the balance equation to set-up the right "molar ratio" to go from the known substance to the unknown substance. 4. Convert from the moles of the unknown susbtandce to tue unknown information (e.g., are you looking for mass, volume, # of particles) A. Again, if you need help on determining the appropriate equation, look at the Graphical Organizer.

Example: Calculate the mass (in grams) of NH₃(ammonia) produced by the reaction of 5.4 grams of hydrogen with nitrogen. The balanced reaction equation is:

1 N₂ (g) + 3 H₂ (g) => 2 NH₃ (g)

Step 1: Question (see balanced reaction above) gave is information

Step 2: Convert the mass of hydrogen (known substance) to moles

moles = grams/molar mass

Molar Mass of H₂ Molar Mass of H₂: (1 g)(2) = 2 g/mol

Step 3: Figure out from the balanced reaction what the Molar Ratio is between hydrogen and ammonia.

2 mole NH₃ / 3 mole H₂

Now use this ratio to convert from known (hydrogen) to unknown (ammonia).

(2.70 mole H₂)(2 mole NH₃ / 3 mole H₂) = 1.80 mole NH₃

Step 4: Convert from unkown (ammonia) moles to unknown information (mass).

mole = grams/molar mass Molar Mass of NH₃: (14 g)(1) + (1 g)(3) = 17 g/mol

1.80 mole NH₃ = X / 17 g/mol NH₃

X = (1.80 mole NH₃)(17 g/mol NH₃) = 30.60 grams NH₃

X = 30.6 grams NH₃