posted by Sara on .
Determine the number of grams of NH3 produced by the reaction of 3.5g of hydrogen gas with sufficient nitrogen gas.
All of these stoichiometry problems are work almost the same. Sometimes an extra step or two are required. Here are the basics.
Step 1. Write the balanced equation.
N2 + 3H2==> 2NH3
Step 2. Convert what you are given to mols remembering that mols = grams/molar mass.
mols H2 = g/molar mass = 3.5/2 = 1.75
Step 3. Using the coefficients in the balanced equation from step 1, convert mols of what you have (in this case mols H2) to mols of what you want (in this case mols NH3).
mols NH3 = mols H2 x (2 mols NH3/3 mols H2) = 1.75 x 2/3 = 1.17 mols NH3. [Note that the 2 and 3 come from the coefficients for the molecules in the balanced equation. Note also that the factor mols NH3/mols H2 is arranged so that the unit we don't want to keep cancels (we want to change H2 to something else) and the unit we want to keep (mols NH3 which is what we want the mols H2 changed to) stays in the numerator.]
Step 4. Now convert mols NH3 to grams with the rearranged equation from step 2. There we had mols = g/molar mass. Rearrange to g = mols x molar mass and g NH3 = mols NH3 x molar mass NH3.
g NH3 = 1.17 x 17 = ??
Remember how to do this. This procedure will solve many many problems for you.