Types of Stoich Problems


Mole-Mole calcs

  • If you know the number of moles of one substance, the balanced chemical equation allows you to determine the number of moles of all other substances in the reaction. This is done by using a mole ratio, which are the coefficients written as conversion factors.

Ex) N2 + 3H2 --> 2NH3

  • Some mole ratios are
  • 1 mol N2           2 mol NH3          3 mol H2            
  •  3 mol H2            1 mol N2          2 mol NH3

 


Ex) N2 + 3H2 --> 2NH3

  • ex) How many moles of ammonia are produced when 0.6 mol of nitrogen reacts with hydrogen?
  • Start with the known : 0.6 mol N2
  • Then set up a mole ratio so that the units you are trying to find are on top and the unit you are starting with are on the bottom:
  • 0.6 mol N2  x 2 mol NH3      = 
  •                      1 mol N2
  • Moles nitrogen cancels out and moles ammonia are left the answer is 1.2 mol NH3
 

Mass – Mass calcs

  • No lab can measure a substance directly from moles so it’s important to be able to determine the mass produced from a starting mass.
  • How this is done is first you convert the mass into moles using the molar mass, then using a mole to mole conversion, figure out the moles of the substance you are trying to find.
  • Use the molar mass of the substance you are trying to find to convert from moles to mass.

Ex) N2 + 3H2 --> 2NH3

  • Ex) How many grams of nitrogen are needed to produce 23 grams of ammonia?
  • Start with what you know: 23 g NH3

Determine the molar mass of the known:

  • N = 14.00 g/mol
  • H = 3( 1.01 g/mol) = 3.03 g/mol
  • Total = 14.00 g/mol + 3.03 g/mol = 17.03 g/mol

Next convert grams to moles of known

  • Convert grams to moles of known:
  • 23 g NH3 x 1 mol NH3    = 1.35 mol NH3
  •             17.03 g NH3
  • notice that you must put grams on the bottom to end up with moles as your answer!
Convert from moles known to moles unknown using a mole ratio:
  • 1.35 mol NH3 x 1 mol N2    =  0.675 mol N2
  •                         2 mol NH3
Then convert from moles unknown to grams unknown:
  •  0.675 mol N2 x  28.0 g N2  =  18.9 g N2
  •                            1 mol N2

You can do this all in one big step

  • You can do this all at once by doing the following :
  • 23 g NH3 x 1 mol NH3    x 1 mol N2     x  28.0 g N2  =  18.9 g N2
  •                   17.03 g NH3     2 mol NH3       1 mol N2
 

Complete the student response section