Stoichiometry Problems

This is the determination of quantitites of component in a chemical reaction when given the quantity of another component.

To work such problems one must have a balanced equation before one starts.

There are several types of stoichiometry problems:

The last three in the above list will be discussed in the gas stoicheometry section.

Mole to Mole Stoichiometry Problem

mole to mole means given the moles of one component in a chemical reaction determine the moles of another component in the same reaction.

For example:

Given the Haber Process: N2 + 3 H2 ---> 2 NH3

How many moles of NH3 could be produced if 4 moles of N2 were reacted with enough Hydrogen.

Here is the sequence of steps one would take.

  1. Check to see if the chemical equation is balanced
  2. Determine the mole relationship between the given component (N2) and the requested component (NH3)

    According to the balanced equation: 1 mole N2 = 2 moles of NH3 produced

    Note: This mole relationship was gotten by considering the coefficients of the given and requested components. That is why it is so very important to have the equation balanced.

    One can formulate two conversion factors (ratios) from the above mole relationship:

    1 mole N2 / 2 moles NH3 or 2 moles NH3 / 1 mole N2

  3. Convert given moles of N2 to moles of requested NH3

    Choose the conversion factor from step 2 that will cancel the given unit (moles of N2) when multiplied by the given component (moles N2)

    4 moles N2 X 2 moles NH3 / 1 mole N2 = ? moles NH3 produced

    Notice that the moles of N2 units will cancel and the moles of NH3 are the only uncanceled unit.

Return to the top of the page.

Mole to Mass Stoichiometry

Mole to Mass Example: Using the same chemical reaction as above determine the grams of H2 that will react with the 4 moles of N2

  1. Convert moles of N2 given to moles of requested H2. This requires the use of the coefficients of N2 and H2 from the balanced equation:

    N2 + 3 H2 ---> 2 NH3

    1 mole N2 = 3 moles H2

    4 moles N2 X 3 moles H2 / 1 mole N2 = 12 moles of H2

  2. Convert the moles of requested H2 to grams of H2 using the fact that 1 mole of any substance = its Formula Mass expressed in gram units:

    1 mole H2 = 2.0 grams H2

    12 moles H2 X 2.0 grams H2 / 1 mole H2 = 24.0 grams H2

    Note that the given moles of H2 cancel

Return to the top of the page

Mass to Mass Stoichiometry

Mass to mass type problem:

Calculate the grams of NH3 that will be produced if 10 grams of H2 are reacted with enough N2

  1. Check to make sure the chemical equation is balanced
  2. Convert grams of given H2 into moles of given H2 using the fact that

    1 mole H2 = Formula Mass of H2 in grams = 2.0 grams H2

    10 grams H2 X 1 mole H2 / 2.0 grams H2 = 5.0 moles H2

  3. Convert moles of given H2 into moles of requested NH3 using the coefficients in the balanced equation: N2 + 3 H2 ---> 2 NH3

    3 moles of H2 = 2 moles NH3

    5.0 moles of H2 (from step 2) X 2 moles NH3 / 3 moles H2 = 3.33 moles of NH3

  4. Convert moles of requested NH3 into grams of requested NH3

    3.33 moles NH3 (answer from step 3) X 17.0 grams / 1 mole NH3 = 56.61 grams NH3.

To summarize:

mole to mole:

moles of given----> moles of requested

mole to mass:

moles of given -----> moles of requested ----> grams of requested

mass to mass:

mass of given ----> moles of given ----> moles of requested ----->grams of requested

Here is a problem that you should solve for practice:

How many grams of FeO are needed to produce 140 grams of Fe?

Given: FeO ------> Fe + O2 (unbalanced)

After working out an acceptable solution,click here for the solution

May I suggest the following Web sites for more problems involving this topic. If you wish to return to this page, click the BACK button or the LEFT ARROW icon on your browser.

  1. stoichiometry problem set (problem sets developed by S.E. Van Bramer for chemistry and environmental science courses at Widener University.)
  2. The ChemTeam's Stoichiometry

Return to Equations and Formulas menu

Return to Home Page

R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

Send Comments to R.H. Logan: AACProfchm@aol.com 

All contents copyrighted (c) 1996
R.H. Logan, Instructor of Chemistry,DCCCD
All Rights reserved


Revised: 10/21/98


Original Date of Creation: 11/17/96






Solution to Stoichiometry Problem



How many grams of FeO are needed to produce 140 grams of Fe?


Given:    FeO ------>   Fe    +   O2   (unbalanced)


First, you must be sure that the equation is balanced.  It is obvious above that the equation is not balanced as written since there are only one Oxygen atom on the left and two on the right.  By adjusting the coefficients of FeO, Fe, and O2 so that you have the same number of atoms of each element represented on the left as you do on the right.  The coefficient of the FeO can be made two and that will balance the Oxygen:


2FeO  ---->   Fe      +     O2


But now we have two Fe on the left and only one on the right so to balance the Fe we need to place a coefficient two to the left of the Fe:


2FeO  ---->   2Fe      +      O2


Now we have a balanced equation.  The problem tells us that we want to produce 140 grams of Fe.  They request the number of grams of FeO needed to do this.  This is a mass to mass type stoicheometry problem.


The following is the conversion map that we should employ for any mass to mass conversion:


The  symbol  ----> means "converts to"


mass given ----> moles of given  ---->  moles of  requested ----> mass of requested


Each conversion is a step:

    

  1. mass of given ----> moles of given:

    
In order to convert a mass of a substance to moles of that same substance we need to divide the mass by the formula mass of the substance.  That is the same thing as molecular weight.

    
moles of Fe = 140 grams Fe  X  1 mole Fe / Formula Mass of Fe in grams = 140 grams Fe / 55.7 grams/mole =  2.51 moles of Fe

    

  2. moles of given ----->moles of requested

    
In order to convert moles of one substance into moles of another substance related to it one uses the coefficients of the two substances that come from the balanced equation that establishes the chemical relationship between the two substances.

    
According to the balanced equation above:

    
2 moles Fe = 2 moles FeO

     
moles of Fe from step 1 = 2.51 moles Fe

    
2.51 moles Fe    X   2 moles FeO / 2 moles Fe = moles FeO = 2.51  moles FeO

    
Notice that I took the mole relationship between the given Fe and the requested FeO (which came from the coefficients of the balanced equation) and I formed a ratio so that the moles of given canceled.  In the above math expression the moles of Fe will cancel and moles of FeO will remain as the unit of the calculation of step 2

    

  3. moles of requested ----->  grams of requested

    
In order to convert a moles of a substance to mass of that same substance we need to multiply the moles by the formula mass of the substance.  That is the same thing as molecular weight.

    
2.51  moles of FeO   X  Formula mass FeO in grams / 1 mole FeO = 2.51 ( 55.7 + 16) = 2.51(71.7) = 180  g FeO

    
Final Answer = 180 grams FeO

    
Return To The Stoichiometry Lesson

URL:http://members.aol.com/profchm/stoic.html