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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: funboy on July 30, 2006, 06:03:03 PM

Title: excess reactant problems
Post by: funboy on July 30, 2006, 06:03:03 PM

Im trying to answer a question in my text about excess reactants.  The question given to me provides the Volume of the reactants but all the examples I have seen on this subject the mass of the reactants are given.  How do I go about solving a problem that uses volume instead of mass??

I have tried to work it out many ways and am failing to get something sensible.

Thanks.

Chris

Title: Re: excess reactant problems
Post by: funboy on July 30, 2006, 06:17:41 PM

I think I figured it out, I will post the Q and A and hopefully someone can confirm

Title: Re: excess reactant problems
Post by: Borek on July 30, 2006, 06:41:31 PM

Both mass and volume (be it gas or solution of given concentration) can be converted to number of moles. Try to think in terms of moles first - reaction equation gives always information about ratios of moles reacting, mass is of secondary importance (even if most answers are expressed in mass units).

Title: Re: excess reactant problems
Post by: funboy on July 30, 2006, 06:50:36 PM

Q) 200 L of NH₃ and 120L of O₂ are placed in an autoclave at 250C and 800atm.  The reaction is allowed to go to completion.  Determin the quantity, in moles, of the gas that remains unreacted.

A)

NH₃ + (7/4) O₂ ---> NO₂ + (3/2) H₂O

using the idea gas law I learn that

NH₃ = 3723.277 moles
and
O₂ = 2235.766 moles

Now (7/4)O₂ = 1.75 O₂
and
2235.766 / 1.75 = 1277.58

sooooooo the equation setup to represent moles looks like this


NH₃ + (7/4) O₂ ---> NO₂ + (3/2) H₂O
1277.6  + 1.75X1277.6 --->  1277.6 + 1.5X1277.6
                   2235.8                                 1916.4

So because a greater amount of moles is shown for O₂ we assume that all the oxygen was used up but we know that there were a greater number of moles for NH₃

So

3726.277 - 1277.58 = 2448.7 moles unreacted.

Now Im certain this is correct, but my book course lessons show me NOTHING along these lines, is it possible to have answered this question another way??

Title: Re: excess reactant problems
Post by: Borek on July 30, 2006, 07:06:40 PM

Quote from: funboy on July 30, 2006, 06:50:36 PM

Now Im certain this is correct

And you are right. Just be careful about significant digits, you should not use more then three in your answer - 2.45*10³.

Quote

but my book course lessons show me NOTHING along these lines, is it possible to have answered this question another way??

This is correct way of doing this question, so there is no need for any other approach. Moles rule in stoichiometry.

One more remark: while the final result is correct, you better use properly balanced equations with integer coefficients:

4NH₃ + 7O₂ -> 4NO₂ + 6H₂O