N2 + 3H2 → 2HN4 If you produce 55.5 grams of ammonia, how many grams of nitrogen will you need?

Question

aaronq · Answer

Stoichiometry

When performing stoichiometric calculations the values to be used $\sf \color{red}{must~be~in~moles}$*. This is because the Molar Masses ($\sf M_{m}$ of two elements are not the same. This is to say that 1 gram of Hydrogen atoms is not the same number of atoms as 1 gram of Nitrogen atoms. We need to be working with numbers of particles (e.g. atoms, molecules, formula units) and using moles is the way this is done. 

General Scheme: 

$\sf \large 1.$First write and balance an equation for the process described.

$\sf \large 2.$Next, use the stoichiometric coefficients to find moles produced. 

$\sf \large 3.$ Set up a ratio using the species of interest, like so:

e.g. for a general reaction:

                      $\sf \large \color{red}{a}A + \color{blue}{b}B \rightleftharpoons \color{green}{c}C$

where upper case are the species (A,B,C), and lower case (a,b,c) are the coefficients ,

                     $\sf \dfrac{n_A}{\color{red}{a}}=\dfrac{n_B}{\color{blue}{b}}=\dfrac{n_C}{\color{green}{c}}$

From here you can isolate what you need. 
For example: if you have 2 moles of B, how many moles of C can you produce?

solve algebraically:      $\sf\dfrac{2}{\color{blue}{b}}=\dfrac{n_C}{\color{green}{c}}\rightarrow n_C=\dfrac{2*\color{green}{c}}{\color{blue}{b}}$


$\sf \large 4.$ Solve

aaronq · Answer

To interconvert between mass and moles, use the relationship:

                                        $\Large \sf n=\dfrac{m}{M_m}$

        where, $M_m$=molar mass, $m$=mass, and $n$= moles.