Question #a1072

A mole ratio is used to convert the number of moles of one substance to number of moles of another substance.

A mole ratio is a fraction made up from the coefficients in a balanced equation.

For example, in the equation

6 Li(s) + N₂(g) → 2 Li₃N(s)

We could have the following mole ratios:

#(6 mol Li)/(1 mol N₂)#; #(6 mol Li)/(2 mol Li₃N)#; #(1 mol N₂)/(6 mol Li)#; #(1 mol N₂)/(2 mol Li₃N)#; #(2 mol Li₃N)/(6 mol Li)#;

#(2 mol Li₃N)/(1 mol N₂)#

EXAMPLE 1:

What is the mole ratio between Li and N₂ in the reaction?

6 Li(s) + N₂(g) → 2 Li₃N(s)

Answer:

#(6 mol Li)/(1 mol N₂)# or #(1 mol N₂)/(6 mol Li)#

We pick the fraction that gives us the correct units in a particular problem.

EXAMPLE 2:

Use the above equation to determine the number of moles of Li₃N that can be prepared using 0.24 mol Li.

Solution

The mole ratio is either #(6 mol Li)/(2 mol Li₃N)# or #(2 mol Li₃N)/(6 mol Li)#.

We choose the one that makes the units cancel and gives the correct units for the answer — the second one.

0.24 mol Li × #(2 mol Li₃N)/(6 mol Li)# = 0.080 mol Li₃N

Detailed explanation and good video here: http://study.com/academy/lesson/mole-to-mole-ratios-and-calculations-of-a-chemical-equation.html