# Stoiciometry. Finding mass and formula

1. Oct 9, 2006

### seru eman

PROBLEM:
0.20 moles of C?H?N? + 60,7 g O2 --> 35,2 g CO2 + 25,2 g H2O + ? g NO2

how many grams of the molecule C?H?N? did we have before the reaction?

Find the forumla.

18,1g, C4H14N2

BUT:
how do you solve it?

2. Oct 9, 2006

### The Bob

Try it mathematically. Start by making everything in the same units, e.g. all in moles. Then compare the coefficents of each compound that contains the same element, e.g. Equate the carbons - 0.2 Cx = 0.8 C. Then think about elements and how they work, e.g. if you have 2CO2 this is the same as saying there are 2 carbon atoms and 4 oxygen atoms or C2O4. Obvious the mole contains more than just one molecule of a substance, which is what I am implying, but it will allow you to make this relationship that will help you to solve the problem. If it really gets to you, say that 2 moles of CO2 is the same as 1 mole of C2O4.

From the equation, I originally wrote, x = 4, which is the number of carbon atoms you need per mole (as per your solutions), as 0.8 C = 0.2 Cx = 0.2x C (if we use the logic from earlier).

Hope this helps with the solution, not necessarily the chemistry.