Stoiciometry. Finding mass and formula

[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 formula.

ANSWER:
18,1g, C4H14N2

BUT:
how do you solve it?

Thanks in advance!

 

[The Bob]

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 formula.

ANSWER:
18,1g, C4H14N2

BUT:
how do you solve it?

Thanks in advance!

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 C_x = 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 C_x = 0.2x C (if we use the logic from earlier).

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

The Bob (2004 ©)

P.S. I reckon the mass of NO₂ is 18.3g not 18.1g