Molecular Formula *Chemistry help*?

Question

The books example of this really sucks because I can't comprehend it like I could with emprical formulas. I have 2 questions on this Section Review where I have to find Molecular formula so I'll write one of the problems and I'm hoping someone could explain it with the answer so I can do the 2nd one on my own?

If 4.04 g of N combine with 1.46 g O to produce a compound with a formula mass of 108.0 amu, what is the molecular formula of this compound?

Sorry, Chemistry has not been good to me this semester :( Parts I understand and parts I don't so I made sure next semester I'm in Biology because I'm better with memorizing than I am with math and stuff for Chemistry.

Answer 1

It's kind of a tricky question. what you need to recognize is what a molecular weight actually is, which is the sum of the atomic weights of all the atoms in the molecule. For each element, you can get the atomic mass from the periodic table. (it's usually a number that is roughly 2 times the atomic number of the element) like for instance, the atomic weight of carbon is 12 grams, which means that one mole of carbon atoms equals 12 grams, or one carbon atom contains 12 amu's of mass.

you've only got 2 elements to work with, nitrogen and oxygen. Their atomic weights are 14, and 16 respectively.

the compound they form can only be composed of nitrogen and oxygen, so it goes some thing like

N2 + O2 --> NxOy, where x and y are subscripts representing the number of atoms of each.

You know that 14*x + 16y = 108

x and y can only be whole numbers greater than or equal to 1, or else you aren't actually forming a compound. so start plugging in numbers for x, and solve for y until you get a whole number... you'll find if you plug in 2 for x, y = 5... thus a compound of N2O5 would have a formula weight of 108 amu.

Note that the weight values they gave you for each element have nothing to do with the formula weight, only the total weight of the compound you could produce, and what if anything would be left in excess. As long as you have 2 nitrogen atoms, and 5 oxygen atoms, you can form a compound with 108 amu formula weight.

Good luck!

Answer 2

4.04 g N = 0.2884 mol N atoms
1.46 g O = 0.09125 mol O atoms
For each O atom you have 0.2884/0.09125 = 3.16 N atoms
The values given imply a empirical formula of N3O (which is nonsense). Reverse the weights that you gave, and-

1.46 g N and 4.04 g O would imply an empirical formula of N2O5 which has a molecular weight of 108.
1.46/14 = 0.1043 mol N atoms
4.04/16 = 0.2525 mol O atoms
1 N for 2.4 O (or 2N for 5 O)

Answer 3

instead of answering it, i'll just do an example step by step, it's easier.
1) find empirical formula ex: H2O
2) sub in atomic weights and multiply so H:1x2 and O:16x1
3) add the two weights
4) divide this number by whatever mass you were given to start, in your problem 108.0, and round to nearest one
5) multiply all elements in empirical formula by this number, so for 9 amu H2O: 16+2=18. 18/9=2. molecular formula would be H4O2
hope this helps some

Answer 4

You have to divide the mass of N by 28 and that of O by 32 to get the moles of each. Then divide 108 by (4.04+1.46). You can use this as a multiplier to multiply the moles you found to come up with a formula.