Help please

[Dark*Phantom]

Answer to questions 1 and 2
Each mole of NH4 contains one mole of nitrogen because no numbers are written after nitrogen
Example: One mole of H2SO4 contains 2 moles of hydrogen, one mole of sulphur and 4 moles of oxygen

Answer to question 3
Let's suppose that the mass of the compound in 100g
Mass of oxygen = 63 gm
Mass of nitrogen = 37 gm
Molar mass of oxygen = 16 gm
Molar mass of nitrogen = 14 gm
No. of moles in oxygen = 63 / 16 = 3.9375 moles
No. of moles in nitrogen = 37 / 14 = 2.643 moles
The ratio between nitrogen and oxygen = 2.643 : 3.9375 = 1 : 1.5 = 2 : 3

The empirical formula is N2O3

Answer to question 4
No. of moles in carbon = 0.02
No. of moles in hydrogen = 0.04
Ratio between carbon and hydrogen =0.02 : 0.04 = 1:2

The empirical formula is CH2

[Modyasoo1]

thanks and can you tell me

Why it's one mole of nitrogen atoms why it's not one mole of nitrogen molecules or ions?


And something more..

How i know if 1=2 and 1.5=3 in the empirical formula because sometimes 1=1 like CH2

Here Carbon is 1

And here C2H3

Carbon is 1 too
and Hydrogen is 1.5

how to know that please and thanks

[Dark*Phantom]

Answer 1
We studies in 1st prep that the molecules of non metals (such as nitrogen) have two atoms. Which means that nitrogen molecule contains 2 nitrogen atoms. Thus, molar mass of nitrogen element (pure nitrogen gas) = 14 + 14 = 28gm. While molar mass of nitrogen , in case it is in other compounds is 14gm

When nitrogen exists in a compound, say nitric acid HNO3, it exists in the form of atoms only. If we said that there is one nitrogen molecule (instead of atom) in nitric acid, we mean that there are two nitrogen atoms - which is wrong. Moreover, atoms (not molecules) bind to each other forming molecules

We cannot say nitrogen ions because the compound didn't dissolve in water

So, we should say "Nitrogen atoms" because of the previous reasons

Answer 2
There is an important rule that we should take in consideration when solving empirical formula questions, which is

The simplest ratio between the atoms forming a compound in an empirical formula should be in whole numbers, not decimals or fractions

In other words, ratios which contain fractions such as 1 :1.5 cannot be used in empirical formula, it is impossible to say that the empirical formula of an organic compound is CH1.5 because we cannot get one and half hydrogen atoms

Thus, we should multiply both sides of the ratio with a number to get the simplest ratio in whole numbers

Example:-
If the ratio between sodium and oxygen in a compound is 2:2.5

To get rid of the decimals, we find the lowest common multiple of both sides, which is 2

The ratio = 4:5

I hope I helped you, Good Luck