# Finding the Chemical Formula and Relative Molecular Mass

#### nineteen

My chemistry teacher gave me this problem. I tried and tried, but I couldn't figure it out and the deadline is tomorrow. Please help me out here friends.

1. This is the problem :

In a compound which is made of element Y, weight percentage of Y is 72% and N( Nitrogen) is 28%. Also, 3 Y atoms are bonded with 2 Nitrogen atoms. Find the followings....
1. Chemical formula of the compound.
2. Relative molecular mass of Y.

2. Relevant equations are :

n of molecular formula = Molar mass/ Formula mass of empirical formula

number of moles = mass/Molar mass

(at least I think these are the only equations used.)
3. The attempt at a solution :

The things is that, even though I tried I couldn't approach the question.... I couldn't find a way to enter and continue doing. Therefore there is nothing like an attempt done to solve the problem. I am sorry if this contradicts with your procedure of this specific template.

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#### DrClaude

Mentor
Isn't the chemical formula more or less given in the problem?

Knowing the mass of N and the its mass percentage in the compound should get you the molecular mass of the compound.

• nineteen

#### Borek

Mentor
If the compound contains 28% of nitrogen, how many grams of nitrogen in 100 grams of the compound? How many moles is that? What is the mass of the element Y present?

• nineteen

#### nineteen

Isn't the chemical formula more or less given in the problem?

Knowing the mass of N and the its mass percentage in the compound should get you the molecular mass of the compound.
Hey, thanks for helping out. Like you said, the chemical formula is the one said in the question itself right? It wasn't obvious to me at first, but now I tend to think so too.

#### nineteen

If the compound contains 28% of nitrogen, how many grams of nitrogen in 100 grams of the compound? How many moles is that? What is the mass of the element Y present?
Thanks for helping out my friend. By the way, the chemical formula is N2Y3 right?

#### Borek

Mentor
By the way, the chemical formula is N2Y3 right?
Yes, that's what you were told.

• nineteen

#### nineteen

If the compound contains 28% of nitrogen, how many grams of nitrogen in 100 grams of the compound? How many moles is that? What is the mass of the element Y present?
Hey I didn't understand what you said here. Except for the molecular formula thing, what you said here is a bit unclear to me. Can you please explain it further if you don't mind? Thank you very much in advance....

#### nineteen

Here's my solution. If there is anything please correct me....

(Mass of N/Total mass) x 100% = 28
Therefore, total mass = 100g

(3 x Mass of Y/100g) x 100% = 72
Mass of Y = 24 g

Chemical formula = Y3N2
The element having a mass of 24 is Mg
Therefore; chemical formula is Mg3N2

R.M.M = (24x3) + (14x2)
= 100

#### Borek

Mentor
Therefore, total mass = 100g
No, that's an incorrect conclusion. I told you to assume mass of the sample to be 100 g as it often simplifies understanding calculations when percentages are involved (but in general is never necessary).

• nineteen

#### nineteen

No, that's an incorrect conclusion. I told you to assume mass of the sample to be 100 g as it often simplifies understanding calculations when percentages are involved (but in general is never necessary).

Thank you very much

#### nineteen

No, that's an incorrect conclusion. I told you to assume mass of the sample to be 100 g as it often simplifies understanding calculations when percentages are involved (but in general is never necessary).

By the way, why did you say we have to assume? I don't understand. In the question it says there are 2 Nitrogen atoms bonded with 3 Y atoms and so, I selected the mass of mass of nitrogen as 28, and thought that it is the amount of mass in the total sample that when taken as a percentage it gives us 28%. See just like this...

[(14x2)/Total Mass] x 100% = 28 14 is the mass of one Nitrogen
(28/Total Mass) x 100% = 28
(28x100)/28 = Total Mass
So, Total Mass = 100 in grams....

I mean like, why is thinking this way wrong? @Borek

#### Borek

Mentor
(28/Total Mass) x 100% = 28
(28x100)/28 = Total Mass
You are just proving that 1=1, which is hardly an interesting and valuable conclusion. You can insert any value for 28 and it will still hold. It also happens this particular compound has a molar mass of 100 g/mol so it looks like 100 g is special here. It is completely accidental and it will not work for other compounds.

• nineteen

#### nineteen

You are just proving that 1=1, which is hardly an interesting and valuable conclusion. You can insert any value for 28 and it will still hold. It also happens this particular compound has a molar mass of 100 g/mol so it looks like 100 g is special here. It is completely accidental and it will not work for other compounds.
Ok, got it. Thank you very much!!!

"Finding the Chemical Formula and Relative Molecular Mass"

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