[lobsidedbob]

Chemical analysis shows that a compound is composed of:
43.64% Phosphorus
56.33% Oxygen

The molar mass of this compound is:
283.9 g/mol.

Find both the empirical and molecular formulas for the following.

[Clough]

Hi, lobsidedbob!

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Thanks!

[lobsidedbob]

Sorry...
I went through the formula and got 1 mol P and 2.5 mol O, but I didn't know what to do with that from there. 2.5 can't be a subscript, can it? I multiplied by two and got P2O5 for the empirical formula, but I don't know if I'm allowed to do that because I didn't take very clear notes. And then I don't have any idea what my notes are telling me about how to find the molecular formula from there.

[Clough]

Hi again, lobsidedbob!

I'm going to private message a member who can potentially help you with this. I don't know when he'll be back on here though. The member who I will contact is unknown008.

Thanks!

[lobsidedbob]

I think I may have figured it out, actually. I watched an instructional YouTube video... Thank you!

[Unknown008]

Ok, you still need help?

You know that:
43.64% Phosphorus
56.33% Oxygen

Dividing 43.64 by the relative atomic mass of phosphorus, 31.0 gives you 1.40.
Dividing 56.33 by the relative atomic mass of oxygen, 16.0 gives you 3.5.

These values are the ratio of P and O in the compound. Since they contain fractions, we will eliminate them by dividing both by the smaller of the two values, giving 1: 2.5 like you had.

Since you have another fraction, you have to remove that too. This time, you will have to multiply by 2 to get rid of the 0.5 in 2.5 which gives 2:5

And that's it, the empirical formula of the compound is

Now to get the molecular formula:
Find the mass of P2O5 = (2x31) + (5x16) = 142.
The molecular formula is a multiple of the empirical formula, so you can write the molecular formula as "n(P2O5)'

You know the molecular mass, 283.9.
Molecular mass = n(Molecular mass of empirical formula)
283.9 = n(142)

You get n = 1.97 = 2 (n is always a whole number)

So, molecular formula =

Did that help? :)

[lobsidedbob]

Yes, thanks!

[Clough]

Great job, Unknown008!

[Unknown008]

You're welcome lobsidedbob! :)

Thanks Clough! :)