[Mayzuh]

Okay so this is my first post and I thought maybe someone could help me solve this problem.

It's for my AP Chemistry Summer assignment, it's one of the problems.

The problem is: Calculate the number of moles of Phosphorus in 15.95g of tetraphosphorus decaoxide.

I honestly forgot how to set it up. My textbook in the back said the answer was .2247 mol P but I can't get that answer. I keep getting .057 or whatever or stuff around that.

If you can help, thanks!

[Arkcon]

OK, lets try.  For starters, how would you determine how many moles of compound were in a certain mass?  What would you use to determine that?

[Borek]

My textbook in the back said the answer was .2247 mol P

Wrong.

but I can't get that answer

Good.

I keep getting .057 or whatever or stuff around that.

Perfect.

Just remember about significant figures. 0.05618 is a correct answer (taking molar mass to be 283.889 g/mol).

Nope, thanks mjc123 and DrCMS for being vigilant.

[mjc123]

That's the number of moles of P₄O₁₀ in 15.95g. How many moles of P are there in 1 mole of P₄O₁₀?

[Borek]

Sigh, I am an idiot.

It was obvious for me at the first sight that the 0.2 number is way too large and the book answer must be wrong, somehow it put me on the wrong track.

[DrCMS]

It all depends whether the question asks how many moles of P or of Phosphorous (P₄)? 

The question has two answers either 0.05618 moles of P₄ or .2247 moles of P

just like if they'd asked about oxygen the answer could be 0.2809moles of O₂ or 0.5618 moles of O

In 1 mole of H₂SO₄ is there 1 mole of S or 0.125moles of S₈?
In 1 mole of HCl is there 1 mole of H (or Cl) or ½ a mole of H₂ (or Cl₂)?

[Mayzuh]

So, I was almost right. I just missed a step. The last person who posted here got the same thing I got for P₄. I forgot to get it for 1 P. Thank you guys!