[cjtm92]

Okay, so I'm getting my Chemistry homework done right now, and I'm really stuck on a problem, which is:

"Tooth enamel, or hydroxyapatite, has a molar mass of 422 g/mol. Its composition is 28.5 percent calcium, 22 percent phosphorus, 49.3 percent oxygen, and 0.2 percent hydrogen. Caclulate the molecular formula."

I found the empirical formula, which I believe to be right -- C₈H₁₁O₃N

Then, when I divide the molar mass by the empirical molar mass (979.36 g/mol), I get a 0.42 number, which wouldn't make any sense when I would multiply the subscripts. If you could guide me in the right direction, I would really appreciate it. Thank you!

[enahs]

Where did you get calcium or nitrogen?

[ARGOS++]

Dear cjtm;

Allow me to ask you where is the phosphorus in your empiric formula?

And from where you get an empirical molar mass of   979.36 g/mole?

And why you need any empirical molar mass, when you have the real molar mass instead?

Have you ever thought about: How many oxygen are 49.3% of 422 g/mole?
(I don’t believe that will be only 3.)

Good Luck!
                    ARGOS⁺⁺

[cjtm92]

Oh jeeze, I was looking at the wrong problem for some areas. The Empirical Formula I got is Ca₇P₇0₃₀H₂. Sorry about that haha, but everything else seems to be right.

[ARGOS++]

Dear cjtm;

In this moment please tell what are 422 g/mole, and what is 0.42 inside your question?
Or where is your question at all?

Good Luck!
                    ARGOS⁺⁺

[cjtm92]

When I divide the Molar Mass, that is given, by the Empirical Molar Mass [(422g/mol / 979.36g/mol)], I get a ratio of 0.43. My ultimate question is, what is the Molecular Formula? I don't know what to do with the 0.43, because it will not give me whole numbers for subscripts. Sorry, for the confusion, and if it's not clear still let me know.

[ARGOS++]

Dear cjtm;

How you got the empirical molar mass of   979.36 g/mole?

Good Luck!
                    ARGOS⁺⁺

[Borek]

When I divide the Molar Mass, that is given, by the Empirical Molar Mass [(422g/mol / 979.36g/mol)], I get a ratio of 0.43.

Why does the empirical mass differ from the given?

[cjtm92]

Because, if you are just told to find the Empirical Formula, it's a given that you are finding the formula of the atoms in the simplest proportion for a given molecule, for Molecular Formula, you are finding the exact number of atoms.

Example: If you found the Empirical Formula for H₂O, but you were given the molar mass is 36g/mol, then it would be a 1:2 ratio, making the Molecular Formula H₄O₂.

But, I think I found the answer to my problem, I would multiply the subscripts by 0.43, so my Molecular Formula would be Ca₃P₃O₁₃H

[ARGOS++]

Dear cjtm;

Your formula is now correct for the given question!

But you could do it next time simpler in case you think:
How many Ca are 28.5% of 422 g/mole? – and so on!
(It gives quite exact numbers, because the correct MW is given!)

Good Luck!
                    ARGOS⁺⁺

[cjtm92]

I'm not sure I fully understand. 0.285 * 422 = 120.27, but how would that help me find how many atoms of each element are in the given compound?

[ARGOS++]

Dear cjtm;

Now divide your result (120.27 g/mole) by the molar mass of  Ca and be surprised how exact the number will be!
The same you can do for each other element too!

Good Luck!
                    ARGOS⁺⁺

[cjtm92]

That's so awesome! I wish our teacher taught us that way... would of helped to check answers. Thanks again so much!

[ARGOS++]

Dear cjtm92;

You 're welcome!   ─   Soon again.

Good Luck!
                    ARGOS⁺⁺

 P.S.: Thank you too!
.