Chemistry Forums for Students > Problem of the Week Archive

Problem of the week - 06/05/2013

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Borek:
What is formula of a basic calcium phosphate present in the tooth enamel, if its molar mass is 502 g/mol?

delta609:
Ca5(PO4)3OH

Big-Daddy:
I think it is Ca5(PO4)3OH. Write equations:

-v[OH]+2v[Ca]-3v[PO4]=0 (total charge is assumed to be 0)
v[OH]·Mr[OH]+v[PO4]·Mr[PO4]+v[Ca]·Ar[Ca]=502

And since we don't have a third equation, we can't solve this system rigorously (i.e. by direct substitution or elimination), so we'll have to guess one of the variables (which is ok because Mr=502 only, hopefully it will not be too high). Start with v[OH]=1 (plan to move onto v[OH]=2, v[OH]=3 etc. if you don't get integer values for the other coefficients when v[OH]=1) and you calculate v[PO4]=3, v[Ca]=5 immediately.

stewie griffin:
I agree with delta609. It's hydroxyapatite.
I'm confused on how to solve this problem though without just "knowing" what hydroxyapatite is.

delta609:
trial and error and the fact that the question states it is a basic phosphate helps to narrow it down a little

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