Homework Help: Chemistry

Posted by Gabby on Saturday, January 19, 2013 at 9:18pm.

Hydroxylapatite, Ca10(PO4)6(OH)2, has a solubility constant of Ksp = 2.34 × 10-59, and dissociates according to

Ca10(PO4)6(OH)2(s) --> 10Ca2+(aq) + 6PO43-(aq) + 2OH-(aq)

Solid hydroxylapatite is dissolved in water to form a saturated solution. What is the concentration of Ca2 in this solution if [OH–] is somehow fixed at 8.40 × 10-6 M?

Chemistry - DrBob222, Saturday, January 19, 2013 at 9:44pm
Ca10(PO4)6(OH)2 >10Ca^2+ + 6PO4^3-+2OH^2
solid.............10x......6x......2x

Ksp = 2.34E-59 = (Ca^2+)^10(PO4^3-)^6*(OH^-)^2.

(Ca^2+)= 10x
(PO4^3-) = 6x
(OH^-) = 8.4E-6 + 2x

Substitute into Ksp expression and solve for x, then Ca^2+ = 10x.
Chemistry - Gabby, Saturday, January 19, 2013 at 9:51pm
Does this require the quadratic formula or is there a simpler way to solve for x?
Chemistry - DrBob222, Saturday, January 19, 2013 at 10:13pm
I would assume 2x+8.4E-6 = 8.4E-6 and see what happens.
Chemistry - Gabby, Sunday, January 20, 2013 at 3:19pm
Is my step here correct mathematically?

2.34e-59 = (10x)^10(6x)^6(8.4e-6+2x)

and then I solve for x?

or is it

2.34e-59 = (10x)(6x)(8.4e-6+2x)

and then I solve for x?
Chemistry - Gabby, Sunday, January 20, 2013 at 3:26pm
nevermind i got it thanks for the help!
Chemistry - andrea, Thursday, April 4, 2013 at 7:10pm
how did you solve it???

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