pt 1066349 CULTY HELP PAGE STUDENT HELP Answer: 7.98 104-6 Remember each solutio

Question

pt 1066349 CULTY HELP PAGE STUDENT HELP Answer: 7.98 104-6 Remember each solution is diluted by the other solution. The Kp of aluminum phosphate is 9.8x1021 The Kap of calcium phosphate is 1.3x10-32 Sodium phosphate is added to a solution that contains 0.0014 M aluminum nitrate and 0.028 M calcium chloride The concentration of the first ion to precipitate (either Alp or Ca) decreases as its precipitate forms. What is th concentration of this ion when the second ion begins to precipitate? t of Answer: 1.027 10A-9 Note that well over 99% of the first ion has precipitated before the second ion starts to precipitate.

Explanation / Answer

Aluminum phosphate dissolves according to

AlPO?(s) ? Al³?(aq) + PO?³?(aq)
=>
Ksp = [Al³?]?[PO?³?]
with Ksp = 9.84×10?²¹

[PO?³?] = Ksp / [Al³?] = 9.8×10?²¹ / 0.0014

= 7.0 * 10^-18 M

Calcium phosphate dissolves according to
Ca?(PO?)?(s) ? 3 Ca²?(aq) + 2 PO?³?(aq)
=>
Ksp = [Ca²?]³?[PO?³?]²
with Ksp = 1.3×10?32

[PO?³?] = ?(Ksp / [Ca²?]²) = ?( 1.3×10?32/ 0.028³) = 2.43×10?¹? M

Because [PO?³?] is smaller for Al³?, aluminum phosphate precipitates first. So at the instant when calcium phosphate starts to precipitate the aluminum ion concentration will be:
[Al³?] = Ksp / [PO?³?] = 9.8×10?²¹ / 2.43×10?¹? = 4.03×10?? M

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