Multiple Choice
What property of water allows it to dissolve salt effectively?
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6. Chemical Quantities & Aqueous Reactions
Solubility Rules
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Which of the following cations will cause a precipitate to form when added to a solution containing phosphate ions?
A
K^{+}
B
Na^{+}
C
NH_4^{+}
D
Ca^{2+}
Verified step by step guidance1
Understand that a precipitate forms when an insoluble compound is produced in solution. To determine if a cation will cause a precipitate with phosphate ions (PO_4^{3-}), we need to consider the solubility rules and the charge of the cation.
Recall the common solubility rules: most phosphates (PO_4^{3-}) are insoluble except those combined with alkali metal cations (such as K^{+}, Na^{+}) and ammonium (NH_4^{+}). This means K^{+}, Na^{+}, and NH_4^{+} generally form soluble phosphate salts.
Recognize that Ca^{2+} is an alkaline earth metal cation, and phosphates of alkaline earth metals are typically insoluble or sparingly soluble, which leads to the formation of a precipitate.
Write the general precipitation reaction for calcium phosphate formation: $\mathrm{3Ca^{2+} + 2PO_4^{3-} \rightarrow Ca_3(PO_4)_2 (s)}$, where $Ca_3(PO_4)_2$ is the solid precipitate.
Conclude that among the given cations, only $Ca^{2+}$ will cause a precipitate to form with phosphate ions due to the low solubility of calcium phosphate.
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