Multiple Choice
Which of the following occurs during the dissolution of an ionic solute in water?
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6. Chemical Quantities & Aqueous Reactions
Solubility Rules
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Which one of the following compounds would form a precipitate when mixed with PO_4^{3-} ions in aqueous solution?
A
NH_4Br
B
CaCl_2
C
K_2SO_4
D
NaNO_3
Verified step by step guidance1
Identify the ions present in each compound when dissolved in aqueous solution. For example, CaCl_2 dissociates into Ca^{2+} and Cl^{-} ions, NH_4Br into NH_4^{+} and Br^{-}, K_2SO_4 into 2 K^{+} and SO_4^{2-}, and NaNO_3 into Na^{+} and NO_3^{-}.
Recall that a precipitate forms when two ions combine to form an insoluble compound. Use the solubility rules to determine which combinations of ions with PO_4^{3-} (phosphate ion) are insoluble.
According to solubility rules, most phosphate salts are insoluble except those with alkali metal ions (like K^{+}, Na^{+}) and ammonium ion (NH_4^{+}). Therefore, compounds containing Ca^{2+} can form insoluble phosphates.
Check the possible combinations: Ca^{2+} with PO_4^{3-} can form Ca_3(PO_4)_2, which is insoluble and precipitates out of solution. The other cations (NH_4^{+}, K^{+}, Na^{+}) form soluble phosphate salts.
Conclude that among the given compounds, only CaCl_2 will form a precipitate when mixed with PO_4^{3-} ions due to the formation of insoluble calcium phosphate.
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