Solubility Rules and Precipitates - College Chemistry

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Question

Write an equation for the precipitation reaction that occurs (if it occurs) when a solution of strontium chloride is mixed with a solution of lithium phosphate.

Answer

Start by writing out the the chemical formulas for the reactants:

$\text{SrCl}_2+\text{Li}_3\text{PO}_4\rightarrow \text{products}$

In order to figure out the possible products, combine the cation of one molecule with the anion of the other molecule.

For this reaction, the possible products are $\text{LiCl}$ and $\text{Sr}_3(\text{PO}_4)_2$ .

Next, use solubility rules to figure out if any precipitate is formed. Since compounds with $\text{Li}^+$ are soluble, $\text{LiCl}$ is soluble. Since $\text{PO}_4^{3-}$ is only soluble when paired with $\text{Li}^+, \text{Na}^+, \text{K}^+, \text{or NH}_4^+$ , $\text{Sr}_3(\text{PO}_4)_2$ is insoluble.

Thus, we can write the final chemical equation:

$\text{SrCl}_2(aq)+\text{Li}_3\text{PO}_4(aq)\rightarrow \text{LiCl}(aq)+\text{Sr}_3(\text{PO}_4)_2(s)$

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Question

Which of the following is insoluble in water?

Answer

Recall the solubility rules:

$\text{Br}^-$ is generally soluble, except when paired with $\text{Ag}^+, \text{Pb}^{2+}, \text{ and Hg}^{2+}$ .

$\text{S}^{2-} \text{ is generally insoluble unless it is paired with Ba}^{2+}, \text{Sr}^{2+}, \text{Ca}^{2+.},\text{Li}^+, \text{NH}_4^+, \text{Na}^+, \text{ or K}^+.$

$\text{CO}_3^{2-} \text{ is generally insoluble unless it is paired with Li}^+, \text{Na}^+, \text{K}^+, \text{ or NH}_4^+$

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Question

Which of the following substances is insoluble in aqueous solution?

Answer

Solubility rules can help us find the answer to this question.

First, solubility rules state that all compounds of Group 1A elements on the periodic table are soluble in aqueous solution. This means that all of the alkali metals, including potassium, form compounds which are soluble in aqueous solution; thus, is soluble in aqueous solution.

Solubility rules also tell us that all ammonium salts (salts of $NH_{4}^{+}$ ) are soluble. This means that $NH_{4}OH$ is soluble.

Next, solubility rules tell us that all bromide salts are soluble, except for those of $Ag^{+}$ , $Pb^{2+}$ , and $Hg_{2}^{2+}$ . Thus, is not soluble in aqueous solution. However, as lithium is not included in that list, is soluble in aqueous solution.

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Question

Which of the following is not readily soluble in water?

Answer

Remember the solubility rules for ionic solids in water:

Salts of group 1 (with few exceptions) and NH4+ are soluble

Nitrates, acetates, and perchlorates are soluble

Salts of silver, lead, and mercury (I) are insoluble

Chlorides, iodides, and bromides are soluble

Carbonates, phosphates, sulfides, oxides, and hydroxides are insoluble. Exceptions: sulfides of group 2 cations and hydroxides of calcium, strontium, and barium are slightly soluble

Sulfates are soluble except for those of calcium, strontium, and barium

Following these rules, we see that MgOH is insoluble in water

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