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In order to predict whether a precipitate will form in a reaction, the solubility of the substances involved must be known. There are rules or guidelines determining solubility of substances. If a substance involved is not soluble, the reaction forms a precipitate.
When a substance is mixed with a solvent, there are several possible results. The determining factor for the result is the solubility of the substance, which is defined as the maximum possible concentration of the solute. The solubility rules help determine which substances are soluble, and to what extent.
Depending on the solubility of a solute, there are three possible results: 1) if the solution has less solute than the maximum amount that it is able to dissolve (its solubility), it is a dilute solution; 2) if the amount of solute is exactly the same amount as its solubility, it is saturated; 3) if there is more solute than is able to be dissolved, the excess solute separates from the solution. If this separation process includes crystallization, it forms a precipitate. Precipitation lowers the concentration of the solute to the saturation in order to increase the stability of the solution.
The following are the solubility rules for common ionic solids. If there two rules appear to contradict each other, the preceding rule takes precedence.
1. Is FeCO3 soluble?
According to Rule #5, carbonates tend to be insoluble. Therefore, FeCO3 is likely to form a precipitate.
2. Does ClO4- tend to form a precipitate?
This is perchlorate, which according to Rule #2 is likely to be soluble. Therefore, it will not form a precipitate.
3. Which of these substances is likely to form a precipitate?
a) CaSO4 b) table salt c) AgBr
Letters a and c are both likely to form precipitates.
Concerning a) CaSO4, although sulfates tend to be soluble, Rule #5 indicates that calcium sulfate is an important exception to this rule.
For b), Rule #1 indicates that table salt (NaCl) is soluble because it is a salt of an alkali metal.
c) is an example of two rules contradicting each other. Rule #4 states that bromides are usually soluble, but Rule #3 states that salts of silver are insoluble. Because Rule #3 precedes Rule #4, the compound is insoluble and will form a precipitate.
4. Predict whether a precipitate will form as a result of this reaction:
\[2AgNO_3 + Na_2S \rightarrow Ag_2S + 2NaNO_3 \]
The products of the reaction must be examined; if either of the substances formed in the reaction is insoluble, a precipitate will form.
Considering NaNO3, Rule #3 states that nitrates tend to be soluble. A precipitate of this compound will not form.
Next, consider Ag2S. According to Rule #5, that sulfides tend to be insoluble. Therefore, because of this compound, a precipitate will form in the course of this reaction.
5. Predict if a precipitate will form as a result of this reaction:
\[2NaOH + K_2CrO_4 \rightarrow KOH + Na_2CrO_4 \]
Consider again the products of the reaction: if either is insoluble, a precipitate will form.
The first product, KOH, is an example of two rules contradicting each other. Although Rule #5 says that hydroxides tend to be insoluble, Rule #1 states that salts of alkali metal cations tend to be soluble, and Rule #1 precedes Rule #5. Therefore, this compound will not contribute to any precipitation being formed.
The second product, Na2CrO4, also adheres to Rule #1, which states that salts of alkali metals tend to be soluble.
Because both products are soluble, no precipitate form as a result of this reaction.
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