Solubility Rules

Whether you use the term molarity, concentration or solubility they all represent the maximum amount of solute that can successfully be dissolved in a solvent. 

Understanding the Solubility Rules

Concept: The Rules for the Solubility of a compound

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Video Transcript

I honestly hate when we have to memorize a lot of things, polyatomic Ions is one and this would be the second. Here we have to learn our solubility rules. Here we’re going to say the solubility of a compound represents the maximum amount of solute you can dissolve in the solvent.
Solubility also has another name. Its other name is concentration. And remember, we said that another name for concentration is molarity. Solubility, concentration, molarity, all three of these terms are basically pointing at the same thing. It’s talking about how much solute do we have in our solvent and together what kind of solution forms?
Now, what we should realize here is these are our solubility rules. Now, here on the left side, we talk about soluble ionic compounds and on the right, we talk about insoluble ionic compounds. Let’s go through this really quickly just to see some of the basic types of ideas we have. Here we’re going to say Group 1A ions, so Group 1A ions: H⁺, Li⁺, Cs⁺, Na⁺, K⁺ or ammonium which is a polyatomic ion, when they’re connected to someone else, that compound will always be soluble.
For example, if I had NH₄ connected with Cl. Soluble means when I throw it into water, it’s going to break up into ions. These will break up into NH₄⁺ (aqueous) plus Cl⁻ (aqueous). It breaks up into the ions that are present before we threw it into the water.
Nitrates, acetates, and remember acetate can be written in two forms, and most perchlorates are soluble. Again, when these guys are connected to a compound, they’re going to be soluble. If we have KClO₄, this breaks up into K⁺ plus ClO₄⁻.
Cl⁻, Br⁻, and I⁻ are soluble. Meaning they break up when they’re connected with something else, except when they’re connected to silver, lead (II), copper (I) or mercury (I), when they’re with these guys they stay as solids. We’re going to say that they’re insoluble. Sulfates are soluble. Meaning they break up except when they’re with one of these guys here.
Now, insoluble ionic compounds basically means that they’re solids. Another name for solid is precipitate. You’re going to hear these words used interchangeably as well. Insoluble means it’s a solid. It doesn’t break up. Solid also means precipitate. All three words are saying the same exact thing. OH⁻, S2-, they’re insoluble except when they’re with Group 1A, ammonium, or one of these Group 2A ions.
You can see how the things fit together. Of course, they would be soluble with these guys because Group 1A ions are always soluble with everyone. Ammonium is always soluble with anyone. Calcium, strontium and barium are soluble with everyone except for sulfate. And then, carbonate and phosphate.Basically, calcium, strontium, barium are insoluble with the -ates: sulfates, carbonates, phosphates.
Then we're going to say carbonates and phosphates are insoluble except when they’re with Group 1A or ammonium. There’s a lot of overlapping of trends. Your best strategy is just to remember Group 1A elements and ammonium are soluble with everyone. Just remember that let’s see the Group 2A ions, calcium, strontium, and barium are insoluble when they’re with the –ates, so sulfate, carbonate, phosphate.
Just remember these little tricks here and there to help you remember what’s what. Other than that it’s really just memorizing and it sucks, I know. But, you're going to have to do it to be able to do a lot of questions, because they’re going to ask you to give us the formula when these two things mix together. What are the products formed? 

Certain positive ions combine with negative ions to form either a liquid, aqueous, solid or gaseous compound. The Solubility Rules help us determine which form is created. 

Concept: The meaning of the terms soluble and aqueous

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Video Transcript

We talked about this already, when we classify a compound as soluble, it means that the compound is aqueous when it breaks up. It means it breaks up into aqueous ions when it’s dissolved in water. It is also known as an electrolyte, which means it can conduct electricity.
Basically, we have pure water here. When we throw this ionic compound in, it breaks up into ions. Why does it break up into ions? Because remember, Na is a Group 1A metal. They’re always soluble with everyone, so they’re going to break up. Nitrates are also always soluble.Both of them are saying they're soluble when connected to anything, so, of course, they’re going to break up into ions.
These ions will be free floating in the water. And because they have a charge, they can conduct electricity. That’s what an electrolyte is. An electrolyte just means we have ions. Those ions are the electrolytes. They conduct electricity.

Saying a compound is soluble means that the compound will dissolve into aqueous ions in a solvent. These ions can conduct electricity and are called electrolytes

Concept: The meaning of the terms insoluble and precipitate

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Video Transcript

When we classify a compound as insoluble, it means that that compound usually is a solid or precipitate. And, we call these guys non-electrolytes. These guys do not conduct electricity because they do not break up into ions when they are in water. A good example here we have what’s called methanol, CH₃OH. Here it’s a liquid before I throw into water. When I throw into water, it becomes aqueous. All that aqueous means is that water is surrounding it. That’s all aqueous means, water surrounds it.
And, what you should realize is that the compound was methanol in the beginning. It stayed intact before I threw it in, even when it’s in the water it’s still intact. It doesn’t break up at all. It stays together. This is what a non-electrolyte does; it doesn’t break up at all.
Barium sulfate is a solid. And remember, sulfates are soluble unless they’re connected to certain ions. One of those ions is barium. That means that this compound would be insoluble, following our solubility rules. When we throw it into water, we shouldn’t expect it to break up. It stays together. Now, it becomes aqueous as well because even though it's still a solid, it surrounded by water. Because it’s surrounded by water, we say its state is now aqueous.
Now, this page really is, again, learning a few trends here and there, but for the most part, the bulk of it is just memorizing what you see for the solubility rules. 

Saying a compound is insoluble means that the compound will not dissolve into aqueous ions and forms a solid called a precipitate, which will not conduct electricity. 

Solubility Rules Additional Practice Problems

Predict the identity of the precipitate that forms when aqueous solutions of BaCl 2 and K2SO4 are mixed.

a) barium chloride (BaCl2)

b) potassium chloride (KCl)

c) barium sulfate (BaSO4)

d) potassium sulfate (K2SO4)

e) no precipitate is formed

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What is the identity of the insoluble solid that is formed when sodium carbonate reacts with aqueous hydrochloric acid?

a) no solid form is formed

b) NaCl

c) NaO

d) NaOH

e) H2CO3

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Which of the following statements is/are INCORRECT?

1. Water soluble ionic compounds, such as NaCl, are all strong electrolytes.

2. Some acids are strong acids while others are weak.  The acids HF and HNO2 are examples of strong acids.

3. Only Group 1 (1A) hydroxides are soluble in water.

 

a. 1 and 2 only

b. 2 and 3 only

c. 1 and 3 only

d. 1 only or 2 only or 3 only

e. 1, 2, and 3

 

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How many of the following compounds are soluble in water?

CuS                 K2SO4             NH4NO3         Ca3(PO4)2          AgBr

A) 1

B) 2

C) 3

D) 4

E) 5

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The SOLUBILITY of a compound represents the maximum amount of solute that dissolves in a solvent.

 

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Which of the following reagents could be used to separate the two cations from a solution containing Lead (IV) acetate and cesium permanganate?
a)    Sr(NO3)2
b)    TiC2H3O2
c)    K2S
d)    NaClO4
e)    KNO3

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Which of the following reagents could be used to separate the two anions from a solution containing magnesium nitrate and cesium hydroxide?
a)    NH4CN
b)    NaCl
c)    KNO3
d)    ZnBr2
e)    CsBrO3

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Which of the following statements is/are true?

i. Na3PO4 is soluble
ii. Cu(C2H3O2)2 is insoluble
iii. Hg2Cl2 is insoluble

a. iii only
b. i, ii and iii
c. ii and iii
d. i and iii
e. i and ii

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Which one of the following reactions does not produce a precipitate?

a) 2LiI (aq) + BaS (aq) →

b) Pb(NO3)2 (aq) + 2NaCl (aq) →

c) Li2CO3 (aq) + CaS (aq) →

d) NH4Br (aq) + AgNO3 (aq) →

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Which interactions are stronger in a dilute aqueous NaCl solution?

A. Solute-solute

B. Solvent-solute

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If you take an ionic compound, dissolve it into water, and then carry out the following steps:

I. Add 50 mL of 2.0 M Ca(NO 3)2 solution. This produces a white precipitate.

II. Next add 50 mL of 2.0 M HCl solution. This causes the precipitate to dissolve and produces a lot of bubbles, but no apparent odor

Which of the following anions (from the original ionic compound) would be consistent with these results.

a) OH -

b) SO2-

c) S 2-

d) CO2-

e) PO3-

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The following aqueous solutions are mixed: 1.00M NaCl(aq) and 1.00M KNO 3(aq). Which statement is true?

 

a. No reaction occurs since all possible species are soluble in water.

b. No reaction occures since NaCl and KNO 3 are both insoluble to begin with.

c. NaNO3 precipitates from the solution

d. KCl precipitates from the solution

e. None of the above statements is true. 

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Equal volumes of 1.00M of NaCl and 1.00M Pb(NO 3)2 are mixed. Which statement below best describes what happens?

 

a. All the Pb(NO3)2 precipitates and excess NaCl remains.

b. All the chloride ions in the NaCl are precipitates as PbCl 2.

c. No nitrate ions remain in solution.

d. No sodium ions remain in solution.

e. No Pb2+ ions remain in solution.

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Which of the following reagents could be used to separate the two cations from a solution containing Cobalt (I) nitrate and sodium hydroxide?

 

a) Ca(C2H3O2)2

b) Sc(NO3)3

c) BaCO3

d) LiClO4

e) NaNO3

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Which of the following reagents could be used to separate the two anions from a solution containing magnesium nitrate and cesium hydroxide?

 

a) NH4CN

b) NaCl

c) KNO3

d) ZnBr2

e) CsBrO3

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Separate samples of a solution of an unknown salt are treated with dilute solutions of HBr, H2SO4, and NaOH. A precipitate forms in all three cases. Which of the following cations could the solution contain:

 

a) Pb2+

b) Ba2+

c) Na+

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Some Pb(NO3)2 (aq) and KI (aq) are combined and a yellow precipitate forms. Which of the following has the highest concentration in the supernatant?

 

a) K+ (aq)

b) NO3 - (aq)

c) Pb2+ (aq)

d) I- (aq)

e) It depends on what was the limiting reactant

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