<p>A sample of a solution of an unknown was treated with dilute HCl. The white precipitate formed was filtered and washed with hot water. A few drops of KI solution were added to the hot water filtrate and a bright yellow precipitate was produced. The white precipitate remaining on the filter paper was readily soluble in NH3 solution. What two ions could have been present in the unknown?</p>
<p>(A) Ag+ and Hg2 2+
(B) Ag+ and Pb2+
(C) Ba2+ and Ag+
(D) Ba2+ and Hg2 2+
(E) Ba2+ and Pb2+</p>
<p>The answer is B, but how on earth do you do that??</p>
<p>Yes, the white precipitate is AgCl and PbCl2. A helpful suggestion - there are quite a few details to the solubility rules, but they vary in significance. One of the important rules you must know is that halides are soluble except for PAM (lead, silver, mercury). In the given question, this main rule lets you eliminate C, D and E. If you can reduce a question to a 50/50 guess, it’s worth flipping the coin and answering the question. The complete answer requires you to remember one of two finer details. You need to either remember that lead halides are the most soluble of the three (sufficiently more soluble to dissolve in hot water) or you need to remember colors of precipitates. (Note - the PAM acronym is in order of decreasing halide solubility.) The ammonia information isn’t necessary to answer the question. Formation of a complex with ammonia confirms that silver was present, and would have let you discard mercury/lead if that option had been provided.</p>
<p>“Formation of a complex with ammonia confirms that silver was present”</p>
<p>The problem states that it was soluble. Salt is oluble in water but forms no complex. </p>
<p>How do you know what is soluble in ammonia?</p>
<p>“One of the important rules you must know is that halides are soluble except for PAM”</p>
<p>You’re oversimplifying halide rules. These need to be known cold.</p>
<p>Here they all are:</p>
<p>Soluble Compounds have:</p>
<p>all nitrates
all acetates except silver acetate
all chlorates (and other chlorates like ClO-)
chlorides except for AgCl, Hg2Cl2, PbCl2
bromides except AgBr, PbBr2, Hg2Br2 and HgBr2
iodides except AgI, Hg2I2, HgI2, and PbI2
Sulfates except Ba, Pb, Hg2SO4, Ca, Ag, Sr
all Alkali metal cations and NH4+
H+ common inorganic acids and low molecular mass organic acids</p>
<p>**Insoluble:<a href=“CO3”>/B</a>2- except group 1 and NH4+
(CrO4)2- except group 1 and NH4+, CaCrO4 and SrCrO4
OH- except group 1 and NH4+, Ba, Sr, Ca
(PO4)3- except group 1 and NH4+
(SO3)2- except group 1 and NH4+
(S)2- except group 1 and NH4+, and group 2</p>
<p>“A few drops of KI solution were added to the hot water filtrate and a bright yellow precipitate was produced. The white precipitate remaining on the filter paper was readily soluble in NH3 solution.”</p>
<p>Hmm… PbI2 and AGI precipitate should have formed. They are both insoluble.</p>
<p>I think (but am not sure) that a key word in this problem was yellow. Just think: lead paint was made of colorful lead compounds.</p>
<p>Thanks for your help. I did have a list of solubility rules, but it wasn’t as comprehensive as what you put. Silver acetate insoluble? – why had I never heard that before? :eek: and chromates and sulfites weren’t even on my list.</p>
<p>“Hmm… PbI2 and AGI precipitate should have formed. They are both insoluble.”
AgI wouldn’t form because KI was added to the <em>filtrate</em>. All of the Ag was left on filter paper in the form of AgCl. Only the PbCl2 washed off the filter paper with the hot water, thus only Pb2+ ions were available to form iodide compounds.</p>
<p>“How do you know what is soluble in ammonia?”
I’ll quote something I posted a few weeks ago.<br>
“Metal ions that commonly form coordination compounds (complex ions):Al3+, Cu2+, Co2+, Zn2+, Fe2+(or 3+), Ni2+, Ag1+. (All Curiously Colored Zebras Felt Nicely Agreeable).
Common ligands: NH3, OH1-, Cl1-, SCN1-, CN1-
Most common coordination number: twice the charge of the metal ion.”
That means that Ag1+ + NH3 –> Ag(NH3)2 1+. Formation of complex ions is a common means to dissolve otherwise insoluble salts. In a solution with only water present, AgCl only dissociates (dissolves) slightly to form Ag1+. When ammonia is added, the Ag1+ complexes with the ammonia, and the removal of the Ag1+ from the solution as it converts Ag(NH3)2 1+ pulls the AgCl dissociation equilibrium to the right (Le Chatelier’s principle). If sufficient ammonia is added to complex all of the silver ions, the AgCl will completely dissolve.</p>
<p>“You’re oversimplifying halide rules. These need to be known cold.”
Only if you want a perfect score. A much simpler set of rules is sufficent to score a five for several reasons. First, <em>textbooks</em> don’t agree on the definition of soluble and insoluble, so every textbook has a unique set of solubility rules, and every teacher teaches a different set of “rules.” Therefore, the AP test can’t test on nuances - no one would agree on the answers.
Second, solubility rules have traditionally been tested most heavily in question 4 (Reaction Writing). Question 4, however, <em>only gives reactions that work.</em> You don’t have to remember which halides form insoluble salts with mercury(I) and which form insoluble salts with mercury(II), because they will only give a reaction that actually forms a precipitate. If the reaction says “a solution of sodium chloride reacts with a solution of mercury(I) nitrate”, you darn well better write Hg2Cl2 as a solid product in the net ionic reaction because “No Reaction” is not a valid response. There’s no need to try and remember whether it’s mercury(I) or mercury(II) that forms insoluble chlorides.
Third, the next most common place that solubility rules shows up is in the MC section with questions such as “100. mL of 0.100M lead(II) nitrate is mixed with 100 mL of .100M sodium chloride. What is the concentration of Pb2+ ions in the resulting solution?” In these types of questions, they aren’t really testing your knowledge of solubility rules, they’re testing your ability to do the calculation. The solubility knowledge required is rudimentary. They <em>always</em> give obvious precipitates in these questions.</p>
<p>“Soluble compounds … H+ common inorganic acids and low molecular mass organic acids.”
Careful - there is a difference between solubility and dissociation. “Solubility rules” are actually usually used to determine whether compounds should be written in the dissociated form in net ionic reactions. Common inorganic and low molec. weight organic acids are soluble, but should <em>not</em> be written dissociated. Only six acids dissociate completely in water.</p>
<p>Simplified dissociation (solubility) rules needed to get a five:
Strong acids and strong bases dissociate.
oxides and hydrides are insoluble
Sulfates are soluble except CaBaSr.
ANA is soluble - Ammoniums, nitrates, and acetates.
Group 1 and Group 7 compounds are soluble except when Group 7 is paired with PAM.
-2 ions are insoluble
-3 ions are insoluble</p>
<p>Note: Strong bases are hydroxides combined with Group 1 metals or CaBaSr. Strong acids are the BIC PeNS. [BIC PeNS stands for hydrobromic, hydroioidic, hydrochloric, perchloric, nitric, and sulfuric. Only the first H comes off of sulfuric.] All other acids are weak.</p>
<p>The above rules are in order of hierarchy. For example, consider sodium carbonate. Rule 5 says it is soluble because it’s a group one metal. Rule 6 says it’s insoluble because it’s a -2 anion. Rule 5 wins because its higher on the list - sodium carbonate is soluble.</p>
<p>Now, I <em>know</em> this is not a complete list of rules, nor does it capture every exception. But remember, there isn’t one set of rules that everyone agrees on, and this one <em>works</em>. The seven rules above cover every precipitation reaction in question 4 (not counting complex ions) since 1981. The seven rules above also cover every solubilty question in the MC section (not counting complex ions) since 1984. These rules also (when combined with some color knowledge) cover all the qual questions that have shown up in free response since 1999. (I haven’t looked much at FR questions older than 1999.) Memorize the more detailed set of rules if you want, or save some time and focus on something else.</p>
<p>Oops - forgot the three oddbals…
Addendum to the dissociation rules: There are 3 oddball chemicals that have unusual dissociations. The three oddballs are:
Carbonic acid (H2CO3) –> H2O + CO2
Sulfurous acid (H2SO3) –> H2O + SO2
Ammonium hydroxide (NH4OH) –> NH3 + H2O</p>
<p>Ex: Solid ammonium carbonate is added to a saturated solution of barium hydroxide. (unbalanced)
(NH4)2CO3 + Ba2+ + OH- –> NH3 + H2O + BaCO3</p>
<p>“Ammonium carbonate decomposes … Are you sure…?”</p>
<p>The example and answer I quoted are from the 1996 AP exam, Question 4, choice f. The key is that ammonium carbonate was added to a solution - this a simple double replacement reaction, with solubility rules applied. For ammonium carbonate to decompose it must be heated.</p>
<p>Ex: 1982 choice (d): “Solid ammonium carbonate is heated.”
Answer: (NH4)2CO3 –> NH3 + CO2 + H2O</p>
A much simpler set of rules is sufficent to score a five for several reasons. First, <em>textbooks</em> don’t agree on the definition of soluble and insoluble, so every textbook has a unique set of solubility rules, and every teacher teaches a different set of “rules.” Therefore, the AP test can’t test on nuances - no one would agree on the answers.