<p>Is it always present in hydrated form?</p>
<p>Excess concentrated HCl is added to a 1.0M solution of cobalt (II) chloride.</p>
<p>I thought it would be Cl- + Co2+ --> [CoCl4]2- </p>
<p>Both of these answers are considered correct:</p>
<p>Cl- + [Co(H2O)6]2+ -> H2O + [CoCl4]2-
(I don't get why that one has the hydrated form)</p>
<p>or</p>
<p>Cl- + Co2+ -> H2O + [CoCl4]2-
(I don't get why that doesn't even seem balanced.)</p>
<p>So are you saying that Cl- + Co2+ --> [CoCl4]2- is not right? I would've written that as well.</p>
<p>Wxmann - your answer was accepted in the scoring rubric. The last answer thecomisar gave (showing H2O appearing out of nowhere) is a mistake. It's given on an answer key that is commonly available on the web, but it's not on the official collegeboard scoring guideline. (The second option he showed with the hydrated cobalt on the left and free water on the right is the other acceptable answer.)</p>
<p>Is the rule you use for complex ions if you have no idea is to have 2 times as many ligands as the central atom's charge then add up the charges for the complex ion charge?</p>
<p>^^Yes, that's correct</p>
<p>If I made up a random complex ion but with correct charge than that is correct? For example: It asks you if sodium cyandie soltuion is added to Iron(II) chloride you could say that
(Fe(CN)4) 2- would form.</p>
<p>Generally, yes, but look for the words excess/concentrated. If either of those is present, and both the cation and anion are on the list of species that typically form complexes, then just trust that that's what they're asking and trust the rule.</p>
<p>Two exceptions.
Al and OH- are exceptions to the twice the charge rule. [Al(OH)4]-
Fe3+ and SCN- is an exception to the "excess/concentrated" guideline. Fe3+ is also ususally only takes one SCN ligand - [Fe(SCN)]2+, but I've seen them give credit for [Fe(SCN)6]3-.</p>
<p>Ex 1997a) Excess potassium hydroxide is added to a solution of aluminum nitrate. (complex ion reaction - "excess" and both species on list)</p>
<p>Ex 1992a) An excess of sodium hydroxide is added to a solution of magnesium nitrate. (not a complex ion reaction - Mg not on list)</p>
<p>Ex 1989f) A solution of ammonium thiocyante is added to a solution of iron(III) chloride. (Complex ion reaction - both Fe and SCN on list, "excess" not required.)</p>