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I was wondering what salts of gold are soluble and which are insoluble. From what I understand, silver is soluble as silver nitrate but insoluble as most other common salts like chlorides and sulphates. How does this work with gold? I was also wondering if there are any rules for solubility of salts, or whether there is some explanation for the solubility that would help in guessing the solubility of a salt?

I started thinking about this because of a question I came across in which there was a mixture of gold, silver and copper to which hot concentrated nitric acid was added, and an undissolved residue of given mass remained. Then $\ce{HCl}$ was added and more precipitate was formed. The question asked you to find the percentage by mass of gold, silver and copper. Judging by the general solubility trend of these metals, I'm guessing the first precipitate was gold and the second silver, but I was also wondering if there are some solubility rules...

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Old question, but I thought I'd add to it:

In your example, the first undissolved solid is indeed gold. It's not easily dissolved by $\ce{HNO3}$ alone, but requires a mixture of $\ce{HNO3}$ and $\ce{HCl}$ (aqua regia). $\ce{Ag}$ and $\ce{Cu}$ would dissolve by hot $\ce{HNO3}$. The precipitate formed by addition of $\ce{HCl}$ is indeed $\ce{AgCl}$, so you're on the right track.

A brief list from experience: [At room temperature in aqueous solution]

$\bullet \ce{Au2S}$: insoluble (quite sure this is the precipitate formed in basic solution when sodium metabisulfite is added to $\ce{HAuCl4}$. Either that or $\ce{Au2S3}$).

$\bullet \ce{AuCl3}$: highly soluble (can be formed by dehydrating and evaporating $\ce{HCl}$ from $\ce{HAuCl4}$, which is the product of $\ce{Au}$ dissolved in aqua regia).

$\bullet \ce{KAu(CN)4}$: soluble

$\bullet \ce{KAu(CN)2}$: soluble

$\bullet \ce{AgCl}$: insoluble

$\bullet \ce{CuCl2}$: highly soluble

$\bullet \ce{CuCl}$: soluble, but harder to synthesize than $\ce{CuCl2}$

$\bullet \ce{(Au/Ag/Cu)(NO3)_x}$: soluble ($x = 1,2$)

I think the overall result here is that gold, silver, and copper salts generally follow one of the primary trends of solubility: that is, that $\ce{NO3-}$ salts are always soluble regardless of cation, and $\ce{Cl-}$ are usually soluble except in cases $\ce{Ag+}$, $\ce{Pb^{2+}}$ and $\ce{Hg+}$.

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Here is What I know:
Gold(III) chloride is Highly Hygroscopic [1] and is highly soluble in water, I don't know which salts are not soluble never researched it, maybe Gold-Mercury amalgam is not soluble in water but its not a salt..
[1] = absorbing or attracting moisture from the air.

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