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If we have a solution that contains water and potassium sulfate (K2SO4).

And we add to this solution :

Du magnesium chloride (MgCl2) and lead nitrate (II) (Pb(NO3)2)

What would the compounds that form the solid (chemical formulas)?

Where is my mistake?

2 H2O + K2SO4 --> 2 KOH + H2SO4

MgCl2 +Pb(NO3)2 --> PbCl2+Mg(NO3)2

I feel something that doesn't make sense

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Your first reaction is wrong. You can't just form sulphuric acid from its salt, or more simply sulfate. It will just form bisulfate in some extent, with an equilibrium reaction, not a one way arrow.

PbCl2 formation is expected since it is poorly soluble it will be precipitated.

Look for the most insoluble union in similar cases, it will be precipitated.

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In the first reaction with water and potassium sulfate, Postassium Sulfate (K2S04) does not react with water(H2O) but rather dissolves in it. And since it readily dissolves in water, it is considered readily soluble and there is no hydrolysis. It neither releases or absorbs and energy and is considered a dissociation reaction. In the second reaction with the magnesium chloride and lead nitrate, the products will be PbCl2(s) and Mg(NO3)2(aq). Lead chloride is a solid because lead and chloride and form a nonsoluble compound whereas NO3 is very soluble so it makes Mg(NO3) soluble in an aqueous solution.

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  • $\begingroup$ Sulfate does react with water to form HSO4- to some extent, you should reconsider the first part of your answer. Check the analytical chemistry textbooks for polyprotic acids dissociation constants. $\endgroup$ – Güray Hatipoğlu Apr 23 '18 at 9:57

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