Please Help!! I don't even know where to start.?

Question

A mixture had 3 compounds found in the table below.

Compound

soluble in cold water
benzoic acid n
Mg (OH)2 n
Zn(OH)2 n

soluble in hot water
benzoic acid y
Mg (OH)2 n
Zn(OH)2 n

soluble in3M HCl:
benzoic acid n
Mg (OH)2 y
Zn(OH)2 y

soluble in 3M NaOH
benzoic acid y
Mg (OH)2 n
Zn(OH)2 y


After the mixture of the 3 solids was extracted with hot water & filtered, Compound A was obtained by evaporating the filtrate to dryness.
When 3M HCL was added to the solid residue, a clear solution resulted.
After adding excess 3M NaOH solution to the acid solution, a precipitate formed.
After filtration, the insoluble solid was found to be compound B. Compound
C was recovered by evaporation of the filtrate.

What is Compond A, B, and C? and please help with the explanation. Thaks.

Answer 1

In the extraction with hot water, benzoic acid dissolved, (Compound A)and was obtained again after the hot water was evaporated. The other 2 did not dissolve so were in the filter paper.

When HCl is added to the residue, both the Mg(OH)2 and the Zn(OH)2 dissolved. When it is neutralized, the Mg(OH)2 precipitates out because it is not soluble in NaOH.Solution B is Mg(OH)2

Compound C is the Zn(OH)2 which is left and obtained on evaporation of the filtrate.

Answer 2

OK. You gotta follow the bouncing ball. Each heading of solubility lists the 3 compounds. If it is soluble, a "y" follows". If not, an "n" follows. If you understand this, the rest is fairly (ha,ha) easy.

If we extract in HOT WATER, going to the hot water entry, the two hydroxides should be solids. The filtrate contains the soluble benzoic acid, which can be evaporated. This is compound "A"

Both hydroxides dissolve when 3N HCl is used to wash them. The subsequent 3M NaOH treatment dissolves the zinc hydroxide, but not the magnesium hydroxide. So the magnesium hydroxide is "B", and the subsequently filtered and evaporated compound is zinc hydroxide.

Answer 3

A = benzoic acid It was soluble in hot water, the other 2 were filtered off.
B= Mg(OH)2 - both this & Zn(OH)2 were sol in HCl but addn of NaOH dissolved Zn(OH)2 leaving Mg(OH)2 as solid recovered.
C=Zn(OH)2 which dissolved in NaOH