Question

A mixture of two white substances is soluble in water. This solution gives brown colour gas on
passing chlorine gas. Another sample of solution gives white precipitate with $BaC{l_2}$ which is
insoluble in concentrated $HCl$. The original solution of the mixture gives white precipitate with large
excess of $NaOH$ solution whose suspension is used as an antacid. After filtering the precipitate, the
filtrate was boiled with excess $NaOH$ solution which gave yellowish precipitate on adding
$NaCl{O_4}$. One of the compounds of the mixture forms alum. Identify the mixture.
(A) Mixture consists of ${K_2}S{O_4}{\text{ }}and{\text{ }}MgB{r_2}$
(B) Mixture consists of ${K_2}S{O_3}{\text{ }}and{\text{ }}MgBr$
(C) Mixture consists of ${K_2}S{O_4}{\text{ }}and{\text{ }}MgBr$
(D) None of these

Answer 1

The brown gas evolved is Bromine gas, which gets displaced due to reaction with chlorine gas.
Another statement says that Barium chloride reacts with this salt to give white precipitate which is of
barium sulphate. Magnesium salts are used as antacids. The substance used in Alums are Potassium and
aluminium.

Complete step by step answer:
The brown gas which is coming out is bromine gas, so the presence of Bromine ion is there in one of the
white salts, which we get when chlorine is replaced.
Now, in antacids we know that magnesium salts are used, so presence of magnesium ion is there in this
salt.
Thus we know now that one of the salts has Magnesium ion and bromine ion which has valency of +2 and
-1 respectively will form a salt called Magnesium bromide which is white colour salt with formula of
$MgB{r_2}$
The reactions can be written in below ways:
$MgB{r_2} + C{l_2} \to MgC{l_2} + B{r_2} \uparrow$
Here brown colour bromine gas is released by displacement with Chlorine gas.
Now, the reaction of salt with large amount of $NaOH$ will give white precipitate whose suspension can
be used as an antacid. This antacid formed is magnesium hydroxide which is white precipitate used as an
antacids. Thus the reaction can be written as:
$MgB{r_2} + 2NaOH \to Mg{(OH)_2} \downarrow + 2NaBr$
Another salt gives white precipitate with barium chloride, so we know that barium chloride is soluble but
barium sulphate salt is not soluble in water and thus it produces white solid precipitate. So this indicates
presence of sulphate ions.
Now one of the salt is used in formation of alum, and we know formula of potash alum is
$KAl{(S{O_4})_2}.12{H_2}O$
Now we may have presence of potassium ion or Aluminium ion, but one more information is given that
this reacts with $NaCl{O_4}$. And we know that only potassium being more reactive can react not
aluminium, as it is less reactive than Sodium. So there is presence of Potassium ion.
Now in second salt we have presence of potassium ions and sulphate ions, which has valency of +1 and -2
respectively. Thus the chemical formula can be written as ${K_2}S{O_4}$
Now we can write reactions of potassium sulphate with barium chloride as:
${K_2}S{O_4} + BaC{l_2} \to 2KCl + BaS{O_4} \downarrow$
Thus white insoluble precipitate of Barium sulphate is formed.
Another reaction of potassium sulphate with sodium perchlorate can be written as:
${K_2}S{O_4} + 2NaCl{O_4} \to N{a_2}S{O_4} + 2KCl{O_4}$
This reaction is taking place as potassium is more reactive than sodium.
Thus the salts which are white in colour are Potassium sulphate and Magnesium bromide with formula of
${K_2}S{O_4}{\text{ }}and{\text{ }}MgB{r_2}$.
Thus the correct option is (A) mixture consists of ${K_2}S{O_4}{\text{ }}and{\text{ }}MgB{r_2}$.

Note:
This question relates to salt analysis, which is a little complicated and confusing. As many ions give
similar results. So we can get confused with ions but if we read carefully the given question or
observation then there is some unique information which relates to specificity of ions and we can come
to conclusions easily.
Another important thing to note is valency, as we may know the presence of ions but if we don’t know
valency then it will be difficult to form salts from the given ions.
We should remember the properties and characteristics of ions, colour, precipitate and other similar
Properties.