Question

An aqueous solution of $FeS{O_4}A{l_2}{\left( {S{O_4}} \right)_3}$ and chromium alum is heated with excess of $N{a_2}{O_2}$ and filtered. The material obtained is:
A.A colourless filtrate and green residue
B.A yellow filtrate and brown residue
C.A yellow filtrate and a green residue
D.A green filtrate and a brown residue

Answer 1

We get the residue and filtrate when we apply the process of filtration. The filtrate is the liquid that passes through the filter whereas residue is the mass remaining on the Filter paper after the filtration process.

Complete step by step answer:
The given salt is a double salt of Ferrous Sulphate and Aluminium Sulphate. It is known as halotrichite and is a double salt as it contains two different cations, $F{e^{2 + }}$ and $N{H^{4 + }}$. Chromium alum alternatively known as Chromium (III) potassium sulphate is represented by the chemical formula $KCr{\left( {S{O_4}} \right)_2}$. When halotrichite and chromium alum is heated in excess of sodium peroxide ($N{a_2}{O_2}$) and the resulting solution is filtered the observation is discussed below:
Chromium ions ($C{r^{2 + }}$) are oxidized to form chromate ions ($Cr{O_4}^{2 - }$) and the Ferrous ($F{e^{2 + }}$) ions are changed to Ferric ions ($F{e^{3 + }}$). The chromate ions formed are a yellow colour and are soluble in water and hence the filtrate is yellow in colour whereas the Ferric ions obtained are brown in colour as is insoluble hence, the brown colour residue is obtained when the resulting solution is filtered.

Hence the correct answer is option B.

Note:
Ferrous Aluminium sulphate also known as feather alum, exists in nature as halotrichite and is prepared when equal moles of ferrous and aluminium sulphates react. The salt is white in colour and is water-soluble. It is naturally available around volcanic vents and is formed by the decomposition of pyrite. Chromium alum exists in aqueous solution in dark violet colour and was used in tanning of leather.