TEST OF PRACTICAL KNOWLEDGE QUESTION (a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glo...

Answer Details

(a) Identifying the zinc salt E

On strong heating E gives a brown pungent gas (nitrogen(IV) oxide, \(NO_2\)), a colourless gas that rekindles a glowing splint (oxygen, \(O_2\)) and a solid residue. These are the products of thermal decomposition of a zinc nitrate.

• (i) E is zinc trioxonitrate(V), \(Zn(NO_3)_2\).
• (ii) Equation for the decomposition:

• (iii) The residue is zinc oxide, which is yellow while hot but turns white on cooling.

(b) Preparing 250 cm\(^3\) of 0.2 mol dm\(^{-3}\) H2SO4 from a 1.0 mol dm\(^{-3}\) stock

Moles of acid required \(= 0.2 \times \dfrac{250}{1000} = 0.05\,mol\).

Volume of stock needed \(= \dfrac{0.05}{1.0} = 0.05\,dm^3 = 50\,cm^3\).

Procedure: using a pipette or measuring cylinder, measure 50 cm\(^3\) of the 1.0 mol dm\(^{-3}\) stock acid and add it slowly, with stirring, to some distilled water already in a 250 cm\(^3\) volumetric flask (always add acid to water). Then make up the solution to the 250 cm\(^3\) mark with more distilled water, stopper and shake to mix thoroughly. (The 150 cm\(^3\) of stock available is more than the 50 cm\(^3\) required.)

(c) Effect of aluminium tetraoxosulphate(VI) solution on litmus

Aqueous \(Al_2(SO_4)_3\) turns blue litmus red, i.e. the solution is acidic. This is because the salt of a strong acid and a weak base undergoes hydrolysis to give an acidic solution.