Q3. Show that the two formulas the Lab Manual gives for potash alum, K2SO4·Al2(SO4)3·24H2O and 2KAl(SO4)2·12H2O, describe the same substance, and find its molar mass per KAl(SO4)2·12H2O. (K = 39.10, Al = 26.98, S = 32.07, O = 16.00, H = 1.008.)
Both contain 2 K, 2 Al, 4 SO4 and 24 H2O: the second is the first divided into two equal halves (Lab Manual page 100). One half, KAl(SO4)2·12H2O, is the formula NCERT gives (NCERT page 120). M(KAl(SO4)2·12H2O) = 39.10 + 26.98 + 2 × 96.07 + 12 × 18.016 = 474.4 g mol−1 (reasoned).
Q4. Why are the crystals washed with a 1 : 1 mixture of cold water and alcohol rather than with plain water?
The wash removes the mother liquor sticking to the crystals (Lab Manual page 101). Cold water mixed with alcohol dissolves less of the salt than plain water would, so less of the product is lost (reasoned).
Q5. Why do we take equimolar quantities of the reacting compounds in the preparation of double salts? (Lab Manual, Discussion Question (i), page 101.)
The double salt holds the two salts in a fixed 1 : 1 ratio (Lab Manual page 100). A near-equimolar feed matches the product composition and limits excess starting salt. Some product remains in the mother liquor because of its solubility, so crystallisation is not automatically quantitative; excess component can also affect purity (reasoned).