A water solution contains 60% Na2S2O2 together with 1% soluble impurity. Upon cooling to 10°C, Na2S2O2 # 5 H2O crystallizes out. The solubility of this hydrate is 1.4 lb Na2S2O2 # 5 H2O/lb free water. The crystals removed carry as adhering solution 0.06 lb solution/lb crystals. When dried to remove the remaining water (but not the water of hydration), the final dry Na2S2O2 # 5 H2O crystals must not contain more than 0.1% impurity. To meet this specification, the original solution, before cooling, is further diluted with water. On the basis of 100 lb of the original solution, calculate (a) the amount of water added before cooling, and (b) the percentage recovery of the Na2S2O2 in the dried hydrated crystals.
Hint: Treat the hydrated crystals as a separate stream leaving the process.
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