Concerete is produced from a mixture of cement , water, sand and small stones. It consists primarily of calcium silicates and calcium aluminates formed by heating and grinding of clay and limestone. In later steps of cement production a small amount of gypsym , `CaSO_(4).2H_(2)O` is added to improve subsequent hardening of concrete . The use of elevated temperature during the final production may lead to formation of unwanted hemihydrate, `CaSO_(4).1//2H_(2)O.` Consider the following reaction:
`CaSO_(4).2H_(2)O(s) rarr CaSO_(4).(1)/(2) H_(2)O(s)+(3)/(2)H_(2)O(g)`
The following themodynamic data apply at `25^(@)C` standard pressure : 1 bar
`{:("Compound","DeltaH_(f(KJ//mol))^(@),S^(@)(JK^(-1)mol^(-1))),(CaSO_(4).2H_(2)O(s),-2021.0,194.0),(CaSO_(4).(1)/(2)H_(2)O(s),-1575.0,130.5),(H_(2)O(g),-242.8,188.6):}`
R=8.314 `JK^(-1)mol^(-1)`
`DeltaH^(@)` for the formation of 1.00 kg of `CaSO_(4).(1)/(2) H_(2)O(s)` from `CaSO_(4).2H_(2)O(s)` is

Options

A. `+446 KJ`
B. `+484 KJ`
C. `-446 KJ`
D. `-484 KJ`

Answer

B

Explanation

`DeltaH^(@) = H[CaSo_(4).(1)/(2)H_(2)O(s)] + H[(3)/(2)H_(2)O(g)] implies -H[CaSO_(4).2H_(2)O(s)] = 833 KJ "mol"^(-1) =+ 484 KJ " for " 1 Kg`