Question

How much heat is evolved when 1249 g of water condenses to a liquid at 100.°C?

Answer 1

n(H₂O) = m(H₂O) / Mr(H₂O)

n(H₂O) = 1249 g / 18 g/mol = 69.4 moles

∆𝐻𝑣𝑎𝑝 = −∆𝐻𝑐𝑜𝑛𝑑

The molar heat of vaporization of water is ∆𝐻𝑣𝑎𝑝 = 40.7𝑘𝐽 𝑚𝑜𝑙

∆𝐻 =69.4 𝑚𝑜𝑙 𝐻2𝑂∙ (−40.7𝑘𝐽/1𝑚𝑜𝑙 𝐻2𝑂) = - 2824.6 𝑘𝐽

The negative sign indicate that heat is given off.

Therefore, the process will release 2824.6 𝒌𝑱 of heat.