Calculate the minimum mass of CaO to attain 21,900 joules.

Question

Calculate the mininum mass of calcium oxide, CaO(s), that a student must use to attain the amount of heat of a 150.0 g sample of water from 25.0 degrees C to 60.0 degrees C. (I already calculated that 150.0 g of water from 25.0 degrees C to 60.0 degrees C is 21,900 J but how do I find the minimum mass of CaO?)

Answer 1

So, now you need to know how much heat is generated by 1 mole of CaO.  Looking up this value, one finds it to be

-635.5 kJ/mole.  You need 21.9 kJ (amount YOU calculated from water), so

635.5 kJ/mole *(x mole) = 21.9 kJ

x = 0.034 moles CaO

mass CaO = 0.034 moles x 56 g/mole = 1.90 grams (3 significant figures)