When 3.0g of ethane, C2H6, is formed, 8.5 kJ of heat is produced. What is the heat of formation of ethane.2C+3H2 --> C2H6
Heat of formation of a compound is the heat produced when one mole of the compound is produced.
The molar mass of ethane, which has a chemical formula of C2H6, is equal to 2*12 + 6*1 = 30 g/mol.
When 3 grams of ethane is formed the heat produced is 8.5 kJ.
3 grams of ethane is equal to 3/30 = 1/10 moles. As the formation of 0.1 moles of ethane requires 8.5 kJ, to produce 1 mole we require 10 times that amount or 10*8.5 = 85 kJ.
The heat of formation of ethane is 85 kJ/mol.