The ΔH∘c value of cyclobutane is given. From this if we substract the ΔH∘c value of cyclobutane calculated when there is no angle strain, we will get the value of ring strain of cyclobutane.
The ΔH∘c value of cyclobutane when there is no ring strain can be calculated from ΔH∘c value of cyclohexane per CH2 group.
Ring strain = (ΔH∘c of cyclobutane - 4×ΔH∘c of per (CH2) group of cyclohexane) = 2744 -
(4×660) = 104 kJ mol−1