Answer:
Because in elastic collisions there is no heat emission or absorption.
Explanation:
A collision is considered elastic when the total kinetic energy of the study system is conserved during the collision. Since the total kinetic energy is conserved, heat is not emitted or absorbed during the collision. Since the emission or absorption of heat is what produces changes in temperature, If the system remains at a constant temperature, there were only elastic collisions.
Isotope 1: 89.905 * 51.45 = 4625.61225 / 100 = 46.2561225
Isotope 2: 90.906 * 11.22 = 1019.96532 / 100 = 10.1996532
Isotope 3: 91.905 * 17.15 = 1576.17175 / 100 = 15.7617075
Isotope 4: 93.906 * 17.38 = 1632.08628 / 100 = 16.3208628
Isotope 5: 95.908 * 2.08 = 268.5424 / 100 = 2.685424
46.2561225 + 10.1996532 + 15.7617075 + 16.3208628 + 2.685424 = 91.22377
actual mass Zr = about 91.22