Problem: I'm trying to calculate the Q-value for the nuclear reaction H-3 + p > He-3 + n, where H-3 is Deuterium, p is a proton, He-3 is Helium 3 and n is of course a neutron. Doing the calculation, I get -1.2715 MeV for the Q, but my textbook and an online 'Q-calculator' both give -763.15 KeV - I cannot work out where I'm going wrong - can anybody help?! My method: The Q-value for a reaction is given by the difference in the initial and final masses of the system (times the speed of light squared), and is equal to the energy 'liberated' by the reaction. I.e. Q = (Mi - Mf)*c^2. Now Mi = M(3H) + M(p) = 3.016049u + 1.00728u = 4.023329u and Mf = M(3He) + M(n) = 3.016029u + 1.008665u = 4.024694u Mi - Mf = -0.001365 Q = -0.001365 * 931.5 MeV/u = -1.2715 MeV But all other sources that I can find listing the Q for this reaction give -0.76375 MeV. Where am I going wrong and how do I make it better?!?