To calculate the standard enthalpy change for the phase transition reaction from solid to liquid for a sample of substance X, we need to consider the enthalpy of fusion for the substance.The enthalpy of fusion is the amount of energy required to change a substance from the solid phase to the liquid phase at its melting point. In this case, the enthalpy of fusion for substance X is given as 50 J/g.Given that the mass of the sample is 10 grams, we can calculate the total enthalpy change for the phase transition by multiplying the mass of the sample by the enthalpy of fusion:Total enthalpy change = mass enthalpy of fusionTotal enthalpy change = 10 g 50 J/gTotal enthalpy change = 500 JTherefore, the standard enthalpy change for the phase transition reaction from solid to liquid for a sample of substance X is 500 J.