In principle, those two things measure different things (mass, and energy). To get the mass associated with a certain amount of energy, divide the number of joules (1 kJ = 1000 joules) by the square of the speed of light, i.e., divide by 9 x 1016 (m2/sec2). The answer is in kilograms; you an multiply this answer by 1000 to convert into grams.
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