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Kg is the SI unit for mass, just as, for example, meters is the SI unit for distance and seconds is the SI unit for time. Mass is sort of unusual as it is one of the only SI units with a prefix. In fact, it might actually be the only SI unit with a prefix, but I'm not sure.
Adding on to the reply above, because kg is the SI unit for mass, it is a part of the units for the constants. Plank's constant has units of J*s, and a joule is kg*m^2*s^-2. Because the units of Plank's constant incorporate kg, it is needed that the mass in de Broglie's equation is measured in kg so the units can cancel in the equation wavelength= h/mv and we end up with an answer in meters.
Planck's constant which is 6.626 times ten to the negative thirty fourth power has Joules. To cancel the joules, which is kilogram times m2 and s-2. This Kg needs to be removed to find the final frequency. You can only do this by having Kg at the denominator.
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