You can't fix that weight to plasterboard walls alone with a simple rawlplug. As Steve says, what is behind the plasterboard? If it is a hollow wall, you need a trip to your DIY store to buy some proper cavity wall fixings. If behind the plaster is a solid wall, you could still use the plugs but use 2.5" or 3" screws and make sure the plugs are trough the plaster into the wall itself.
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