A100 watt microwave oven takes 90 secondd to heat a bowl f soup how many joules of energy does it use ?

Before I answer the question, please let me strip away all the color and decoration of the microwave story:

      (100 watts) = 100 joules per second

        (100 joules/second) x (90 seconds)  =  9,000 joules .

I did that because the "100 watts" on the label on the front of the
microwave oven is the "cooking power" ... the power of the radio
waves that it shoots into the chamber to heat the soup or the meatloaf. 

The whole appliance actually uses a lot more power than that to do its job. 
The light, the turntable, and the cooling fan all use some power, and the
magnetron is not 100% efficient, so the appliance has to put way more
than 100 watts into the magnetron to wind up with 100 watts available
to shoot to the meatloaf.  

When you ask "How much energy does it use ?", the answer is: 
A lot more than what eventually comes blasting into the chamber
to actually cook with.