1 litre is 1000ml therefore 15g per 100ml would equal 150g per 1000ml. So 3 Litres would contain 450g.
2.5 x 6.022 x 1023 = 1.5055 x 1024 molecules of sucrose are in 2.5 moles.
The composition is the same because the natural sugar in the cane stalk or beet root is separated from the rest of the plant material by physical methods. Also they both require heating up the juice...
Yes it can. http://physics.nist.gov/PhysRefData/Handbook/Tables/argontable6.htm