I've read that Pentacel has a higher aluminum content than the separate vaccines, so I was trying to find exact numbers of the separate parts, but what's the difference between aluminum phosphate and just aluminum?
That means that the aluminum atom (Al) is attached to a phosphate group (PO4) The phosphate group contains one phosphorus atom and 4 oxygen atoms.
Those phosphorus and oxygen atoms all add to the weight of the compound. So 1 molecule of aluminum phosphate is going to weigh more then 1 Aluminum atom. Therefore 1.5 mg of AlPO4 (aluminum phosphate) will weigh more then just the aluminum in that compound.
It's kind of like saying a batch of chocolate chip cookie dough will weigh more then just the chocolate chips that are in that cookie dough. Make sense?