What is the number of moles in 0.025 g (NH4)2Cr2O7? A) 1.5 x 1022
B) 4.2 x 10-26
C) 6.3
D) 1.0 x 10-4
E) 1.0 x 104

Vander waals forces are inter molecular bonds that exist within molecules of the same substance. a water molecule is made up of oxygen and hydrogen atoms which share electrons between each other. the vander waals forces are forces that exist between multitude of different  molecules of water. dipole dipole forces is the type of vander waals force that exist in water. the oxygen atom which is more electronegative pulls the shared electron closer to itself thus generating a partial negative charge while the hydrogen acquire a partial positive charge. these two different charges causes attraction to occur in water molecules.

Remember that rust prevented by placing a sacrificial electrode in electrical contact with the iron. the sacrificial electrode must be composed of a metal that  oxidizes  more easily than iron (that is, it must be below iron in table 18.1).

The image seems to be behind the mirror, but nothing is really there. if someone looks behind the mirror, there is no image there.