Question

If the mass of oxygen is 16 times as large as the mass of hydrogen, what is the distance between the centre of the oxygen atom and the centre of mass of the water molecule? (Note: 1 pm = 1 x 10-12 m).

Answer 1

The water molecule has two hydrogen atoms, spaced apart at 105 deg), not linearly opposed. So you have to first consider that geometry. If one just looks at the center of mass of the molecule along the line bisecting the two hydrogen, then
cm=(2*1*cos(105/2) + 16*O)/18 and that is the distance for the center of mass from the oxygen center of mass.