How many moles are there in 545g of fe2o3 ?

To calculate the number of moles, the equation is mass of substance divided by the molar mass of substance.

Molar mass of Fe2O3 is the relative atomic mas of all the elements from the substance added up, which makes 55.845x2 + 16.0 x3 = 159.69g mol-1

So the number of moles is
545g / 159.69gmol-1
=3.41 mol (corrected to 3 sig. fig.)

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water