If 1.216g of cobalt metal react with 0.495g of oxygen gas what is empirical formula of the cobalt oxide product

First you divide each mass by the mass number of the element:

Cobalt        1.216 / 59 = 0.02

Oxygen      0.495 / 32 = 0.03

Then the result is divided by the lowest number:

Cobalt        0.02 / 0.02 = 1

Oxygen      0.03 / 0.02 = 1.5

After that you multiply with a number to obtain an integer number:

Cobalt        1 × 2 = 2

Oxygen      1.5 × 2 = 3

The empirical formula of the oxide:  

CoO

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.