Question

What mass of oxygen is present in this sample of ZnCO3, which contains 3.11×1022 O atoms?

Answer 1

The given compound is ZnCO3

So each mole of ZnCO3 will have three moles of Oxygen atoms

So if the number of oxygen atoms in the sample = 3.11×10^22

It means the moles of oxygen atoms = number of atoms / Na (avagadro's number)

moles of oxygen atoms = 3.11×10^22 / 6.023 X 10^23 = 0.0516 moles

Mass of oxygen atoms = moles X atomic mass = 0.0516 X 16 = 0.8256 g

Answer 2

Atomic mass of an element is the average relative mass of an atom of th element as compared to the mass of ¹²C atom taken as 12 units.  Thus atomic mass of an element is the number of times one atom of the element is heavier than 1/12th of a ¹²C atom.

Atomic mass of oxygen is 16 atomic mass unit which indicates mass of Avogadro's number of atoms is 16.

Mass of 6.023 X 10²³ number of atoms is equal to 16, so mass of 3.11 X 10²² number of atoms is $\frac{16 X 3.11 X 10^{22} }{6.023 X 10^{23} }$= 0.826 g.