calculate the mass of sodium that contain the same no. of atoms as in 4g of calcium.

The molar mass (i.e. the mass of one mole of particles) of Calcium is 40g. This means that there are 6.023 X 10²³ atoms of calcium in 40g of calcium. Thus

Number of atoms in 40g of calcium = 6.023 X 10²³

number of atoms in 1g of calcium = (6.023 X 10²³) / 40

number of atoms in 4g if calcium = (6.023 X 10²³ X 4) / 40

= 6.023 X 10²²

So, the number of atoms in 4g of Ca are 6.023 X 10²². We have to calculate the mass of sodium which contains the same number of atoms as 4g of Ca. Now the molar mass of sodium is 23g. This means that there are 6.023 X 10²³ atoms of sodium in 23g of sodium. Thus

Mass of 6.023 X 10²³ atoms of sodium = 23g 

Mass of 1 atom of sodium = [23 / (6.023 X 10²³)]

Mass of 6.023 X 10²² atoms of sodium = [23 X (6.023 X 10²²)] / (6.023 X 10²³)

= 2.3g