The atomic weights of two elements A and B are 40 and 80 respectively. If x g of A contains y atoms, how many atoms are present in 2x g of B?

We are given that the atomic masses of the two elements A and B are 40 and 80 respectively. Also, we are given that the number of atoms in amount x g of element A is y. Therefore

number of moles of A in x g of A = mass / molar mass = (x / 40)

number of atoms in x grams of A = number of moles X Avogadro's number

= (x / 40) X 6.023 X 10²³ = y (1)

mass of element B = 2x

number of moles of B in 2x g of B = mass / molar mass = (2x / 80)

number of atoms in 2x grams of B = number of moles X Avogadro's number

= (2x / 80) X 6.023 X 10²³

= (x / 40) X 6.023 X 10²³

But this is equal to y [from (1)]

Therefore the number of atoms in 2x grams of B is the same as x grams of A that is y.