Question

Calculate the mass of two helium atoms.
(A) $\dfrac{4}{{{N}_{a}}}$
(B) $\dfrac{2}{{{N}_{a}}}$
(C) $\dfrac{8}{{{N}_{a}}}$
(D) $\dfrac{1}{{{N}_{a}}}$

Answer 1

Hint: The number of moles of the substance can be calculated by dividing the given number of atoms to the Avogadro's number. The mass of n number of atoms can be calculated by multiplying the mass of one atom to the number of moles of the atom.

Complete step by step solution:
A mole of a substance has several meanings.
We can say that a mole of the substance has a mass equal to the gram atomic mass (if it is atoms) or gram molecular mass (if it is a molecule).
In other definition we can also say that, a mole of the substance is equal to the amount which has Avogadro’s number ($6.022\text{ x 1}{{\text{0}}^{23}}$) of atoms (if the substance is atomic), or Avogadro’s number ($6.022\text{ x 1}{{\text{0}}^{23}}$) of molecules (if the substance is molecular).
Avogadro's number is the same as that number of atoms present in one gram atom of the element or it is the same as that number of molecules present in one gram molecule of the substance.
Or we can say that moles are equal to the ratio of the number of atoms or molecules to the Avogadro's number.
Here we are given 2 atoms of helium:
So, the number of moles will be: $\dfrac{2}{{{N}_{a}}}$
We know that the mass of one atom of helium is equal to $4.00 g$.
$\dfrac{2}{{{N}_{a}}}$moles of helium will have: atomic weight x $\dfrac{2}{{{N}_{a}}}$
$Mass=4\text{ x }\dfrac{2}{{{N}_{a}}}=\dfrac{8}{{{N}_{a}}}$

So, the correct answer is an option (C)- $\dfrac{8}{{{N}_{a}}}$.

Note: In the volume form we can say that a mole of the substance is equal to the amount of the gas that has a volume of 22.4 litres at STP. STP is the standard temperature and pressure, where temperature $273 K$ and pressure is $1$ atmosphere.