Question

What weight of Sodium contains the same number of atoms as those in 8 grams of Oxygen?
a.) 10.5g
b.) 13.5g
c.) 11.5g
d.) 14.2g

Answer 1

Hint: The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol).
Each mole of a substance contains ${{N}_{0}}$ number of molecules of that particular substance, where ${{N}_{0}}$is Avogadro’s number which is given by:
${{N}_{0}}=6.023\times {{10}^{23}}$
Try and apply this knowledge to the given question.

Complete step-by-step answer:
Let us first discuss why the concept of molar mass is so important to help us solve this question.
The molar mass of a compound can be calculated by adding the standard atomic masses (in g/mol) of the constituent atoms.
Molar mass serves as a bridge between the mass of a material and the number of moles since it is not possible to measure the number of moles directly.
Now, we know that the molar mass of Oxygen (O) is 16.
$\begin{align}
  & \because 16g\to {{N}_{0}}\text{ }atoms \\
 & \Rightarrow 8g\to \dfrac{{{N}_{0}}}{2}atoms \\
\end{align}$
Let us now apply the same logic to Sodium (Na).
We know that the molar mass of Na is 23.
Therefore,
$\begin{align}
  & \because 23g\to {{N}_{0}}\text{ }atoms \\
 & \Rightarrow 11.5g\to \dfrac{{{N}_{0}}}{2}atoms \\
\end{align}$
Therefore, the amount of mass of Sodium required to have the same atoms as 8g of Oxygen (which is ${{N}_{0}}$/2) is 11.5g
Thus, the answer to this question as a result of the above calculation is c)11.5g.


Note: Do not get confused between the concepts of molar and molecular mass so as to ensure that you make the correct calculations. For future reference, the atomic mass tells you which mass one mole of any atom/molecule has, so divide by the Avogadro's number to know how many moles the required mass corresponds to.