Question

How many moles are in 15 grams of lithium?

Answer 1

Hint As we know that the concept of mole is mainly to calculate the entities at the microscopic level that is ions, particles, molecules, electrons or atoms etc. It is found that mole is having the symbol mol.

Complete Step by step solution:
- As we are being provided with the information that there is 15 grams of lithium. As we know that the molar mass of lithium is 6.94 g/mol.
- As we know that mole is the amount of substance that has entities as there are atoms exactly in 12 g of carbon isotope. We should note that the number of entities in one mole is important because it is called the Avogadro constant. The numeric value of this constant is $6.022\times {{10}^{23}}$.
- Firstly we will write the given mass as:
$\dfrac{15}{6.022\times {{10}^{23}}}$
- Now, we can find the number of moles by the formula of moles that is given mass of the substance divided by the molar mass of the substance.
$\begin{align}
& moles=\dfrac{given\text{ }mass}{molar\text{ }mass} \\
& \implies\dfrac{\dfrac{15}{6.022\times {{10}^{23}}}}{6.94} \\
& \implies\dfrac{15\times 6.94}{6.022\times {{10}^{23}}} \\
&\implies 17.28\times {{10}^{-23}}moles \\
\end{align}$

- Hence, we can conclude that there are $17.28\times {{10}^{23}}$ moles in 15 grams of lithium.

Note:
- If we want to calculate the number of moles of an individual entity, like say A, that is dissolved in a solution of an entity say B (A+B), then we can do so by using the concept of mole fraction. The formula of mole fraction is given as: moles of substance divided by the total number of moles.
- We should not forget to write the unit after solving the solution.