Question

How many moles of $KCl$ are in $27.5g$ of $KCl$?

Answer 1

Hint: As we are well aware with the term mole which is given as the ratio of the given mass of any substance or compound to the molecular mass of the given substance or compound. We can calculate it by knowing the mass and molecular mass of the compound.

Formula used: $moles = \dfrac{{mass}}{{molecular\;mass}}$

Complete step-by-step answer:
As we already know the mole concept where a mole is defined when mass of a substance is divided by the molecular mass of that substance. It is expressed as shown below:
$\Rightarrow moles = \dfrac{{mass}}{{molecular\;mass}}$

Now, we are given the mass of Potassium chloride which is $27.5g$ and we also know the molecular mass of Potassium chloride which is $74.55g$. Now, using the above formula we can easily calculate the number of moles of Potassium chloride and we will get:
$\Rightarrow moles = \dfrac{{27.5}}{{77.55}}$
$\Rightarrow moles = 0.354mol$
Therefore, the correct answer is that approximately $0.354mol$ are present in $27.5g$ of Potassium chloride.

Additional information: Along with moles, we can also calculate other parameters such as the volume of any substance at NTP or STP, number of atoms, number of molecules etc. in a given mass of any substance using the below relationships between these parameters as:
$\Rightarrow moles = \dfrac{{mass}}{{molecular\;mass}} = \dfrac{{vol.\;at\;STP}}{{22.4L}} = \dfrac{{no.of\;molecules\;or\;atoms}}{{{N_A}}}$
Where ${N_A}$ is Avogadro’s number.

Therefore, if we are given volume of any substance and we are asked to find out the mass of that substance then we can use the above relation between mass and volume and similarly we can identify every given condition we are asked in the question.

Note: Always remember that the ratio of mass to the molecular mass is equivalent to its number of moles which in turn is equivalent to the ratio of number of atoms or number of molecules to the Avogadro’s number. We can also calculate the volume of a substance if we know its number of moles.