Question

If 1 grain is equal to $642mg$, how many moles of aspirin(mot wt.= $169$) are present in a 5 grain aspirin tablet ?

Answer 1

The mole (symbol= mol) is the unit of measurement for amount of substance in the International System of Units (SI). A mole of a substance or a mole of particles is defined as exactly $6.02214076\times {{10}^{23}}$ particles, which may be atoms, molecules, ions, or electrons. In short, for particles. $1mol=6.02214076\times {{10}^{23}}$.

Complete step by step answer:
Here, we have given that five grains of aspirin of molecular mass of $169g$. Molecular weight means the mass of 1 mole of substance. Here we have given that, $169g$ as molecular weight of the aspirin such that 1 mole of aspirin contains $169g$
Also, we have given that 1 grain of aspirin is equal to $642mg$. So, we have to find the number of moles of aspirin present in a 5 grain aspirin tablet.
1 grain of aspirins is equal to $642mg$. Hence 5 grain aspirin tablet is equal to
$\Rightarrow 5\times 642=3210mg=3.21g$
Hence we have a total of $3.21g$of aspirin
Hence, we know that the equation of number of moles when total mass and molecular mass is given;
$\Rightarrow Number.of.moles=\dfrac{Total.mass}{Molar.mass}$
$=\dfrac{3.21}{169}=1.89\times {{10}^{-2}}mol$

Hence, 5 grains of aspirin tablet contain $1.89\times {{10}^{-2}}$ mol of aspirin.

Note: Here the mass of aspirin is given in milligram and molecular mass of aspirin is given in grams. So, we have to convert the unit of milligrams to grams while calculating the number of moles. In some problems the amount of substance may give in volume instead of mass(so we have to use the standard volume measurement instead of molecular masses). So we have to be very clear about the quantities given in the questions.