Question

How many moles are in $42.5g$ sample of magnesium oxide MgO?

Answer 1

The question is based on the concept of number of moles and Molar Mass of compound/element. One mole of any compound contains mass equal to its molar mass. In this question we are asked to find out the number of moles in $42.5g$ of MgO. For this we will use the relation which relates the number of moles with the molar mass and given mass.

Complete step-by-step answer: We know that number of moles in any compound is given by the formula mentioned below,
Number of moles = $\dfrac{given\,mass}{Molar\,mass}$
In the question the given mass of magnesium oxide MgO is given as $42.5$grams.
Now, let us calculate the molar mass of the MgO
Molar Mass of magnesium $(Mg)\,=\,24gmo{{l}^{-1}}$
Molar Mass of Oxygen (O)= $16gmo{{l}^{-1}}$
Molar mass of MgO= $24+16=40g/mol$
Using the Formula $number\,of\,moles=\,\,\dfrac{given\,mass}{molar\,mass}$we get
$number\,of\,moles\,=\,\,\dfrac{42.5}{40}$ = $1.0625$ moles of magnesium oxide (MgO)
Hence the number of moles present in $42.5$ grams of magnesium oxide are $1.0625$moles.

Note: The questions based on moles concept are often misunderstood by many students. Determine your starting point and ending point to determine the number of corresponding steps that will be necessary. Always be careful in calculating the molar mass of compounds as a slight mistake can make the entire question wrong. Whenever the answer comes out in decimal always use the rules of significant figures to round your answer to the proper nearest value possible.