Question

Calculate the number of moles of magnesium present in a magnesium ribbon weighing $12g$ molar atomic mass of magnesium is $24{\text{gmol}}{{\text{e}}^{{\text{ - 1}}}}$

Answer 1

The formula of magnesium ribbon is $Mg$. We know that one mole of an element weighs equal to its gram atomic mass. Thus, for any given amount of an element number of corresponding moles can be found using unitary method.

Complete step by step answer:
We know that,
Molar mass of magnesium $= {\text{24g/mol}}$
So, $24gm$ of $Mg$ has $1$ mole
Then, $1gm$of $Mg$ has $\frac{1}{{24}}$ mole
$12gm$ of $Mg$ has $\frac{{12}}{{24}}$ mole
From the above information, $\frac{{12}}{{24}} = 0.5$ moles of magnesium are present in magnesium ribbon.
We can do it also in an alternative method. i.e.
${\text{Number of moles of magnesium = }}\dfrac{{{\text{Given Mass}}}}{{{\text{Molar Mass}}}} = \frac{{12g}}{{24{\text{gmo}}{{\text{l}}^{{\text{ - 1}}}}}} = 0.5{\text{mol}}$

Additional information:
The oxidation or combustion of magnesium in air has long been used as a source of intense light in photography and for other photochemical reactions. The energy released in this combustion occurs as the magnesium reacts with oxygen in the air according to the equation:
$2Mg + {O_2} \to 2MgO$
The heat of formation of magnesium oxide is $- 6{\text{01}}{\text{.83 KJ/mol}}$. When magnesium burns approximately $10\%$ The energy of combustion occurs as light, a value unapproached among known transformations of energy used in production of light.

Note: The burning of magnesium ribbon produces light of sufficient intensity to cause temporary loss of sight. So we should avoid looking directly at the light source. Magnesium ribbon burns with a brilliant white flame.
$2Mg + {O_2} \to 2MgO$