Question

The number of moles of magnesium present in a magnesium ribbon weighing 12g is ……….
(molar atomic mass of magnesium is 24g/mol):
A. 0.5 mole
B. 1 mole
C. 2 mole
D. 0.25 mole

Answer 1

Hint: The number of moles are found by knowing the values of the weight of the substance given in grams and also the molecular weight of the same substance which is always given in grams. This way, the units cancel each other giving us a unitless quantity.

Complete step by step solution:
-Mole is the amount of substance that contains the atoms, molecules or other particles in an entity equal to the atoms present in 12 g of C-12 isotope. It defines the basis of physical chemistry and so its number is called Avogadro's number which is equal to 6.023x${{10}^{23}}$entities. It is denoted by the symbol ${{N}_{A}}$ .
-Some standard terms used in mole concept are
1 gram-atom = 1 mole atom
1 gram-molecule = 1 mole molecule
1 gram-ion = 1 mole ion
- Avogadro’s number is the number of molecules present in 1 mole which is equal to 6.023x${{10}^{23}}$entities. It is denoted by the symbol ${{N}_{A}}$ . This value is directly used to find the number of atoms in certain different types of molecules which have different representational units.
-The number of moles is given by the formula
\[\text{moles =}\dfrac{wt.\text{ }in\text{ }grams}{\text{molecular wt}\text{.}}\]
-In the question, we are given the weight of magnesium to be 12 grams. Also the molecular weight of magnesium is given to be 24 grams. So we can directly find the value of the number of moles using the above formula.
-So the number of moles will be
$moles=\dfrac{12}{24}=0.5$

Therefore the correct option is A.

Note: Mole is not only related to mass but also to volume as any matter that has a certain mass will occupy a certain volume. 1 mole is the amount of molecules present in 22.4 litres of a gas calculated at standard temperature and pressure. Always ensure that the weight is in the same unit both in numerator and denominator.