Question

Convert $ \text{20g} $ of water into moles.

Answer 1

Hint A mole is the amount of a substance that contains as many atoms or molecule as there are atoms in exactly $ \text{12}\,\text{gm} $ of the $ \text{carbon-12} $ . In a simple way one mole is the collection of $ \text{6}\text{.022 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{23}}}\text{atom/molecule} $ .
One mole of every substance contains $ \text{6}\text{.022 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{23}}}\text{molecules} $ and this is given a separate name and symbol known as ‘Avogadro constant’ denoted by $ {{\text{N}}_{\text{A}}} $ .
In chemistry atoms, molecules and ions are subatomic particles and we count the number of atoms or molecules by weighing the amount of substance. In a chemical reaction the moles of two substances reacts in a definite ratio, so we use following formula to calculate the number of moles in a reaction –
 $ \text{mole}\,\text{atom = }\dfrac{\text{weight}}{\text{atomic}\,\text{weight}}....(i) $
 $ \text{mole}\,\text{molecules = }\dfrac{\text{weight}}{\text{molecular}\,\text{weight}}.....(ii) $
 $ \text{mole}\,\text{molecules =}\dfrac{\text{number}\,\text{of}\,\text{molecule}}{{{\text{N}}_{\text{A}}}}......(iii) $

Complete Step by step solution:
This calculation is takes place in two steps-
In first step we will calculate the molecular weight of water molecule –
 $ \begin{align}
& \text{molecular}\,\text{weight}\,\text{of}\,{{\text{H}}_{\text{2}}}\text{O =}\,\text{2 }\!\!\times\!\!\text{ 1+16}\,\,\,\,\text{ }\!\!\{\!\!\text{ atomic}\,\text{weight}\,\text{of}\,\text{H=1}\,\text{and O =16 }\!\!\}\!\!\text{ } \\
&\implies 18\,\text{gm}
\end{align} $
In the second step we will apply equation (ii) to calculate the number of moles of water molecules present in $ \text{20g} $ of water, after putting molecular weight $ \text{18gm} $ .
 $ \begin{align}
& \text{No of moles}\,\text{ = }\dfrac{\text{weight}}{\text{molecular}\,\text{weight}} \\
& \text{No of moles}=\dfrac{\text{20gm}}{\text{18gm}} \\
&\implies \text{1}\text{.11}\,\text{mole}
\end{align} $

Additional information:
One mole of water contains two moles of hydrogen and one mole of oxygen, so total no of moles of hydrogen and oxygen present in $ \text{1}\text{.11}\,\text{mole} $ of water-
\[\begin{align}
 & \text{moles}\,\text{of}\,\text{hydrogen}\,\text{=}\,\text{2 }\!\!\times\!\!\text{ 1}\text{.11} \\
 & \text{=2}\text{.22}\,\text{mole} \\
& \text{moles}\,\text{of}\,\text{oxygen}\,\text{=}\,\text{1 }\!\!\times\!\!\text{ 1}\text{.11} \\
& \text{=1}\text{.11}\,\text{mole}
\end{align}\]
After applying the equation (III) we will get the number of molecules present in the $ 1.11\,\text{mole} $ of water.

Note: Weight of one mole is the molecular weight in grams. Equal moles of different substances contain the same number of constituent particles but equal weight of different substances does not contain the same number of constituent particles.