Question

The actual weight of a molecule of water is:
A.$18g$
B.$2.99 \times {10^{ - 23}}g$
C.Both (A) and (B) are correct
D.$1.66 \times {10^{ - 4}}g$

Answer 1

We can calculate the actual weight of the molecule of water using the mole concept. We can calculate the actual mass occupied by one molecule of water by the mass of one mole of water divided by the Avogadro number multiplied by the given molecules. We know that the value of Avogadro number is $6.022 \times {10^{23}}$.

Complete answer:
Based on the mole concept, we have to know that one mole of a substance contains $6.022 \times {10^{23}}$ molecules.
We have to know that the molar mass of a substance is the mass of one mole of that substance. The number of moles in the sample gives the amount of substance.
We have that molar mass of water is $18g/mol$. Therefore, the mass of one mole of water is $18g$.
Therefore, $18g$ of mass of water is occupied by $6.022 \times {10^{23}}$ number of molecules.
Therefore, we can calculate the mass of one molecule of water by dividing the mass of one mole of water and Avogadro number.
Number of molecules=$\dfrac{{18g}}{{6.022 \times {{10}^{23}}}} \times 1$
Number of molecules=$2.989 \times {10^{ - 23}}g$
Number of molecules=$2.99 \times {10^{ - 23}}g$
The actual weight of a molecule of water is calculated as $2.99 \times {10^{ - 23}}g$.
Therefore, the option (B) is correct.

Note:
-We should not take the actual mass of a molecule of water as $18g$ because one mole of water is $18g$ and not one molecule of water. One mole is equal to $6.022 \times {10^{23}}$. We can say $6.022 \times {10^{23}}$ particles include molecules, ions, atoms (or) electrons.
-We can convert moles to molecules using the Avogadro number. Consider the example,
Example: Calculate the number of molecules of $2.5mol{S_8}$.
Given,
-The number of moles of ${{\text{S}}_{\text{8}}}$ is $2.5mol$
-The Avogadro’s number is $6.022 \times {10^{23}}molecules$
-The number of molecules can be calculated as,
$2.5mol\left( {\dfrac{{6.022 \times {{10}^{23}}molecule}}{{mol}}} \right) = 15.055 \times {10^{23}}molecule$
-The number of molecules $2.5mol{S_8}$ is $15.055 \times {10^{23}}molecules$.