Question

What is the mass of a molecule of water?
a) 18 grams
b) 2.99 × 10^-23 grams
c) Both of the above
d) None of the above

Answer 1

Hint: Each mole of a substance contains $N_0$ number of molecules of that particular substance, where $N_0$ is Avogadro’s number which is given by:
$N_0=6.023\times {10}^{23}$
Try and apply this knowledge to the given question.

Complete step-by-step answer:
Before solving this question, let us first establish the differences between the molar and molecular mass of a chemical substance.
The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol).
The molar mass of a compound can be calculated by adding the standard atomic masses (in g/mol) of the constituent atoms.
Molar mass serves as a bridge between the mass of a material and the number of moles since it is not possible to measure the number of moles directly.
The molecular mass is the mass of a given molecule: it is measured in u.
Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element.
The related quantity relative molecular mass, as defined by IUPAC, is the ratio of the mass of a molecule to the unified atomic mass unit and is unitless.
The molecular mass and relative molecular mass are distinct from but related to the molar mass.
Now with the help of the hint, let us try and solve this question.
We know that the molar mass of water is 18g.
Now,
$\begin{array}{rl}& MolecularMass={\displaystyle \frac{MolarMass}{N_0}}\\ & \Rightarrow MolecularMass={\displaystyle \frac{18}{6.023\times {10}^{23}}}=2.99\times {10}^{-23}g\end{array}$
Therefore, the answer to this question is b).

NOTE: Be very careful of the difference between molar and molecular masses. This question requires many equations to be solved before you arrive at the solution, therefore to make sure no mistakes are made at any of the elementary steps.