Question

The mass of one molecule of water is approximately?
A. 1 g
B. 0.5 g
C. ${ 1.66\times 10 }^{ -24 }$ g
D. ${ 3\times 10 }^{ -23 }$ g

Answer 1

Hint: For this question, knowing the mass of one mole molecules of water can help you. Now, since you know mass for one mole, just convert it to mass of one molecule.

Complete step by step answer:

Here, first we calculate mass of one mole of water molecule and then simply we will convert it to mass of one molecule of water dividing it by avogadro number.
Molecular Mass - Molecular mass is a number equal to the sum of the atomic masses of the atoms in a molecule.
We know that in a water molecule, there are two hydrogen and one oxygen atom present. So, the total mass of water molecule, in g/mol we can calculate like this:
2H = 2 x 1.008 = 2.016 g/mol
O = 1 x 16.000 = 16 g/mol
So, the total mass of ${ H }_{ 2 }O$ = 18.016 g/mol
Now, we know that this is the mass of ${ 6.022\times 10 }^{ 23 }$ molecules of water. Therefore, we have to convert it to a mass of one molecule of water.
Mass of one molecule of water = 18.016 / ${ 6.022\times 10 }^{ 23 }$
Mass of one molecule of water = ${ 2.99\times 10 }^{ -23 }$ g .
So, we can say that the final answer would be approximately, ${ 3\times 10 }^{ -23 }$ g .

Hence, the correct answer for this question is option D.

Note: Never confuse yourself with the gram molecular mass, it also seems a similar term. But the gram molecular mass of water is 18 grams per mole. This is because a water molecule contains two hydrogen atoms (one proton each) and one oxygen atom (8 protons and 8 neutrons), for a total of 18 nucleons.
In option C, the value given is equal to 1 Dalton (Da or u).