Question

Which of the following is Heaviest?
A.25 g of ${\text{Hg}}$
B.2 moles of ${{\text{H}}_2}{\text{O}}$
C.2 moles of ${\text{C}}{{\text{O}}_2}$
D.4 g atom of ${{\text{O}}_2}$

Answer 1

First of all we need to convert the number of moles in option B and C in mass using the formula for number of moles and molecular mass. Then we will compare the masses, the one with the maximum mass will be the heaviest.

Formula used: ${\text{number of moles }} = \dfrac{{{\text{given mass}}}}{{{\text{molar mass}}}}$

Complete step by step solution:
The molar mass of any substance is the mass of one mole of that substance. For example the molar mass of water will be the mass of one mole of water molecules. Molar mass can be calculated by adding the molar mass of each of the elements involved multiplying by their respective atomicity. Atomicity is the number of atoms that are contained in a particular type of an atom.
The molar mass of hydrogen is 1, the molar mass of oxygen is 16 and the molar mass of carbon is 12. Hence the molar mass of water molecule will be: $1 \times 2 + 16 = 18$ g
The mass of one mole of water is 18 amu. We have been given two moles of water, the mass of 2 moles of water will be $2 \times 18 = 36$ g.
In the same way the mass of the molar mass of carbon dioxide will be $12 + 2 \times 16 = 44$ g
The mass of one mole of carbon dioxide is 44 amu. We have been given two moles of carbon dioxide, the mass of 2 moles of water will be $2 \times 44 = 88$ g.
Hence, among all 2 moles of carbon dioxide has maximum mass.

So, the correct option is C.

Note: Whenever we need to compare the mass, we cannot directly compare it from the concentration or amount given. Mass is an absolute quantity while mole is a relative concept.