Question

Which is heaviest?
A. 25 gm mercury
B. 2 moles of water
C. 2 moles of carbon dioxides
D. 4 gm atoms of oxygen

Answer 1

Elements must have identical units like gram or kilogram to compare their masses.So we need to calculate the mass of carbon dioxide and water from the moles given.

Complete step by step answer:
In option A the mass of mercury (Hg) is given as: 25 gm
But in option B mass of water is not given so first we have to convert moles of water into mass of water.
For this we will use the formulae given below:
$mass\;of\;water = moles\;of water \times molecular\;mass\;of water$
Molecular mass of water $\left( {{H_2}O} \right)$ can be find out as:
.${M_{{H_2}O}} = 1 \times 2 + 16 \times 1 \\\ {M_{{H_2}O}} = 18\dfrac{{gm}}{{mole}} \\\$.
On substituting the values:
$mass\;of\;water = 2 \times 18 = 32\;gm$
In option C also the mass of carbon dioxide $\left( {C{O_2}} \right)$ is not given so first we have to convert moles of carbon dioxide into mass of carbon dioxide.
For this we will again use the same formulae which explained earlier, as:
$mass\;of\;C{O_2} = moles\;ofC{O_2} \times molecular\;weight\;ofC{O_2}$
Molecular mass of $C{O_2}$ can be find out as:
${M_{C{O_2}}} = 12 \times 1 + 16 \times 2 \\\ {M_{C{O_2}}} = 44\dfrac{{gm}}{{mole}} \\\$
On substituting the values:
$mass\;of\;C{O_2} = 2 \times 44 = 88\;gm$
In option D the mass of mercury $\left( {{O_2}} \right)$ is given as: 4 gm

Option	Component	Symbol	Mass (gm)
A	Mercury	$Hg$	2
B	Water	${H_2}O$	32
C	Carbon dioxide	$C{O_2}$	88
D	Oxygen	${O_2}$	4

Hence the correct option will be (C) “2 moles of carbon dioxide” is heaviest.

Note:
Water and Mercury which is a silvery-white, shiny metal, are liquid at room temperature. Carbon dioxide and oxygen are gas at room temperature.