Question

Which of the following would weigh the least if an equal number of moles of each substance is taken?

• A. $C_{12}{H}_{22}{O}_{11}$
• B. $N{O}_2$
• C. $CaC{O}_3$
• D. $H_{2}O$

Answer 1

The correct option is D $H_{2}O$

Number of moles $=\frac{\text{Total mass}}{\text{Molecular mass}}$

Let Y moles of each substance is taken.

Mass of $CaC{O}_3$ = Y$\times$ Molecular mass of $CaC{O}_3$

= Y$\times$ (atomic mass of Ca + atomic mass of C + 3$\times$ atomic mass of O)
= Y$\times$$(40+12+3\times 16)$
= $100$Y

Similarly,

Mass of $C_{12}{H}_{22}{O}_{11}$ = Y$\times$ $(12\times 12+22\times 1+11\times 16)$
= $342$Y

Mass of $N{O}_2$ = Y$\times$$(1\times 14+2\times 160)$ = $46$Y

Mass of $H_{2}O$ = Y$\times$$(2\times 1+16)$ = $18$Y

From the above calculation water would weigh the least.