A student has a 1 g sample of each of the following compounds: NaCl, KBr, and KCl. Which list shows the samples in order of increasing number of moles? A) NaCl < KCl < KBr B) NaCl < KBr < KCl C) KCl < NaCl < KBr D) KBr < KCl < NaCl

What decides “more moles” when the mass is the same?

All three samples have the same mass (1 g). Moles depend on molar mass: $$n = \frac{m}{M}$$ So if $m$ is fixed, a smaller $M$ means a larger number of moles.

Compare molar masses of NaCl, KCl, and KBr

Use approximate atomic masses (g/mol): $\mathrm{Na}=22.99$, $\mathrm{K}=39.10$, $\mathrm{Cl}=35.45$, $\mathrm{Br}=79.90$.

• $$M(\mathrm{NaCl}) = 22.99 + 35.45 = 58.44\,\mathrm{g/mol}$$
• $$M(\mathrm{KCl}) = 39.10 + 35.45 = 74.55\,\mathrm{g/mol}$$
• $$M(\mathrm{KBr}) = 39.10 + 79.90 = 119.00\,\mathrm{g/mol}$$

Turn molar mass into an order of moles

Because $n = \frac{1\,\mathrm{g}}{M}$:

• Largest $M$ gives smallest $n$, so $\mathrm{KBr}$ has the fewest moles.
• Next is $\mathrm{KCl}$.
• Smallest $M$ gives largest $n$, so $\mathrm{NaCl}$ has the most moles.

Therefore, increasing moles: $$\mathrm{KBr} < \mathrm{KCl} < \mathrm{NaCl}$$ That matches choice D.