Return to MCAT® Blog Homepage

MCAT Chem Practice Question – Ion to Atoms Ratio

A 1-L beaker is filled with 2 moles of liquid methanol and 0.25 moles of NaCl. The ratio of ions to atoms in this solution is:

A. 0.5:2

B. 1:6

C. 1:25

D. 3:1



C is correct. Atoms are the tiny units that compose matter; they consist of electron(s) and a central nucleus. The term “ion” refers specifically to charged atoms. Luckily, the number of moles of a substance directly relates to the number of particles by way of Avogadro’s number, so a direct comparison of moles is all we need to find our answer. However, two moles of a molecule is not equivalent to two moles of atoms. For example, since methanol has a formula of CH3OH, it contains a total of six atoms per molecule. In contrast, NaCl will dissociate into two ions: Na+ and Cl. In this solution, we have 0.25 moles of NaCl, which can be thought of as (0.25 moles * 2) or 0.5 ions. For atoms, we have (2 moles * 6) + (0.25 * 2) or 12.5; remember that ions are technically atoms as well! This gives us a ratio of 0.5:12.5 or 1:25.

A. 0.5:2: incorrect. This is simply the ratio of moles of ions to moles of methanol.

B. 1:6; incorrect. This is the ratio of molecules of NaCl to atoms in methanol, but neglects to involve moles or account for the two ions that NaCl produces.

D. 3:1; incorrect. This answer is incorrect in several ways. It likely was derived from a 6:2 ratio, or the ratio of atoms in methanol to ions in NaCl. However, this question asks for the ion-to-atom ratio. Additionally, this answer does not account for the moles present.


Want more MCAT practice?

We’ve got options for every schedule and learning style!

From the best online MCAT course created by top instructors with 524+ MCAT scores to the most representative full-length practice exams and private tutoring, we can custom tailor your MCAT prep to your goals!

Not sure which option is right for you? Schedule a free MCAT consultation with an MCAT expert using the form below. No obligation, just expert advice.

Schedule My Free Consultation

Create your Free Account to access our MCAT Flashcards


MCAT is a registered trademark of the Association of American Medical Colleges (AAMC), which is not affiliated with Blueprint.