Units and Molarity

[Samantha Loc 1B]

I noticed during lecture that Professor Lavelle kept converting units of volume to liters in order for the calculations to coincide with our understanding of molarity (mol/L); however, I also noticed that after calculating initial volumes (when the problem called for doing so), he also converted these units back to mL in addition to stating what volume flask this should be measured in. Are both of these steps necessary when stating our final answers for questions like these, or would leaving the answer as plainly a volumetric measure of liters work as well?

[Chem_Mod]

Hello Samantha,

In general, it always good to convert into the appropriate SI Units in order to ensure the units cancel out accordingly to solve the question.

For instance, if you had to multiply the volume of a solution by molarity to figure out the number of moles please refer to the example below.

Correct:

Incorrect:

[Emily Wu 2G]

Hey Samantha, this is a very good question. I was also wondering how can we know which equation with volume measurement is in L and which is in mL. Is there a trick into knowing or memorizing?

Emily

[Nancy_La_1H]

The reason he converted L back to mL was in the case of using the measurements in a lab, taking out a sample of 9.87 x 10^-3 L is a lot harder to do compared to 9.87 mL. Therefore I believe that as long you convert it correctly during the calculations that leaving it L is fine too unless the question specifically stated to answer in a specific unit.

[Tess_Fleser_1D]

Liters (L) is the base unit, so it is mostly used for equations (like molarity), but questions will be phrased in other units because they are more appropriate for the size of the measurement. It's best to just convert to L for your work and equations, and then convert to whatever unit makes most sense for your answer.

[Anika Scott 3A]

My high school teacher used to have a clever way to remember molarity, but I can not remember what it was. By any chance did anyone have anything similar that they could share?

[Allison Peng 1D]

It is not necessary to convert the answer back to mL, since I asked Prof. Lavelle after class and he said that when possible you can leave it in SI units (unless the problem asks otherwise). I think for the dilution question it was converted to mL since we want the steps to make this solution (250.0 mL of final solution), which would be done in a volumetric flask that measures in mL. I would generally leave it in SI units unless it's otherwise stated, though, since even in this case you can't really go wrong with 9.xx * 10^-3 L.

[Alejandro Oliva 2F]

Converting back to mL or to L for that matter is not necessarily a required step. More often than not, more questions would be satisfied by simply leaving the units as L, however as Lavelle noted, some questions ask that you convert the units to mL or any desired measurement or that in some cases converting to other units make your answers easier to quantify relative to the substance in question.

[madeleinewright]

I believe Professor Lavelle only changed the units back to mL so that it would make sense in a lab setting, since generally you would not tell someone to measure out a tiny fraction of a liter, but rather that amount in milliliters. He stated in the lecture that it was not necessary to convert the final answer back to mL (unless that is what the question asks for).

[Ryan Lafferty 1L]

I believe Dr. Lavelle converted his final answer from L to mL in order to make the measurements more practical in a lab environment, as it is far easier to measure out 9.87 mL rather than 9.87x10^-3 L. Unless the question states otherwise, I feel as a rule of thumb you should normally leave your answer in SI units. Hope this was helpful!

[Emily Quill 2H]

Hi! I believe that leaving your answer in L would be fine as Dr. Lavelle stated he converted back into mL for the context of a lab experiment to measure that amount.