To calculate the molarity of acidic solutions (wt/v%), Igenerally use the formula,
N.o. moles = (relative density (in g/ml) *1000(ml) * mass% of solute) / (100*molar mass of the solute(g/mol))
The obtained moles is the n.o. moles per litre solution, hence its molarity.
However, I recently came through some reference websites, that does not involve relative density in the calculation. The following formula is given:
M = (mass of the solute in 1L (g/L))/(molar mass of the solute (g/mol))
Refer:
https://www.labce.com/spg938571_interchanging_between_percent_concentration_and_mo.aspx
http://abacus.bates.edu/~ganderso/biology/resources/dilutions.html#:~:text=To%20convert%20from%20%25%20solution%20to,divide%20by%20the%20formula%20weight.
I'm confused now. Which is the correct one?
Both are correct because they convert different units to mol/L. If you want to convert from wt% to mol/L you have to consider the density of the solution. Starting from a volume-based concentration (e.g. g/L), there is no need to consider the density as you simply have to convert the solute mass (g) to mol. Be carefull: Sometimes concentrations are given as amount of solute per amount of solvent. In such a case the above formulas are not applicable. Converting true volumetric concentrations (vol%) e.g. for mixed liquids is also not that easy.
You take into account of relative density or specific gravity in calculation largely when dealing with chemicals supplied in their liquid form such as all conc. acids and solvents. Each compound will have its own specific gravity per Kg of liquid. for example;
for acids
https://scrippslabs.com/molarity-specific-gravity-of-concentrated-acids-ammonia/
for liquid in general
https://www.binmaster.com/_resources/dyn/files/75340090z876d0c14/_fn/Specific+Gravity.pdf
This issue is very important but often overlook by the beginner researcher/students.
For example, you wish to accurately prepare 0.01 M orthophosphoric acid as part of the elution gradient for an HPLC experiment
Article Determination limits in high-performance liquid chromatograp...
Orthophosphoric acid is generally supplied as an 85% pure solution. To carry out the standard dilution using M1V1=M2V2, you need the initial Molarity of the orthophosphoric acid and you don't find this printed on the bottle. You have to use a generic information from say;
https://www.nestgrp.com/protocols/trng/molarity.shtml
or you can calculate the molarity using the specific gravity (Density: 1.685 g/mL at 25 °C) and take into account the purity as the following example
Article How to calculate concentration of concentrated acid
Try to understand the rudimentary aspects of the calculation. This way you can calculate the molarity of any liquid chemicals in the lab.
Hi Dr G. Srividhya . What I studied, taught and used is the equation:
Molarity = (10 x percent x density)/Molecular mass .
During calculations and working with numbers and measurements, we need to consider:
SI units and their conversions (from one to other unit), scientific notations (standard notations), and related concepts (precision, accuracy, errors...)...
Thanks!
Please note that what you have called «mass% of solute» (at the enunciate) should in principle be understood as wt/wt % concentration. It is not to be confused with wt/v% concentration.
We should generally take not fully specified % concentrations as wt% ― where wt is often omitted or can be substituted by either wt., wt/wt, wt./wt., or m/m ― meaning: (mass of pure solute / mass of solution)·100%; even if this is not clearly stated.
Example calculation:
The concentration of some given commercial HCl conc. sol. is 37 wt. %. The label indicated density is 1.18 g aq. sol. / cm3. The molarity of this conc. sol. is given by: 0.37(g HCl / g aq. sol.)·1.18(g aq. sol. / cm3)·(1000 cm3/dm3)/(36.46 g HCl/mol HCl) = 12.0 M (approximate).
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