Sunday, 20 January 2013

First Lab Session - Understanding chemicals bottles

Fco Javier Mondaza Hernández and Maite Pérez Lagares

Objective: To calculate the molarity of a substance by only having its molecular mass, richness and density.


All liquid substances we could find in the lab have different labels where we can find their different characteristics. For example, let’s see the next label:

-NAME: Name of the compound

-CHEMICAL REPRESENTATION: Formula of the substance/compound.

-MOLECULAR MASS: Mass of just one molecule of the substance/compound.

-RICHNESS: Percentage of concentration of the liquid.

-DENSITY: It is a measure of the mass of the substance in a standard unit of volume. It is measured in g/ml or g/cm3   

-MINIMUM PURITY: It could be:

-Pro Analysi (Pro)
-Very pure (PRS)
-Pure  (PR)
-Pro synthesi (PS)


Now, knowing all the data showed in the background, we are going to calculate its molarity (M):

Nitric acid: HNO3

R.m.m.:63.01 g/mol

Density: 1.395 kg/L

Richness: 65%

1.-) 1L x 1.395 kg/L = 1.395 kg

2.-) Now, we are going to multiply it by its richness (65%):

1.395kg x 65%= 0.907 kg HNO3

3.-) Then, we divide it by its molecular mass, which is 63.01 g/mol:

moles: g. substance/ relative molecular mass = 907 g. / 63.01 g/mol = 14, 394 moles HNO3

4.-) FinalLy, we calculate its molarity:
M= moles /L.solution // M = 14,394 / 1 L. // M HNO3 = 14, 394

So we can claim that this Nitric acid, has 14, 394 moles per litre.


We think this method is a very simple and easy one, because in order to calculate the molarity (M) of a substance you only need to make simple mathematical calculations (multiplications). For this reason, percentage error is very low, because you have less probability of having mistakes.

On the other hand, in order to use this method, you need to know many characteristics of the compound, such as, the molecular mass and its richness. If you do not have all this data, this process of calculating its molarity will be impossible to do; and we will need to resort to the common equation:

Molarity Equation

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