Chemistry

Stoichiometry (continued)


Molecular Mass (MM)

It is the mass of the molecule measured in atomic mass units. For stoichiometric calculations, the unit grams (g) is used.

The calculation of the molecular mass is made from the atomic masses of the elements and the sum of their atoms in the molecule. Like this:

H2O (water)

O = 1x 16 = 16
H = 2 x 1 = 2
MM = 16 + 2 = 18g or 18u

In the formula of water, there is 1 atom of O, which is multiplied by its atomic mass (16), resulting in 16. There are two atoms of H, which multiplied by its atomic mass (1), resulting in 2.

These results are summed and thus we find the value of molecular mass, 18g or 18u. See other examples:

CO2 (carbon dioxide)

O = 2 x 16 = 32
C = 1 x 12 = 12
MM = 32 + 12 = 44g or 44u

Ç12H22O11 (sucrose)

O = 11 x 16 = 176
H = 22 x 1 = 22
C = 12 x 12 = 144
MM = 176 + 22 + 144 = 342g or 342u

Mg (OH)2 (magnesium hydroxide)

H = 2 x 1 = 2
O = 2 x 16 = 32
Mg = 1 x 24 = 24
MM = 2 + 32 + 24 = 58g or 58u

Ca (NO3)2 (calcium nitrate)

O = 6 x 16 = 96
N = 2 x 14 = 28
Ca = 1 x 40 = 40
MM = 96 + 28 + 40 = 164g or 164u

CuSO4.5H2O (cupric sulphate pentahydrate)

O = 5 x 16 = 80
H = 10 x 1 = 10
O = 4 x 16 = 64
S = 1 x 32 = 32
Cu = 1 x 63.5 = 63.5
MM = 80 + 10 + 64 + 32 + 63.5 = 249.5g or 249.5u

Minimum formula

It is a formula that gives the relative number between the atoms of the substance. Shows the ratio of atoms of elements to integers and the smallest possible.

See the minimum formula of some substances and their molecular formula:

Substance

Molecular formula

Minimum formula

Hydrogen peroxide

H2O2

HO

Glucose

Ç6H12O6

CH2O

Sulfuric acid

H2ONLY4

H2ONLY4

Generally, the minimum formulas are a "mathematical simplification" of the molecular formula. Hydrogen peroxide can be divided by 2, forming the minimum formula above.

In glucose, the molecular formula has been divided by 6 and in sulfuric acid it is not possible to divide by an integer, so the minimum formula is equal to the molecular formula.

Centesimal composition or elemental analysis

The proximate formula gives the percentage of atoms that make up the substance. Represents the mass ratio that exists in the substance. It is always constant and follows Proust's Law. Example:

C: 85.6%
H: 14.4%

Here's how to calculate the centesimal formula from substance analysis data:

Analysis of 0.40g of a certain iron oxide revealed that it has 0.28g of iron and 0.12g of oxygen. What is your proximate formula?

x = 70% Fe

x = 30%

So this oxide has 70% Fe and 30% O.