Stoichiometry - Formulas and Tips

Before making a stoichiometric calculation it is important to know how to calculate the atomic mass of substances.

Its unit is in grams (g). The value of the atomic mass of the chemical element in the periodic table is sought.


He = 4.00g

Ne = 20.18g

If the substance has more than one element or the same element, it is calculated by summing the atomic masses of these elements. If you have it, it multiplies. Ex.

H2O = 16 + 2. (1) = 18g

Ç12H22O11 = 11. (16) + 22. (1) + 12. (12) = 342g

Ca (NO3)2 = 2.3. (16) + 2. (14) + 40 = 164g

The mol always indicates:
- amount
- pasta
- volume

The amount is a very large number that was experimentally determined, the Avogadro Number (6.02.1023).

Just as there are a dozen, there is the Avogadro Number.

If the dozen indicates 12 units of anything, the Avogadro Number indicates 6.02.1023 units of anything. In this case, it is used to quantify atoms, molecules, ions and many other very small subatomic particles.

The mol also indicates mass. It is the same mass that we find in the Periodic Table, but in grams (g). therefore one mole of a substance is equal to its atomic mass.

Mole indicates volume in CNTP, which means normal temperature and pressure conditions. The temperature must be 0 ° C or 273K and the pressure 1 atm. If these conditions are met, one mole of a gas will be 22.4L.
This constant is for gases. If the gas is not in CNTP, it can be calculated using the following formula for ideal gases:

P.V = n. R. T

P = pressure (atm)
V = volume (L)
n = number of moles
R = Clapeyron constant = 0.082 atm.L / mol.K
T = temperature (K)

These calculations are relations of quantities. Three simple rules are used.

Always put the data we already know on the first line and on the second line the data to be calculated.

See the example:

- How many grams of water are there in 3 mol of water?

If 1 mol is 18 grams (calculate molecular weight with the help of the periodic table) then 3 mol is how many grams?
In the first line, we put the known data, that is, that a mol has 18 grams:
In the second line, we put the data we want to calculate, that 3 moles will have x grams. Always placing unit under the same unit.

Thus we have:

1 mol - 18g
3 mol - x (g)

x = 54g H2O

For these calculations, you can follow some steps:

1. Balancing the chemical equation (adjusting the stoichiometric coefficients);
2. make mol count of each substance;
3. read in the problem what you ask for;
4. relate the quantities;
5. calculate with rule of three (proportion).

It is always important to list the substances you have data with and the substance you want to calculate some quantity.

This calculation is widely used in chemical laboratories as no substance is 100% pure. There is always some impurity. For this reason, some problems already indicate the amount of impurity or how pure the substance is.

If a 40g sample of NaCl is 70% pure, how much NaCl is in the sample?

40g - 100%
x (g) - 70%

x = 28g NaCl

This is the first step for stoichiometric calculations involving chemical reactions with purity calculation.

No chemical reaction has 100% utilization. Usually the amount of product may be less than the expected value. In this case, the yield was not total. This can happen for a variety of reasons, such as poor instrument or reagent quality, poor operator readiness, and so on.

The yield calculation is made by relating the expected value and the obtained product value.

- In a given chemical reaction one should obtain 500g. However, the reaction only had 60% yield. What is the value of the mass of product obtained?

100% - 500g
60% - x (g)

x = 300g

Clapeyron Constant:
R = 0.082 atm.L / mol.K
R = 8.314 / mol.K
R = 1.987cal / mol.K

Avogadro Number: 6.02.1023

1atm = 760mmHg = 101325Pa
1Torr = 1mmHg

1mL = 1cm³
1dm³ = 1L = 1000mL

1000Kg = 1ton
1Kg = 1000g
1g = 1000mg

1nm = 1.10-9m

Formula for calculating number of moles (n):

n = number of moles
m = mass (g)
MM = molar mass (g / mol)