Quantities of chemical Species

When we study stoichiometry we have to calculate the quantities of Species in several ways:

Take the reaction:

 2HCl + Fe -> FeCl2 + H2

As you can appreciate, it is already balanced 
We now have to study the quantities of species. Stoichiometry is all about this. There are up to 4 magnitudes related to the quantity of substance:

• Mass(m): the basic amount of substance. We measure mass in g. We can use mass to express the quantity of a solid, a gas, or a liquid, but we shouldn't use it in a  chemical equation

• Volume, V: is used for gasses. A mole of gas in normal conditions fills 22,4L.

• Number of moles,n: which is the magnitude we handle in a chemical equation. A mole is a group which contains 600000 trillion!!!! particles. This is Avogadros number. Stoichiometric quotients refer to the number of moles

• Concentration, C. for solutions. It is the number of moles in a volume of 1L. The units are mol/L 2mol/L=2M

Now: How do we change from a magnitude to other???????: We have to use these fórmula

• From mass m to number of moles. We use the formula n=m/Pm. Pm is the mollecular mass: sum of the masses of all the atoms (Periodic table) of the compound. For instance, for HCl; Pm=1+35.5=36,5g/mol If we have 56g of HCl: n=m/Pm=56/35,5=1.58mol. If we have 2 mol of HCl, its mass will be m=n·Pm=2*36.5=73g
• From volume in a gas to number of moles: n=V/22.4L. 10L of H2 contain n=10/22.4L=0.45 moles. 2.5 moles of H2 fill V=22.4×n=22.4×2.5=56L. Then, to calculate Volume of a certain number of moles of a gas: V=22.4·n; 3 moles of H2 fill: V=22.4*3=67.2L 

• From concentration C and volume V of a disolution to number of moles n=CV. 2L of a 0.5M disolution have n=CV=0.5×2=1mol. 4 moles of a 2M substance fill V=n÷C=4÷2=2L

Now do these exercises in a piece of paper: You will need a PT


1. Calculate the mollecular mass of FeCl2, H2, AlCl3


2. Calculate the number of moles in 25g FeCl2, 4g H2, 80g AlCl3.


3. Determine the mass in 2.5 moles of FeCl2, 0.7 moles of HCL, 6 moles of H2O


4. Determine the volume of 0.3 mol of H2, of 1.5 mol H2 and 3.2 mol of H2. Determine the number of moles in 11.2L of H2 and 20L of H2. Determine the mass of 25L of H2


5. Determine the number of a)moles of 2L of HCl 2M. b) 500 mL of HCl 0.3 M How much mass of HCl would this solution contain (you must determine the number of moles of HCl first)


6. How much volume of HCL 0.6 M do we need to get a)0.3mol b)1.8 mol