The Mole (4) Using the mole to determine equation stoichiometry

Ok so what is a chemical equation?

If a chemical formula reveals the mole ratio of elements in  compound then a chemical equation reveals the ratio of reactants to each other and to the products of a chemical change.

This mole ratio is called the equation stoichiometry.

You always wanted know what that word meant, didn't you!!

Here's an example, a simple one:

2H2O2     =      2H2O   +     O2

You can see here that the mole ratio or stoichiometry is given by the large numbers in front of the chemical formulas.

That's 2 moles hydrogen peroxide decomposing to form 2 moles water and a mole of oxygen gas.

Each large number represents the number of moles (or the amount) of that compound in the balanced symbol equation for the chemical change.

(This blog post describes how to balance combustion equations)

Question is how can anyone measure an equation stoichiometry?

Here is one approach using the mass of a precipitate formed in a chemical reaction.

You can find this example here

But we need a simpler example to illustrate the idea.

Suppose we react a known mass of magnesium with excess hydrochloric acid and measure the volume of hydrogen evolved.

Here's the apparatus you could use.

Then we have a way of comparing the mass of magnesium used with the volume of hydrogen formed.

Here are a typical set of results that could be obtained:

Magnesium (g)       Hydrogen (cm3)

0.24                         240
0.12                         120
0.06                          60
0.10                         100

 If these mass results are converted into amounts in moles then this is what we find:

Amt Magnesium (mol)   Amt Hydrogen (mol)
0.01                                 0.01
0.005                               0.005
0.0025                             0.0025
0.00417                           0.00417

The mole ratio of magnesium to hydrogen in this chemical change is 1:1

We can begin to construct the chemical equation for the reaction between magnesium and hydrogen like this:

Mg    +    x HCl    =      XMgCl2     +     H2

It follows that x is then 2 and X  is 1 and the completed chemical equation for this reaction is

Mg    +     2HCl     =   MgCl2       +     H2

Calculations both at GCSE and Advanced level often require you to calculate masses of reacting substances or products formed.

These questions often rely on you using the molar quantities in the chemical equation for the reaction

Here is a typical example for you:

What mass of carbon dioxide is formed from the complete thermal decomposition of 25g of sodium hydrogen carbonate  NaHCO3?


Write the equation:                            2NaHCO3     =      Na2CO3    +     H2O     +     CO2

Put molar quantities to the symbols:   168g                      106g                 18g               44g                  
(you need to calculate the molar
mass (Mr) of each compound)

From the question. identify the           168g                                                                    44g
compounds involved

Work out what 1g would give               1g                                                                      44/168

Scale up by the quantity in                    25g                                                                  (25*44)/168
the question

                                                                                  Answer 6.55g carbon dioxide
Some example problems:

Zinc chloride can also be prepared in the laboratory by the reaction between zinc and hydrogen chloride gas.
Zn + 2HCl =  ZnCl2 + H2
An impure sample of zinc powder with a mass of 5.68 g was reacted with
hydrogen chloride gas until the reaction was complete. The zinc chloride produced had a mass of 10.7 g.
Calculate the percentage purity of the zinc metal. Give your answer to 3 significant figures.

People who have a zinc deficiency can take hydrated zinc sulfate (ZnSO4.xH2O) as a dietary supplement.
A student heated 4.38 g of hydrated zinc sulfate and obtained 2.46 g of anhydrous zinc sulfate.

Use these data to calculate the value of the integer x in ZnSO4.xH2O Show your working. 

The Mole (3) Using the mole to determine the simplest (empirical) formulae
The Mole (1) Relative Atomic Mass and the Mass Spectrometer
Mole (2) Amount of substance and Molar Mass