Mass-Mass Stoichiometry

In a mass-mass calculation you are given information in grams and asked for information in grams.  To complete stoichiometry problems you must know the ratio of particles.  The only way to determine this is by writing a balanced reaction.  The coefficients will provide you with the MOLE RATIO.

Remember that a reaction tells you the ratio of particles or moles, NOT MASS!  A gram of one substance will not have the same number of particles as a gram of another substance. That means you will need to convert grams to moles using molar mass.

We approach this type of problem using the same problem solving steps that we've been using all semester.

1. What are we given?
2. What are we looking for?
3. What additional information is needed?

Let's walk through an example problem:

     Example 1:  An excess amount of sodium chloride reacts with 25.0 grams of lead (II) nitrate in water.  How many grams of precipitate will be formed?

1. What are we given?
   excess NaCl, 25.0 g Pb(NO3)2
2. What are we looking for?
   grams of solid (?)
3. What additional information is needed?
   reacts means a reaction, so you need a balanced reaction
   grams means at some point molar mass will be needed
   solid, means you need to know what precipitates, so you need states

Example 2: How many grams of NaCl are actually needed to react all 25.0 g of lead (II) nitrate?

Ionic Reactions

In a molecular or complete reaction, all substances are shown in neutral (molecular) form.

In an ionic reaction, the substances that all always dissociate (strong) are broken into their ions.

Because lead (II) nitrate, sodium chloride and sodium nitrate all all soluble salt (strong electrolytes), they will all completely dissociate in water.  They will not exist as molecules, but as ions.  Remember you can't lose any mass in a reaction!  The Law of Conservation of Matter still applies!

 Notice that sodium and nitrate are EXACTLY the same on the reactant and product sides?  They are spectator ions.  Spectator ions do not change in a reaction.  When you cancel the spectator ions, you're left with the net ionic reaction.

This shows us what is really changing in a reaction- the driving force for the reaction to proceed.

Aqueous Solutions

Water is a very polar molecule, meaning it has a partial positive charge on the hydrogen end and a partial negative charge on the oxygen end.  This is caused by the unequal sharing of electrons by the hydrogen and oxygen atoms.

Because water is polar, it will dissolve most ionic compounds.  Since ionic compounds are composed of a positive ion (cation) and a negative ion (anion), the opposite charged end of a water molecule will be attracted and break a large crystal into smaller pieces.  This is called hydration. If the molecules are completely broken into their ions by water, it is called dissociation.

For instance, table salt (NaCl) will completely dissociate in water. Every single molecule will be broken apart into ions and kept apart by the water molecules.

NaCl (aq) --> Na+ (aq) + Cl- (aq)

In chemistry, STRONG means that every molecule will dissociate when dissolved in water.  WEAK means that it partially dissociates, or that only some of the particles will dissociate while others will remain in neutral/molecular form.

Strong acids, strong bases and strong electrolytes will always dissociate when dissolved in water.  Weak acids, weak bases and weak electrolytes may or may not dissociate.