Stoichiometry of Reactions Between Ions in Solutions
Key Questions

Answer:
Well, stoichiometry requires EQUIVALENCE with respect to mass and charge....
Explanation:
And for an older treatment of the principles involved, see this old answer. And the fundamental principle of stoichiometry is
#"garbage in equals garbage out"# . Every chemical reaction must be balanced with respect to mass and charge. A#10*g# mass of reactants from all sources yields at most a#10*g# mass of products.#"Molarity"# is a concentration term, i.e.#"molarity"="moles of solute"/"volume of solution"# ...and as such it has the units of#mol*L^1# .. And so if we have TWO of the three quantities, say#"molarity"# and#"volume"# , we can get the third..#"moles of solute"# ...…
#"molarity"xx"volume"="moles"# ....and this is certainly consistent dimensionally. What do I mean by this?And a practical example? Well suppose I gots a
#100*mL# volume of#HCl(aq)# , that is#1*mol*L^1# with respect to#HCl# . What mass of sodium hydroxide is required for equivalence?We write out the stoichiometric equation as a preliminary:
#HCl(aq) + NaOH(aq)rarr NaCl(aq) +H_2O(l)# #n_"HCl"=0.100*Lxx1*mol*L^1=0.100*mol# And for equivalence we require equimolar sodium hydroxide...
#0.100*molxxunderbrace(40.0*g*mol^1)_"molar mass of NaOH"=4.00*g# ...i.e. a#4*g# mass of hydroxide is required for equivalence. 
Answer:
How? The idea that not only mass is conserved but charge is conserved is a tenet of stoichiometry.
Explanation:
At its simplest level, stoichiometry embodies conservation of mass. If there are 10 g of reactant, at most there can be 10 g of product. But while mass is conserved in a chemical reaction; charge is also conserved. When we represent a redox reaction we use the addition/removal of electrons to represent reduction/oxidation respectively. Cross multiplication of the individual redox reactions shows how charge is conserved.