How would you balance HCI + NaOH -->H20 + NaCI?
Well in fact you have already balanced this.
In every chemical reaction, mass is conserved, and it is conserved in the acid/base reaction above. If there are 5 g of reactant there must be 5 g of product. This is an example of stoichiometry. To put this in another context, suppose you went to a shop with a £20-00 note, and made a purchase worth £8-15. If you received £5-85 in change, would you have cause for complaint? I think you would because the value of the purchase plus the value of your change must equal £20-00. This transaction is not stoichiometric, whereas the acid/base reaction is stoichiometric. For every reactant particle, there is a corresponding product particle.