nacid added = Macid x (25.00 mL).

The antacid now consumes a portion of that acid, schematically shown by

HCl (aq) + antacid (aq) m HCl (left over) + salts.

The amount left over is then titrated with NaOH, in a one-to-one reaction

HCl (left over) + NaOH (added) m H2O + NaCl + pink solution (except for Mylanta)

where nacid left over = nbase added = Mbase x Vhase.

The number of moles of acid used (or consumed) by the antacid is just the difference, that, is

nacid used = nacid added - nacid left over.

Knowing the number of moles of acid used, we can now calculate the number of milligrams of acid used.

Direct-titration: in the direct titration, the number of moles of acid used is simply the molarity of the standard acid times the volume added, i.e.,

nacid used = Macid x Vacid added.

Since the amount of each active ingredient is printed on the side of the container, we can calculate the number of moles of the active ingredients in a single pill.

For example, consider an antacid that is composed of NaHCO3 (let there by "x" moles of it) and Ca(OH)2 (let there by "y" moles of it). We can represent the reactions by

NaHCO3 (aq) + HCl (aq) m NaCl (aq) + H2CO3 (aq)

Ca(OH)2 (aq) + 2 HCl (aq) m CaCl2 (aq) + 2 H2O (liq).

Hydrochloric acid reacts one-to-one with NaHCO3, therefore "x" moles of HCl will disappear in the first reaction. The acid, however, reacts two-to-one with Ca(OH)2, therefore "2 y" moles of HCl will disappear in the second reaction. Combining the two,

nexpected acid use = x + 2y .

We know the amount of acid actually used; and we can calculate the amount of acid which should be used, based upon the ingredients in each dose. A percent yield is then calculated as per its definition.

(written by BKR)