0.4 Antacids  (Page 3/4)

9. Titrate with your standardized NaOH solution until you reach the endpoint, a change in color from purple to green (or from purple to clear). Some samples may not give color changes as sharp as for the HCl standardizations; for these use your best judgment to estimate the endpoint. Endpoints will generally be sharper for quick titrations than for slow ones.

10. Repeat the above procedure with a new sample of the same antacid. Enter your data in an excel spreadsheet in the workrooms, check the web page later on the week and make a final comment on the overall cost to neutralize one mole of HCl for various brands. THIS IS PART OF YOUR REPORT AND IF DATA IS NOT FOUND ON THE WEB YOU SHOULD CONTACT YOUR TA. (No excuses)

Retail cost maximum

 *Active ingredient in all of these antacids is ${\text{CaCO}}_3$ .

Supporting information on standardization:

Molarity is the most commonly used concentration term when one is interested in the amount of materials involved in a chemical reaction in solution. Molarity (M) is defined as the number of moles of solute per liter of solution.

M = $\frac{\text{moles}(\text{solute})}{\text{Liters}(\text{solution})}$ (1)

The number of moles is calculated by dividing the mass of the sample in grams by the gram formula weight (GFW or molar mass). One GFW is the same as one mole.

Number moles = $\frac{\text{grams}(\text{solute})}{\text{GFW}}$ (2)

For example in a 0.150 M ${\text{HNO}}_3$ solution, there are 0.150 moles of ${\text{HNO}}_3$ in one liter of this solution. The following factors may then be used in chemical calculations: 

$\frac{0\text{.}\text{150}{\text{molHNO}}_3}{\mathrm{1L}(\text{solution})}$ or $\frac{\mathrm{1L}(\text{solution})}{0\text{.}\text{150}{\text{molHNO}}_3}$

In a chemical reaction that takes place in solution, the volume and the molarity of one reactant and the molarity of the second reactant can be used together with the stoichiometry of the equation to find the volume of the second reactant needed to react completely with the first reactant.

Titration is a process in which a solution of one reagent, usually the base, is added to an accurately measured volume of another solution, usually the acid, until the reaction is complete. The concentration of one of the reagents is known. From the known concentration and the measured volumes, the concentration of the second solution can be calculated.

In acid-base reactions the end of the reaction or equivalence point is detected by adding a compound that undergoes a color change as it changes from its acid form to its basic form. This compound is called an indicator. An indicator is an organic dye that changes color at a characteristic $H^{+}$ ion concentration. A dye can be an indicator if it has an intense color that changes when it gains or loses $H^{+}$ ions.

$\text{HIn}+{\text{OH}}^{-}\to {\text{In}}^{-}+H_{2}O$ (3)

The change of an indicator dye from Color A to Color B depends upon the concentration of $H^{+}$ (or ${\text{OH}}^{-}$ ) ions. Care must be taken in selecting an indicator to be sure that the color change (endpoint) occurs at the $H^{+}$ ion concentration that corresponds to the equivalence point. Phenolphthalein, methyl red, methyl orange, and litmus are examples of indicators.