Chesapeake Campus – Chemistry 112 Laboratory

Objectives

• Calculate the concentration of a NaOH solution by titrating with a primary standard.
• Determine the mol base present in a commercially available antacid.
• Determine and balance neutralization reactions.

Introduction

Acid –Base reactions are an important portion of chemistry. In aqueous solutions, a compound that produces H+ ions upon dissolution is termed an acid. A compound that produces OH– ions when dissolved in water is called a base. The reaction of an acid and base is a neutralization reaction, the products of which are a salt and water. In an aqueous solution, virtually all of the OH– ions present will react with all of the H+ ions that are present to give:

H+ + OH− → H2O Equation 1 (𝑎𝑞) (𝑎𝑞) (𝑙)

It is possible to determine the concentration of an acid or base in an aqueous solution with high accuracy because this reaction is quantitative.

When a solution of hydrochloric acid, HCl, is exactly neutralized with a solution of sodium hydroxide, NaOH, the number of moles of NaOH used will equal the number of moles of HCl originally present. The following relationship then holds true:

molesNaOH = molesHCl Equation 2 (M )(V = (M )(V ) Equation 3

or

where M = concentration in molarity and V = volume. If three of the above quantities are known, the fourth can be calculated.

In order to determine when a solution has been exactly neutralized, an acid-base indicator is used that changes color in a certain pH range (pH is a scale used to measure acidity). This color change is termed the endpoint of the titration. Because the pH of a neutral solution is 7, an indicator that changes color near this pH should be used for an acid-base titration. Phenolphthalein indicator changes color in the range pH = 8.3 – 10.0 and can be used to determine when the correct amount of base has been added to an acidic solution to exactly neutralize it. An endpoint using phenolphthalein

NaOH (NaOH in L) HCl HCl in L can be observed at 1-2 drops past the equivalence point which is within a small margin of error and allows for us to perform chemical calculations with reasonable accuracy.

Preparation of Sodium Hydroxide Solution
Sodium hydroxide is hydroscopic which means it constantly gains water. This continued volume change has big implications for the solution’s concentration. Therefore a solution that was made yesterday may not have the same concentration today. We must standardize or calculate the concentration of the NaOH solution for this lab every day the lab is performed. To standardize a solution we use a compound that is stable, has a large molar mass, and reacts in a predictable way. For this experiment, a solution of NaOH, which has an approximate concentration of 0.1 M, will be standardized using potassium acid phthalate, (which is abbreviated KHP). The molecular weight of KHP is 204.23 g/mole, and it has one acidic proton, which will react quantitatively with OH–:
𝐾𝐻𝑃 + 𝑂𝐻 − → H O + KP+ Equation 4

(𝑎𝑞) (𝑎𝑞) 2 (𝑙) (𝑎𝑞)

For the highest accuracy, a sample size is chosen such that it will consume as large a volume of the base as possible without exceeding the capacity of the buret. If a 25 mL buret is used, the amount of KHP is chosen such that it will require approximately 20 mL of 0.1 M NaOH solution to reach the endpoint. Thus, about 0.002 moles, or 0.4 g, of KHP is needed. Once you measure the mass of KHP used, it can easily convert be converted to mols using the molar mass of KHP.

g KHP = 1molKHP =molKHP Equation5 204.23 g KHP

At the endpoint, the number of moles of NaOH equals the number of moles of KHP used.

moles NaOH = moles KHP

This will allow us to find the molarity of the NaOH solution.

Equation 6

Equation 7

Equation 8

or

mol NaOH = M NaOH V NaOH used (in L)

𝑀 𝑁𝑎𝑂𝐻 = 𝑚𝑜𝑙 𝐾𝐻𝑃 𝑉𝑁𝑎𝑂𝐻 𝑖𝑛 𝐿

Once the NaOH solution has been standardized, it can be used to determine the acid neutralizing capacity of an antacid tablet.

Determination of the Acid Neutralizing Capacity of an Antacid Tablet
The stomach has an acidic interior generated by dilute HCl, “stomach acid”,

which insures proper digestion. When the acidity of the stomach becomes high enough to cause discomfort, brought about by the ingestion of certain types of food, an antacid preparation can be taken to neutralize the excess stomach acid. The active ingredient in every antacid is a base, the most common being metal hydroxides, metal carbonates or a mixture of the two. Table 1 lists the active ingredients in several commercial brands of antacid.

Brand Name Major Ingredient Recommended Dose

Table 1. Brands of antacid tablets and their major ingredients

Antacids containing the carbonate ion actually form a buffer (of carbon dioxide and bicarbonate) solution in the stomach. Water gains dissolved carbon dioxide from the air which means solutions containing the antacids will initially have a buffer system. Adding excess acid helps eliminate this. In addition, it is important to heat samples containing the carbonate ion to remove as much dissolved carbon dioxide as possible. Skipping this step would greatly affect calculations.

The acid neutralizing capacity of a tablet is the amount of hydrochloric acid that it can neutralize. It is the quantity that is referred to in some advertisements when it is stated that the tablet “neutralizes x times its weight in stomach acid”. This capacity can be determined by a technique called back-titration. A known amount of antacid is dissolved in an excess of HCl, and then the excess acid is back-titrated with standardized NaOH solution.

When the endpoint is reached, the number of moles of acid that was added to the antacid sample is equal to the number of moles of base present (NaOH solution plus the antacid). Therefore, the number of moles of HCl that was neutralized by the antacid is equal to the total number of moles of HCl added minus the number of moles that were neutralized by the NaOH:

moles acid neutralized = (moles of HCl added) – (moles of NaOH required for back-titration)

molacidneutralized= (M x V )– (M x V ) 𝐻𝐶𝑙 𝐻𝐶𝑙 𝑖𝑛 𝐿 𝑁𝑎𝑂𝐻 𝑁𝑎𝑂𝐻 𝑖𝑛 𝐿

where M = molarity and V = volume in liters or milliliters.