Chapter 6 Ions: Charged Particles in Solution 101 3. Dissolve 40 g NaOH in 1 liter of distilled water. (This is a very exothermic process. Because of the heat generated it must be done gradually for safety’s sake.) One mole of any substance always has the same number of particles. Therefore, chemical equations can also represent the number of moles of each of two combined substances. The number preceding the elements or molecules tells scientists how many moles to combine for the desired result. For example, look at the equation below. How many moles of oxygen are needed for the body to use 1 mole of glucose? C6H12O6 + 6O2 The number 6 preceding the O2 represents 6 moles. Applications of pH The relationship of acids and bases to pH is helpful in understanding how the body digests food. More important to the food scientist is the role of pH in food preservation and baking. pH and Digestion The balance of pH in the human body is critical for many life processes. It is especially critical for digestion. The digestive process begins in the mouth. There, food is mixed with saliva, which has a pH of about 6.5. After food is swallowed, it travels to the stomach. As food enters the stomach, the stomach begins to secrete gastric juice. Gastric juice is a combination of water and other liquids that help break down proteins and fats. It is composed of the following: • Hydrochloric acid. This substance makes the stomach extremely acidic to kill harmful bacteria and aid in the digestion of protein. • Pepsin. This enzyme begins breaking down protein molecules into smaller pieces. • Gastric lipase. This enzyme begins fat digestion. • Gastrin. Gastrin is a hormone that controls the secretion of hydrochloric acid and helps keep food moving through the digestive process. • Intrinsic factor. This substance helps the body absorb vitamin B12 in the intestines. The pH in the stomach is between 1.5 and 1.7. The stomach has a special lining that produces large amounts of mucus. This mucus enables the acid to digest food without eating through the stomach wall. The same principle applies to molecules. One mole of a compound has a mass in grams equal to the combined atomic masses of the elements in the compound. Think about water. Each water molecule contains two hydrogen atoms and one oxygen atom. Each hydrogen has an atomic mass of 1 g for a total of 2 g. Oxygen has an atomic mass of 16 g. These numbers are added together to get the atomic mass of a mole of water, which equals 18 grams. The following example illustrates how to determine the number of grams of C6H8O6 (ascorbic acid) equal 1 mole. 1. Using the periodic table, find the atomic mass of each element in the compound. C = 12 H = 1 O = 16 2. Multiply the atomic mass of each element by the number of atoms of that element in the compound. C = 6 × 12 = 72 H = 8 × 1 = 8 O = 6 × 16 = 96 176 grams Molarity Suppose a scientist needs a known amount of sodium hydroxide (NaOH). He or she can simply calculate the number of grams in a mole. In titrations, however, the acid or base is in water solution. This is because diluted acids and bases are safer to work with in case of spills, and some acids and bases exist only as solutions. Therefore, scientists measure solute concentrations in terms of molarity (M). The formula for molarity is molarity = moles liters A one molar (1 M) solution is 1 mole of an element or compound dissolved in 1 liter of distilled water. You would write the chemical formula for a one molar solution of sodium hydroxide as 1 M NaOH. The steps to preparing this solution are as follows: 1. Determine the atomic mass of 1 mole of NaOH. Na = 23 O = 16 H = 1 40 grams 2. Mass 1 mole or 40 grams of NaOH. Copyright Goodheart-Willcox Co., Inc.