Chapter 6 Ions: Charged Particles in Solution 99 molecules are in a known solution. Scientists cannot sit down and count out atoms of an element as a child counts out different colored candies. Atoms are too small. In 1811, Amedeo Avogadro, an Italian physicist, made an observation about gases. He observed that equal volumes of different gases at the same temper- ature and pressure contained the same number of particles. This principle led scientists to discover that 22.4 liters of any gas at 0°C contains approximately 602,000,000,000,000,000,000,000 particles. This number is usually written in scientific notation as 6.02×1023. This number is known as Avogadro’s number. Avogadro’s number of particles of any substance is known as a mole. A mole of one element always has the same number of atoms or particles as a mole of another element. Avogadro’s work led to another discovery. One mole of an element has a mass in grams equal to the atomic mass of the element. You can find the atomic mass of an element on the periodic table. When looking at the periodic table, you will notice that the atomic mass is the largest number describing each element. Remember that the other number is the atomic number and represents the number of protons. The atomic mass is the average number of protons and neutrons for each element. Oxygen has an atomic mass of 15.9994 g. For the purposes of this class, round the atomic mass to the nearest tenth. Thus, 1 mole of oxygen has a mass of 16.0 grams. See 6-9. After neutralization occurs, it is impossible to tell how a solution was formed. It could have been made by mixing an acid and a base. The solution could also have been made by dissolving a salt in water. Measuring Concentrations of Acids and Bases Titration can work only if you have a known solution. You must know the volume and concentration of this solution. Concentration is the measure of parts of one substance to the known volume of another. You may wonder how a scientist knows how many Characteristics of Acids and Bases Acids are any substances that… Bases are any substances that… produce excess hydronium or hydrogen ions produce excess hydroxide ions have a positive electrical charge have a negative electrical charge are proton donors are proton acceptors are electron acceptors are electron donors taste sour taste bitter turn litmus red turn litmus blue have a pH of 1.0 to 6.9 have a pH of 7.1 to 14.0 6-7 Remembering these characteristics will help you distinguish acids from bases. Janet Ward 6-8 You can measure pH with pH paper. For more accurate pH readings, however, use a digital pH meter. Mole Mass of Elements 55.847 Fe 26 Atomic mass Element (iron) Atomic number 6-9 By looking at the atomic mass of iron in a periodic table, you can calculate that 1 mole of iron equals 55.8 g. Copyright Goodheart-Willcox Co., Inc.