MCTP Maryland Collaborative for Teacher Preparation "pH Balanced": the meaning of pH Thomas C. O'Haver Department of Chemistry and Biochemistry University of Maryland College Park, MD 20742 (301) 4051831 to2@umail.umd.edu NSF Cooperative Agreement No. DUE 9255745 Topic Acids, bases, pH, dissociation equilibria in water solutions. Primary Expected Outcomes The student should be able to describe acids, bases, and neutralization; use chemical equations to represent the dissociation of acids and bases; describe solutions as acidic, basic, or neutral based on their [H+], [OH-], or pH; interpret pH values as being acidic, basic, or neutral; describe the quantitative meaning of pH; understand the logarithmic nature of the pH scale; give common examples of familiar substances that are acidic or basic. Scientific and/or Math Concepts Special nature of the hydrogen ion, centrality of water as a reference point, nature of acids and bases; dissociation; neutralization; reversable ractions, dynamic equilibrium, logarithmic scales, mathematical modeling. Disciplines integrated Chemistry, mathematics, earth science. Prerequisite knowledge Basic knowledge of atomic structure (relative size of nucleus, nature of ions); the concept of solution concentration; the nature of water. Student's Preconceptions Student may have a naive notion of an acid as simply something that "dissolves things". Activities and Experiences Equipment and supplies. Distilled water, tap water, rain water, vinegar, lemon juice, orange juice, tomato juice, milk, carbonated water, various soft drinks (Coke, etc.), sugar water, salt water, milk of magnesia, sodium bicarbonate, shampoo, household ammonia, wood ashes, lye, a 0.01 molar solution of hydrochloric acid, a 0.01 molar solution of sodium hydroxide. A sample of local rainwater, if possible. A pH meter for each group of students. Buffer solutions for calibration. pH test paper. Dropper (medicine) bottles. Goggles \ for all students. Calculators with log and antilog functions. Student handout: ---------------------------------------------------------------------- Chemistry 121/122, Fall, 1994 Name________________________________ Acids, Bases, and pH Partner ____________________________ 1. You will be using a pH meter, a device that measures the concentration of hydrogen ions (H+) in a solution. It has a probe that is immersed in a solution to be measured. You will also be provided with a wide array of samples of common substances. Working with your partner, measure and record the pH of each of the samples, using both the pH meter and the pH test paper strips provided. Between measurements carefully rinse off the pH meter probe with water from the squeeze washbottle, in order to avoid contaminating the samples. -------------------+-----------+-------------+------------------------ Substance | pH meter | pH paper | Acid/base/neutral -------------------+-----------+-------------+------------------------ distilled water | | | -------------------+-----------+-------------+------------------------ tap water | | | -------------------+-----------+-------------+------------------------ rain water | | | -------------------+-----------+-------------+------------------------ vinegar | | | -------------------+-----------+-------------+------------------------ lemon juice | | | -------------------+-----------+-------------+------------------------ orange juice | | | -------------------+-----------+-------------+------------------------ tomato juice | | | -------------------+-----------+-------------+------------------------ milk | | | -------------------+-----------+-------------+------------------------ carbonated water | | | -------------------+-----------+-------------+------------------------ Coke | | | -------------------+-----------+-------------+------------------------ sugar water | | | -------------------+-----------+-------------+------------------------ salt water | | | -------------------+-----------+-------------+------------------------ milk of magnesia | | | -------------------+-----------+-------------+------------------------ sodium bicarbonate | | | -------------------+-----------+-------------+------------------------ shampoo | | | -------------------+-----------+-------------+------------------------ household ammonia | | | -------------------+-----------+-------------+------------------------ wood ashes | | | -------------------+-----------+-------------+------------------------ lye | | | -------------------+-----------+-------------+------------------------ .01 M HCl solution | | | -------------------+-----------+-------------+------------------------ .01 M NaOH solution| | | -------------------+-----------+-------------+------------------------ 2. The definition of pH is such that small values of pH correspond to large values of hydrogen ion concentration, and vice versa. Which of these substances has the largest hydrogen ion concentration? Which substance has the smallest hydrogen ion concentration? 3. Solutions with a pH about equal to that of pure water are called neutral solutions; those with a pH below that of pure water, and therefore have relatively high hydrogen ion concentration, are called acidic solutions; those with a pH above that of pure water, and therefore have relatively low hydrogen ion concentration, are called alkaline or basic. a. In the table of pH values you just measured, classify each substance as being acidic, basic, or neutral by writing the appropriate term in the third column of that table. b. Which of of these substances are associated with foods or drinks? What can you say in general about the pH of those substances? c. Which of of these substances are associated with household cleaning or washing? What can you say in general about the pH of those substances? d. Compare the pH meter readings in your table with the pH test paper readings, according to the color chart on the box. Do you feel that the pH test paper is a good substitute for a pH meter? 4. Measure out roughly 20 mL of water into a 50 mL beaker and add ten drops of 0.01 molar hydrochloric acid solution, using the medicine dropper provided. Stir the solution and measure and record its pH. Add 0.01 molar sodium hydroxide solution one drop at a time, with stirring, and measure and record the pH and total number of drops, until a total of 20 drops have been added. ------------+--------- Drops NaOH | pH added | reading ------------+--------- 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ---------------------- a. Describe your observations in your own words. b. Prepare a line graph showing drops added as the manipulated variable (on the x or horizontal axis) and pH as the response variable (on the y or vertical axis). Label the axes. c. On the basis of your pH measurements, what happens to the hydrogen ion concentration of the hydrochloric acid solution as sodium hydroxide solution is added to it? d. The chemical formula for sodium hydroxide is NaOH. It is a white solid which is very soluble in water. A sodium hydroxide solution is a good conductor, which suggests that ions are formed in solution when NaOH solid is dissolved. Write down all the ways you can think of that NaOH might ionize in solution. e. You observed that the addition of a sodium hydroxide solution to a strongly acidic solution containing a large concentration of H+ ions causes an increase in pH (reduction in concentration of H+ ions) and even produced a neutral solution with a pH near 7, like pure water, H2O. Of your various possible schemes for the ionization of NaOH, which one do you think would produce ions that are likely to be attracted to and to react with H+ ions to produce a neutral product? f. Write the reaction that shows the consumption of H+ from the acid to form a neutral product. g. The addition of a basic solution to an acidic solution, such that the resulting solution has a pH near that of water, is called neutralization. Why is that an appropriate name? 5. Pour 10 mL of the 0.01 moles/liter hydrochloric acid solution into a beaker and label this solution A. (Moles/liter is a unit of concentration that expresses how many molecules of HCl were dissolved in one liter of solution. One mole is a very large number of molecules). Pour exactly 1.0 mL of solution A into a graduated cylinder and add water to the 10 mL mark. Pour this into a second beaker and label it B. What is the resulting concentration in moles/liter of solution B? Carefully rinse out the graduate cylinder. Pour exactly 1.0 mL of solution B into the graduated cylinder and add water to the 10 mL mark. Pour this into another beaker and label it C. What is the concentration in moles/liter of solution C? Carefully rinse out the graduate cylinder. Pour exactly 1.0 mL of solution C into the graduated cylinder and add water to the 10 mL mark. Pour this into another beaker and label it D. What is the concentration in moles/liter of solution D? Measure the pH of solutions A, B, C, and D, carefully rising the probe between measurements (why?). Enter the concentrations, in moles/liter. and the measured pH values of the four solution into the table. ----------+----------------------+---------------+------------------- Solution | [H+] calculated by | Measured pH | [H+] calculated | dilution of original | of solution | from measured pH ----------+----------------------+---------------+------------------- A | 0.01 moles/liter | | ----------+----------------------+---------------+------------------- B | | | ----------+----------------------+---------------+------------------- C | | | ----------+----------------------+---------------+------------------- D | | | ----------+----------------------+---------------+------------------- b. Describe the relationship between concentration and pH in your own words. c. Propose a simple mathematical relationship between concentration, expressed in moles/liter, and pH. Write down an expressions for pH as a function of concentration, and for concentration as a function of pH. d. What would you predict would happen to the pH if the dilution process were continued to concentrations down to 10-8 or 10-9 molar, that is, where the diluted concentration of the [H+] added by the HCl becomes smaller than the [H+] already present in the water. In the limit of infinite dilution, what would you predict to be the pH of a diluted HCl solution? 6. a. You observed that pure water has a pH of 7, which implies an H+ concentration of 10-7 moles/liter. You also observed that when you added NaOH solution to an acid solution, the H+ ions were consumed in a neutralization reaction. In a pure NaOH solution, if the OH- ions from the ionization NaOH react with the H+ ions that are present in pure water, then would you predict the H+ concentration of NaOH solutions to be higher or lower than pure water? b. Pour 10 mL of the 0.01 moles/liter NaOH solution into a beaker and label this solution A. Pour exactly 1.0 mL of solution A into a graduated cylinder and add water to the 10 mL mark. Pour this into a second beaker and label it B. What is the concentration in moles/liter of solution B? Carefully rinse out the graduate cylinder. Pour exactly 1.0 mL of solution B into the graduated cylinder and add water to the 10 mL mark. Pour this into another beaker and label it C. What is the concentration in moles/liter of solution C? Carefully rinse out the graduate cylinder. Pour exactly 1.0 mL of solution C into the graduated cylinder and add water to the 10 mL mark. Pour this into another beaker and label it D. What is the concentration in moles/liter of solution D? Measure the pH of solutions A, B, C, and D, carefully rising the probe between measurements (why?). Enter the concentrations, in moles/liter. and the measured pH values of the four solution into the table. ----------+----------------------+---------------+------------------- Solution | [OH+] calculated by | Measured pH | [H+] calculated | dilution of original | of solution | from measured pH ----------+----------------------+---------------+------------------- A | 0.01 moles/liter | | ----------+----------------------+---------------+------------------- B | | | ----------+----------------------+---------------+------------------- C | | | ----------+----------------------+---------------+------------------- D | | | ----------+----------------------+---------------+------------------- c. Describe the relationship between concentration of sodium hydroxide and pH. d. Devise an expression that gives the pH as a function of the concentration of the base, Cb, in moles per liter. e. Plot [H+] vs [OH-] using a piece of standard graph paper. Label the axis. f. Describe qualitatively the relationship between these two quantities as NaOH concentration is varied. g. Propose a mathematical model for the relationship between the concentration of OH- and the concentration of H+. Hint: Try computing the product of these two concentrations and see if you notice anything.