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pHMy Goals for this Lesson:Determine the relationships between pH, hydronium (H3O+), and hydroxide (OH-) ions in a given solution.Describe the process of titration and apply it to a laboratory investigation.I’m preparing to understand what pH means and how it is determined in calculations and a titration. pH Many of the solutions that you encounter every day can be categorized as acidic or basic. Use the pH paper provided to test the pH value of some common substances. Substances with pH values greater than 7 are basic, equal to 7 are neutral, and less than 7 are acidic. Record your findings in your notebook.514510941021000Fill in this table with the pH of the everyday solutions you found in the Introduction. Classify each as an acid, base or neutral solution. SolutionpHAcid, base or neutral?Orange juiceMilkDistilled water330390510731500Toilet cleanerDetergentDish soapWindow cleanerDeodorantVinegarDrain cleanerWaterFill in the blanks using the lesson. -425452984500Water has the ability to behave as both an and as a , depending on the properties of the other reactant. This means that water molecules can react with each other in a sample of pure water. One water molecule, acting as an acid, donates a ion to another water molecule, acting as a base. This reaction forms a ion and a ion, which can also react together to re-form the water molecules. H2O + H2O OH- + H3O+ When water molecules react together, numbers of hydroxide (OH-) and hydronium (H3O+) ions are formed. This means that in pure water, the concentrations of hydroxide (OH-) and hydronium (H3O+) ions are, making the solution . Experiments have revealed that the concentrations of hydroxide (OH-) and hydronium (H3O+) ions in pure water are extremely low, around 0.0000001 molar for each at room temperature. This is why pure water behaves as a very weak acid or base, and is also a very poor electrolyte. Water is because it contains equal amounts of and ions, not because of the low concentrations of these ions. The acidic, basic, or neutral property of a solution is determined by the between the hydronium (H3O+) and hydroxide (OH-) ions in the solution. When the concentrations of the hydronium (H3O+) and hydroxide (OH-) ions are , the solution is neutral. In an acidic solution, the concentration of ions is greater than the concentration of hydroxide (OH-) ions. Adding an acid, such as hydrochloric acid, to pure water will increase the concentration of ions present. HCl + H2O → Cl- + H3O+ When the hydronium (H3O+) ion concentration is than the hydroxide (OH-) ion concentration, the solution is acidic. In a basic solution, the concentration of ions is greater than the concentration of hydronium (H3O+) ions. Adding a base, such as ammonia, to pure water will increase the concentration of the ions present. NH3 + H2O → NH4+ + OH-When the hydroxide (OH-) ion concentration is greater than the hydronium (H3O+) ion concentration, the solution is basic.pH ScaleFill in the blanks using the lesson.In addition to describing a solution as acidic, basic, or neutral, scientists use numerical values to express the acidity of a solution in more detail. Because the of hydronium (H3O+) and hydroxide (OH-) ions in a solution can vary greatly, chemists use values called pH to conveniently express a solution’s ion concentration.The letters pH stand for the French words pouvoir hydrogène, meaning “ .” The pH scale is a numeric scale used to indicate the ion concentration of a solution. The pH of a solution is determined by calculating the negative base-10 logarithm of the hydronium (H3O+) ion concentration (in molarity).According to the Br?nsted-Lowry definition of acids and bases, acids increase the concentration of hydronium (H3O+) ions in a solution by donating hydrogen ions, while bases decrease the concentration of hydronium (H3O+) ions by accepting hydrogen ions. This means that the acidic or basic nature of a substance can both be measured and described by its hydrogen ion, or hydronium (H3O+) ion, concentration. This is why pH values can be used to describe acidic and basic solutions, even though the values are calculated using the concentration of hydronium (H3O+) ions. The range of pH values of aqueous solutions generally falls between 0 and 14, which is a more reasonable range for comparison than the possible range of concentrations. The pH scale and its values are dependent on, so we will be comparing and calculating pH values at 25 degrees Celsius.The pH of a solution, when the concentrations of hydroxide (OH-) and hydronium (H3O+) ions are equal, at 25°C is 7.0. When the amount of hydronium (H3O+) ions is greater than the amount of hydroxide (OH-) ions in the solution, the solution is and will have a pH value that is lower than 7.0. In a solution, the amount of hydroxide (OH-) ions is greater than the amount of hydronium (H3O+) ions, and the pH will be greater than 7.0. Refer to the pH scale to compare the pH values of some common solutions. Notice that as the concentration of hydronium (H3O+) ions increases, the pH value decreases (becomes more acidic). It can be easy to confuse this, so be careful when you use the pH scale or compare pH values.372046510287000Fill in the table using the pH in the lesson. H3O+ concentrationpH valueExampleConcentration of H+ compared to water.01234567891011121314Determining pH and Concentration (this is the TEXT VERSION from the interactive at the bottom of the lesson)There are two main types of pH problems that will concern you: the calculation of pH when the hydronium ion concentration is knownthe calculation of the hydronium and hydroxide ion concentration when the pH is knownUsing concentration to solve for pH When you are given the concentration of hydronium (H3O+) ions in a solution, or the concentration of a strong acid (assumed to ionize 100 percent), you can determine the pH by using the following formula: pH = -log [H3O+]For example, we can determine the pH of a 1.2 × 10-3 molar nitric acid solution by plugging into the formula above because nitric acid is a strong acid that will ionize 100 percent to produce 1.2 × 10-3 molar H3O+.pH = -log [1.2 × 10-3]pH = 2.92Be sure to notice that there is a negative sign in the formula for calculating pH values. In most cases, the pH value will come out to be a positive number. If the concentration of hydronium ions is high, the pH value may come out to be less than zero. When you are given the concentration of hydroxide ions in a basic solution, you can use that value to find the concentration of the hydronium ions present using the following relationship: [H3O+] × [OH-] = 1.0 × 10-14 MThe value 1.0 × 10-14 is called the ionization constant of water. The relationship above can be used to compare the hydronium and hydroxide ion concentrations in any aqueous solution at 25°C. For example, we can determine the pH of a 2.3 × 10-2 molar NaOH solution by first using the hydroxide ion concentration to determine the hydronium ion concentration in the solution. NaOH is a strong base that ionizes completely in water, so the concentration of the base is equal to the concentration of the hydroxide ions in the solution. Dividing the ionization constant of water by the concentration of the hydroxide ions will give you the concentration of hydronium ions in the same solution. [H3O+] × [OH-] = 1.0 × 10-14 M[H3O+] = 1.0 x 10-14/ [OH-][H3O+] = 1.0 x 10-14/ 2.3 x 10-2[H3O+] = 4.3 × 10-13 M H3O+pH = -log [4.3 × 10-13]pH = 12.4Notice that the concentrations of hydronium and hydroxide are inversely related—the higher the concentration of one, the lower the concentration of the other. On Your Scientific CalculatorA base-10 logarithm is simply the power to which 10 is raised. For example, the logarithm of 103 (1,000) is three, which is the power to which 10 is raised. Any positive number has a logarithm. It might be helpful for you to use a scientific calculator to determine the logarithm of a given number when that number is not given as a number raised to a given exponent.Find the log button on your scientific calculator. Some calculators require you to press the log button after typing in the number, while others require you to press the log button before typing in the number. Because calculators can work differently, try the following problem to make sure you understand how to calculate logarithms using your calculator. What is the logarithm of 1.2?Answer: 0.079Did your calculator give you the correct answer when you pressed the log button before the number 1.2, or after? Write this order of operations down in your notebook as a reminder for when you need to solve logarithm problems later on. Practice Problem OneWhat is the pH of a solution with a concentration of 6.8 × 10-4 molar H3O+? pH = -log [6.8 × 10-4]pH = 3.17Practice Problem TwoWhat is the pH of a solution with a concentration of 4.2 × 10-5 molar OH--? Formula: [H3O+] × [OH-] = 1.0 × 10-14 MRearrange to solve for [H3O+]: 1.0 × 10-14 M / [OH-]Plug in [OH-] from the problem: 1.0 × 10-14 M /4.2 × 10-5 M OH--[H3O+] = -2.4 × 10-10 MSolve for pH:pH = -log [H3O+]pH = -log (-2.4 × 10-10 M)pH = 9.62Using pH to solve for concentrationWhen you are given the pH value of a solution, this value is a logarithm. To determine the concentration of hydronium ions from a given pH, you must take the negative antilogarithm of the pH value. [H3O+] = 10-pHFor example, we can determine the concentration of hydronium and hydroxide ions in a solution that has a pH of 2.3 by plugging into the formula above. [H3O+] = 10-2.3[H3O+] = 0.0050 MUsing the ionization constant of water, we can use the concentration of hydronium ions to determine the concentration of hydroxide ions in the solution. On Your Scientific CalculatorOn your scientific calculator, the antilog button may appear as one of the following: Once you find this button on your scientific calculator, you should practice using it so you know the correct order of operations for your calculator. Try the following problem to make sure you are using your calculator correctly. What is the antilog of -3.2?Answer: 0.000631Did your calculator give you the correct answer by pressing the antilog button before or after the number -3.2? Write this down in your notebook as a reminder for future problems. Practice Problem OneWhat is the concentration of hydronium ions in a solution with a pH of 6.3? [H3O+] = 10-6.3[H3O+] = 5.0 × 10-7 MPractice Problem TwoWhat is the concentration of hydroxide ions in a solution with a pH of 12.6?[H3O+] = 10-12.6[H3O+] = 2.51 × 10-13 M[OH-] = 1.0 × 10-14 M / [H3O+][OH-] = 1.0 × 10-14 M / 2.51 × 10-13 M[OH-] = 3.98 × 10-2 M600710019558000TitrationTitration is a laboratory method that allows you to determine the precise amount of one reactant needed to react with a given amount of another reactant. A buret is a long, narrow piece of glassware that is used to deliver small amounts of one reactant to a flask containing the other reactant. The bottom of the buret can be adjusted, using the stopcock, to release a small stream of liquid or add the reactant drop by drop. This allows you to carefully add the reactant until you reach the end of the reaction. For an acid-base titration, an acid-base indicator or pH meter is used to determine when you have reached the end of the reaction, called the endpoint.Be sure to do the “Titration” interactive section on the Activity page before doing the virtual lab. ................
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