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Lab 13 pH Chemistry |
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| Procedural Tips
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Print a copy of the Grading Sheet for this lab. Use of a Buret and Perfoming a Titration Don't forget to add indicator to your titration flasks. Why do I have to wear safety goggles? I have added twice as much base as everyone else and still haven't reached the endpoint. What is wrong?
Why is the NaOH in my buret pink?
Introduction: This experiment will introduce you to a method that can be used to measure the pH of a solution. The pH meter is an instrument that determines the pH of a substance based upon voltage changes in a electrochemical cell. Use of the pH meter is particularly useful when rapid accurate pH measurements are required, or when indicator colors may be obscured (dark or highly colored solutions). The behavior of buffers with the addition of acids and bases will be investigated. In the second part of the experiment, the molarity of an acid solution will be determined by titrating the acid solution with a standardized NaOH solution. Phenolphthalein indicator is used to detect the endpoint. Goals:
Arrehnius described an acid as any substance that produces hydrogen ions (H+) in aqueous solution and a base as any substance that produces hydroxide ions (OH-) in aqueous solution. Today we realize that H+ does not exit as such, but rather it exists as the H3O+ (hydronium) ion. Hydronium ion concentrations vary from relatively high values such as 10 M to very low values such as 1 X 10-14 M. Because it is inconvenient to work with exponential numbers, the pH scale was developed as a practical alternative. The pH of a solution is defined as the -log[H3O+]. Values for pH range from 0 to 14. Acidic solutions have pH values less than 7. Basic pH values are those pH is greater than 7. A neutral solution (neither acidic or basic) has a pH of 7.
Buffer solutions: A buffer is a solution that is prepared to resist large changes in pH when small amounts of acid or base are added to it. Many commercial products contain buffers to maintain optimum pH conditions. You may be familiar with buffered aspirin or pH-controlled shampoos. Buffers are also important physiologically to help maintain the pH of your blood and other tissues. In the lab, buffers are used to maintain optimum pH conditions for chemical reactions. Buffers are typically prepared by combining a weak acid and its conjugate base, or a weak base and its conjugate acid. Acetate and phosphate buffers are two commonly used buffer systems.
Titration: The concentration of acid or base concentration in a solution is determined by a procedure called a titration. An acid-base titration is based on the neutralization reaction between an acid and a base. In a neutralization reaction the point at which all of the acid in the sample is completely reacted with the base is called the equivalence point. At the equivalence point the moles of OH- equal the moles of H+. In the titration process one solution of known concentration is added incrementally to a measured volume of a solution of unknown concentration. The volume of the added solution is measured by using a buret. The titration is stopped at the endpoint of the titration. The endpoint is often indicated by a color change of an indicator such as phenolphthalein. In doing a titration, it is important that the the endpoint is very close to the equivalence point. Because the equivalence point and the endpoint are usually very close to the same value, the terms are often used interchangeably. In this experiment, the concentration of vinegar (an acid) will be determined by titrating it with a NaOH (base) solution of known concentration. The volume of NaOH required to react with a known volume of acid is found by reading a buret. If the concentration and volume of NaOH and the volume of acid is known, the concentration of the acid can be calculated as follows: For example: When a 25.0 mL sample of acid is titrated, 16.3 mL of a 0.200 M NaOH solution is required to reach the endpoint. What is the concentration (M) of the acid?
Read the introductory material on the webpage. Read Experiment Thirteen in the laboratory manual. Answer the pre-lab study questions on for Experiment Thirteen. You are encouraged to do the Application Questions at the end of Experiment Thirteen before coming to lab.
1.The titration of a 20.0 mL solution of HCl required 15.3 mL of 0.011 M NaOH. Calculate the concentration of the HCl. 2. Tell whether the following solution would be buffers?
3.Write a chemical equation to show the neutralization reaction between Mg(OH)2 and HCl. 4. The formula for acetic acid is CH3COOH. There are 4 hydrogen atoms in the structure. However, only one hydrogen is considered to be an acidic hydrogen. Indicate which of the four hydrogens is acidic. 5. What is the pH of a solution that has a [OH-] of 2.4 X10 -5? 6. Answer 10.101 in your McMurrary textbook.
SAFETY GOGGLES MUST BE WORN AT ALL TIMES DURING THE LAB.
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