Experiment 30/31/32

ACID - BASE Equilibrium

Learning Objectives

Upon completion of this experiment, students will have experienced:

1. 1. Predict and balance the reactants and products in an acid - base reaction
  2. Calculate the mass, mole and molarity of each species within the balanced chemical reaction.
  3. Use a primary standard to titrate a solution and determine its molarity
  4. Use a standardized solution to determine the molar mass of an unknown substance.
  5. Identify by inspection acids, bases and salts.
  6. Understand the concept of buffers and how a buffer works
  7. Use a pH meter to measure the pH of acid, base and salt solutions
  8. Generate the titration curve for a strong acid and a weak acid with a strong base
  9. Use the titration curve to determine the equivalent mass and pK of the weak acid.

The following discussion and videos will show you real titrations and calculations of an acid base titration used to complete the combined Experiment 30/31/32 laboratory assignments.

The assignments that must be completed are:

1. Complete the pre laboratory problems
2. Complete the following parts I through VI for the report using the simulations, virtual labs and class data given.
  1. - Part I - complete the setup of 30+ test-tubes to see indicator dependence on pH
    - Part II - Titrate NaOH with KHP. Input and use the class data to complete the calculation and average for the standardization of the NaOH solution.
    - Part III - Measure the pH of several acid, base and salt solutions using a pH meter, pH paper and a simulation at http://billvining.com/mmlib_sims/#gen_15_2
    - Part IV - prepare a buffer solution and determine how the addition of NaOH or HCl affect the pH of the buffer solution.
    - Part V - Use the simulation at http://chemcollective.org/vlab/103 to create a pH plot of the unknown acid being titrated with NaOH and calculate the molarity and the Ka of the unknown acid solution.
    - Part VI - Titrate a strong, weak and polyprotic acid with NaOH.
3. Complete the conclusion paragraph for the experiments (30, 31, and 32).

Experiment 30

Experiment 31

Experiment 32

Discussion

Acids by definition are substances that generally contain a hydrogen atom that when placed in water becomes a hydrogen ion, H+, in solution. When a compound creates charge in an aqueous solution, it is also called an electrolytic solution. By the definition of Brönsted-Lowery, an acid is a hydrogen ion donator; therefore, it increases the hydrogen ion in solution, this does not mean that an acid has to contain a hydrogen atom. A strong acid is also a strong electrolyte. Several examples of common strong acids are HCl, HNO3, H2SO4, HClO4 and H3PO4. A base can be defined in two ways, first as a substance that contains the hydroxide ion, OH1-, or by Brönsted-Lowery, a substance that accepts a hydrogen ion and therefore, increases the hydroxide ion concentration in solution. Examples of strong bases include, NaOH, KOH. Examples of weak bases are NH3 and CO32- ion. Salts are electrolytes, which are ionic compounds where, in general, a metal bonds with a non-metal or polyatomic ion. We can also look at a salt as the product of an acid - base reaction.

When an acid reacts with a base, a double replacement reaction occurs and the products are water and a salt.

Acid + Base ⇄ Salt + Water

Examples of an acid base reaction include:

HCl + NaOH ⇄ NaCl + HOH Reaction 1

2 H3PO4 + 3 Ca(OH)2 ⇄ Ca3(PO4)2 + 6 H2O Reaction 2

When the moles of H+ = moles of OH1-, this is called the equivalence point. The second reaction has up to 6 equivalence points and is a very difficult problem to solve. In this lab we will use a monoprotic acid, which has only one equivalence and the strong base, NaOH, which also has only one equivalence. Therefore the equivalence point is the stoichiometric point at which the moles of acid = moles of NaOH.

The difficulty in this reaction is how we determine the equivalence point. This point can be calculated but when using an unknown, we need to see this point visually. Therefore, we use an indicator that when the pH of the solution changes, due to the changing hydrogen ion concentration, the indicator changes color and allows a visual indication of the endpoint, which if the indicator is chosen correctly is with very little error, can be considered to be the same as the equivalence point. In a titration of an acid with a base, the indicator shows the end point, at which point we can read a volume of the titrant and use this volume to determine the moles at the equivalence point.

Experiment 30-31-32 Discussion

Pre laboratory Writing Assignment

Complete the pre laboratory writing assignment in your laboratory notebook. The prewriting must be completed before the laboratory session begins. Use the procedure document below to explain what has to be done to collect the necessary data of the lab. The writing should include a purpose/hypothesis of the experiment, a discussion or theory, a detailed step by step summary of the procedure explaining what data will be collected and how you will collect it. Data Tables or space to collect the data should be included within the procedure discussion. Look up any SDS on chemicals that you are not familiar.

Pre laboratory problems

The pre laboratory problems ask questions or problem solving to help understand the laboratory assignment.

Attempt the pre laboratory problems either as the document provided in the experiment or as the laboratory quiz in canvas before coming to the class meeting.

O-PreLabExperiment 30/31/32

notes from April 14 - lab 31 and buffers.pdf

These are scanned notes of the work done for the pre laboratory problems

Titration of a sodium hydroxide solution simulation

Open the virtual lab at http://chemcollective.org/vlab/101

In this question, you will determine the concentration of a NaOH solution using a 0.500 M KHP solution and an acid-base indicator, phenolphthalein.

The directions for using the simulation are in the document to the right.

Experiment 30 Virtual Lab

Procedure

Experiment 30-31-32 Procedure

O-AcidBaseClassData

Part I. Indicators

Use the following pictures of the 30 test tubes. To the first column of test tubes, 1 mL of 0.01 M HCl was added. To a last column 1 mL of 0.01 M NaOH was added. To the second, third, and fourth columns, 1 mL of the pH 4, 7 and 10 buffer solutions were added. 5 drops of each indicator solution was then added to each set of test tubes to show how the pH affects the color of the indicator.

Part II. Standardization of sodium hydroxide solution.

A sodium hydroxide with an approximate 0.2 M concentration will be used to determine the molar mass of an unknown acid in Part III. The sodium hydroxide solution cannot be prepared with a precise determination of the concentration; therefore, the solution must be standardized with a primary standard. A titration will be performed to determine the concentration of the sodium hydroxide solution, accurately. There are several solid acids that could be used as a primary standard for this purpose; however, potassium hydrogen phthalate (KHP) is a very common compound used for this purpose.

Calculating the molarity of the NaOH solution from the titration of mass of KHP: https://youtu.be/J8gDdy6y_j8

Part III: pH of Acids, Bases and Salts

We will be measuring and calculating the pH of several acid, base and salt solutions. The first method we will employ is using pH paper to determine the pH for each of the following solutions. Compare the color of the pH paper to color key to determine the estimated pH.

The second method we will employ is the pH meter. After the pH meter has been calibrated with 3 buffer solutions of pH 4, 7 and 10, measure the pH of each solution below using the pH meter. Report the pH of each solution to the nearest 0.1 pH unit.

A video of me completing these two methods is below. Use the video to record the measure pH using the paper and the meter.

pH of solutions using pH paper

Color key of pH paper

0.1 M HCl;
0.1 M NaCl;
0.1 M NH3;
0.1 M NaH2PO4;
0.1 M NaClO

0.01 M HCl;
0.1 M NaOH;
0.1 M NH4Cl;
0.1 M Na2HPO4;
0.1 M C5H5NHCl

0.001 M HCl;
deionized water;
0.1 M (NH4)2C2O4;
0.1 M Na3PO4;
0.1 M C5H5NHClO

0.1 M HC2H3O2;
tap water;
0.1 M NaC2H3O2;
0.1 M H3PO4

Virtual Experiment 30/31/32 part III - watch the video to complete the pH measurements of the above solutions

The third method we will employ to measure the pH of several of the salts solutions is a simulation of the Hydrolysis of Salts.

The simulation is available at http://billvining.com/mmlib_sims/#gen_15_2 . The simulation is limited to a few of the salts provided, Measure the pH of the salt solutions above. An older flash version of this simulation is available at http://employees.oneonta.edu/viningwj/sims/hydrolysis_s.html

Exp 31-32 pH probe setup

Calculating the molarity of the NaOH solution from the titration of mass of KHP: https://youtu.be/J8gDdy6y_j8

FlinnScientific

Determine the pH of salt solutions using acid--base indicators. Certain cations or anions in salts react with water to produce H+ or OH-- ions, respectively.

This video is part of the Flinn Scientific Best Practices for Teaching Chemistry Video Series, a collection of over 125 hours of free professional development training for chemistry teachers - http://elearning.flinnsci.com

ATTENTION: This demonstration is intended for and should only be performed by certified science instructors in a safe laboratory/classroom setting.

Part IV. Buffers.

In the laboratory assignments, we will prepare a Buffer solution

Prepare a buffer solution using acetic acid and sodium acetate. Calculate the concentration of the acetic acid used in the preparation of the buffer and the concentration of the sodium acetate solution used to prepare the buffer. Determine from these calculations the pH of the buffer solution.

In the laboratory assignment, the pH of water will be measures as we add NaOH or HCl,

We will add 1 M NaOH and 1 M HCl to 25 mL of water by drops and measure the pH of the mixture, then look at how the pH changed with the addition of the NaOH versus the addition of the the HCl.

In the laboratory, we will prepare a buffer solution using acetic acid and sodium acetate and then the pH of buffer solution will be measured as we add NaOH or HCl

We will add 1 M NaOH and 1 M HCl to 25 mL of an acetic buffer solution by 1 mL increments, measuring the pH. We will look at how the pH changes in the buffer solutions versus how it changed in the water.

Buffer Demonstration 2 0 for Avid

Water versus buffer simulation http://employees.oneonta.edu/viningwj/sims/buffer_solutions_s.html

Part V - Titration of an unknown acid

Use the following virtual simulation to determine the concentration and Ka of an unknown monoprotic acid. http://chemcollective.org/vlab/103 Use the document at https://docs.google.com/document/d/1JHO5HSCHLl9_Ckx0OOE5oIsQxki8cGz_9kFIxNXAYCM or linked below to follow the directions for collecting pH and volume of NaOH added to determine the Ka and concentration of the unknown acid solution.

Create a pH plot of the unknown acid being titrated with NaOH. Measure the volume of NaOH added against pH (y axis) from a volume of 0 mL added to a pH of ~12, calculate the molarity of the unknown acid solution)

Titration simulation

Part VI: pH Titration curves

We will use the MicroLab system to create pH titration curves for a strong acid, weak acid and polyprotic acid titration with NaOH.

pH Titraion maleic acid acetic acid and HCl

This spreadsheet contains the pH and drops or volume of NaOH added so that you can produce the titration curve of the acid with NaOH.

Report and Conclusion Paragraph

Complete the data and result tables, graphs, calculations and answer all required questions.

Complete a conclusion paragraph using the RERUNS method. A discussion of how to write a conclusion paragraph is given in Appendix D: How to Write a Formal Laboratory Report

Experiment 30-31-32 Report

O-Experiment 30-31-32

On Campus Experiment 30/31/32 document

Experiment 30

Virtual Experiment 30/31/32 document

Page updated

Report abuse