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CHM 1020L Introduction to pH lab

This lab uses the pH Scale Basics simulation from PhET Interactive Simulations at University of Colorado Boulder, under the CC-BY 4.0 license.

https://phet.colorado.edu/sims/html/ph-scale-basics/latest/ph-scale-basics_en.html Learning objectives

1) Understand the pH scale 2) Relate the value of pH of a substance to whether a given solution is acidic or basic. 3) Predict the effect of volume on pH of pure substances 4) Predict the effect of dilution on pH of pure substances 5) Organize a given list of substances from most acidic to most basic with supporting

evidence. 6) Apply knowledge gained in the experiment to explain relevant real world situations.

Introduction We use a multitude of acids and bases during the course of everyday activities, for example in cooking, cleaning, and even in the food we consume. Acids and bases have very different properties from each other.

The simplest definition of an acid is a substance that produces H+ ions in solution. Acids are sour, sting the skin, and are corrosive to metals. Acids turn blue litmus red and phenolphthalein colorless. The simplest definition of a base is a substance that produces OH- ions in solution. Bases are bitter to taste and slippery to touch. Bases turn red litmus blue and phenolphthalein pink.

The pH of a substance is defined as follows pH = -log [H3O+] The pH scale is a 14 points scale. The middle number 7 means the solution is neutral and pure distilled water is considered neutral. Lower numbers indicate increasing acidity and higher numbers indicate increasing basicity.

An acid and a base will react to neutralize each other and produce salt and water.

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Lets get oriented first : ) Go to the following website https://phet.colorado.edu/en/simulation/ph-scale-basics You need not download the simulation but if you click on it, it will open on your computer. If you have technical issues please reach out to FIU Online technical help to get the issues resolved in a timely manner (details on how to reach FIU technical help are on the course homepage). Once you have the simulation open it should look like the figure shown below. Examine the figure shown below and ensure that you identify the parts on the simulation that are clearly marked. This will allow you follow directions and to perform the experiment stress free

Please familiarize yourself with the following parts of the simulation (play with the simulation a bit to familiarize yourself with it)

1) Location where you select the liquid from the dropdown menu. 2) Faucet to add water when required 3) Dropper to add the liquid 4) Container 5) Stopper to drain container 6) pH probe.

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Furthermore, there are a couple of precautions you must take care with the pH probe specifically

Failure to correctly place the pH probe will result in faulty data which can be very confusing. Additionally, if you check carefully, you will see that the pH meter is not giving you a reading which should also alert you to the fact that something is not right.

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1. Ensure the pH probe is inside the liquid in the container (I would put it at the bottom) so that it can give you a pH reading.

2. Always ensure that there is enough liquid to completely cover the pH probe so that all parts of the probe are fully submerged.

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So the correct placement of the probe is shown below

Note how the pH probe is completely submerged in the liquid and how there is actually a reading on the pH meter. Procedure Now for each liquid in the dropdown menu you have to follow the same directions. I am using drain cleaner as an example to show how you need to work with the simulation to gather data. Section 1 – pH of pure solution

1) Make sure the container is empty. If necessary open the stopper to drain any remaining liquid.

2) Choose the liquid (in this example the drain cleaner) from the dropdown menu 3) Place the pH probe into the container 4) Press the red button on the dropper to start adding the chosen liquid (in this example the

drain cleaner) into the container and add up to a volume of 0.30 L and record the pH in the table provided under observations.

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5) Then add more of the chosen liquid (in this example the drain cleaner) into the container upto a volume of 0.50 L and record the pH in the table provided under observations.

6) Add more of the chosen liquid (in this example the drain cleaner) into the container upto a volume of 1.00 L and record the pH in the tables 1 and 2 provided under observations.

7) Drain the liquid in the container completely. Section 2: What happens when you dilute the solution? Dilution generally means adding water to something.

1) Make sure the container is empty. If necessary open the stopper to drain any remaining liquid.

2) Choose the liquid (in this example the drain cleaner) from the dropdown menu 3) Place the pH probe into the container. 4) Press the red button on the dropper to start adding the chosen liquid (in this example the

drain cleaner) and add up to a volume of 0.50 L. Check to see if the value of pH is the same as recorded in table 1. Record the value in table 2.

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5) Then add water into the container from the faucet upto a volume of 1.00 L and record the pH in table 2 provided under observations.

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Repeat sections 1 and 2 for all the liquids in the dropdown menu and record your results in tables 1 and 2 in the observation section.