11 chem, week 14 - Stoic(h) (Ti)traitors

Hey y'all, it's Liam again.  Let's go straight into it!

My friend's favourite chemistry meme

Was I trying too hard with the title?  I, uh... I like puns.

ANYWHO, as you may have guessed, this blog post is about using stoichiometry in titration problems.  Titration is defined as "a process or method to determine the concentration of a substance in an unknown solution, in which a known reagent is added to an unknown solution to produce a known reaction such as a change in colour or electrical measurement".  The first thing we did was look at an article about titration in everyday life.  The article cited tests such as pregnancy and blood sugar tests as common uses of titration, and I came up with my own example in anesthetics: when knocking people out for surgery, doctors have to come up with the right concentrations of anesthesia for the person, and that all has to do with titration.

A fairly relevant meme

I also learned that there are four types of titration.  They are listed as follows:

• Acid-Base Titration: a reactant is added until the sample solution reaches a specified pH level. This type of titration relies on a pH meter or a dye to track the change in pH
• the dye will change to a certain color once the correct pH has been reached
• Oxidation-Reduction Titration: relies on a gain or loss of electrons within a sample to find out what is in the sample
• Precipitation Titration: a reactant is added to a sample until a reaction occurs that causes a solid to precipitate from -- or "fall out" of -- the sample
• Complexometric Titrations: in complexometric titration, the solid is formed more quickly and more completely than in precipitation titration, which reduces errors in measurement

The next step in learning about titration was actually learning about how to do a real live titration, complete with equipment and reactants.  The equipment needed is as follows: a burette; a ringstand; a burette lamp; a small funnel; a 125 mL Erlenmeyer flask; a volumetric pipette & pipette bulb; a wash bottle w/ deionized water; a beaker/flask, one filled with analyte and one filled with titrant; indicator; a reading card to see meniscus clearly; and a sheet of white paper to visualize endpoint.  I'll try to condense the steps for a proper titration here:

1. Rinse equipment with deionized water, then with tiny amount of titrant.
2. Mount burette vertically, close stopcock, place funnel in top of burette and fill burette with titrant, almost to the top
3. Gently tap burette to free any air bubble within, and remove funnel
4. Place flask under burette, and allow a few mL of titrant into flask and rinse tip of burette with water
5. Empty flask into waste container and rinse the flask.  Drying not necessary
6. Record starting volume from bottom of meniscus in burette (make sure proper sigfigs (+1 sigfig because that’s what you do with graduated glassware) are used)
7. Use pipette to transfer analyte to flask, add few drops of indicator to flask and swirl
8. Place flask under tip of burette; record this volume
9. Place white paper under flask to make colour detection easier
10. With one hand, open stopcock, and with other swirl the flask
11. When done, record final volume of titrand in burette.  Subtract initial from final to get estimated volume of titrant needed for the titration
12. Subtract 5mL from THAT number to get amount that can be safely added before a slower, more measured addition is required
13. To reduce risk of passing desired endpoint, slow down the addition of titrant when flashes of colour begin to appear in the analyte
14. Adjust stopcock to slow flow of titrant down to drop-wise rates, and continue swirling, with other hand waiting to shut off stopcock
15. When indicator starts taking longer to fade, shut off stopcock.  Rinse burette tip with deionized water and swirl flask
16. Record final burette volume.  If analyte remains faintly coloured, end point was reached
1. If analyte is colourless:
1. Add drop
2. Rinse
3. Swirl
4. Record

Typically, titrations are performed three times.  Use average volume of titrant required for calculations

Hmm... that's not very condensed. The thing is, all of these steps are very important to a titration as the procedure is very precise, and must be followed accurately to avoid screwing up measurements.

The feeling of pure, stoic bliss

After writing all of this fun stuff down, we had to do some stoichiometry problems with titration. First off, here are the steps in any procedure:

1. Balanced chemical equation
2. Determine # of moles of known variable
3. Mole ratio to find mole # of unknown variable
4. Solve for concentration

To show an example of this, I solved one of the problems and then wrote down my solution in order to show the rest of the class.  As it is a fairly long solution, I will just put the video link here.

The last question in the module was about trying to complete a titration with an unknown concentration of sulfuric acid and 23.2mL of 0.30 M sodium hydroxide. The question asks if it is okay to assume that, because C1V1 = C2V2, the concentration of sulfuric acid will be the same as the concentration of the sodium hydroxide. The answer is NO, because were you to actually go ahead and create a balanced chemical equation for this question, you would see that the mole ratio of sulfuric acid to sodium hydroxide is 1:2, meaning that there is half as many moles of H2SO4 in this titration as there are moles of NaOH.  This means that C1V1 ≠ C2V2.

Thank you for reading, hopefully you found some joy with this nice peak into my chemistrical life (Is that a word?  I don't think it's a word) and I hope to keep providing said entertainment until we finish our little journey together!

 - L