Titrations and Buffers

Titration is a technique that helps determine concentration of an unknown acid or base. The reactions are called neutralization reactions, but don't actually form neutral solutions.

When titrating, the solution in the buret is the titrant and it always has a known concentration.

This solution then is combined with a solution called the analyte, which has an unknown concentration.

To know when the chemical reaction is complete, look for a color change in the analyte due to an indicator that is pH sensitive.

Once the analyte changes color, the endpoint has been reached.

Given all of the volumes and concentrations that we know, we can then calculate the concentration of the unknown.

Percent Acetic Acid in Vinegar Lab

Last week in chemistry class, we completed a three day lab to determine the percent of acetic acid in vinegar. In order to be able to do this lab, we had to memorize the procedure. First, we added 100mL of NaOH to our buret and then mixed around .5 grams of KHP with water in a flask. After the KHP was dissolved, we added 3 drops of phenolphthalein to act as our indicator. Then we titrated this using the NaOH in the buret, until the solution turned and stayed pink. Then we recorded how much NaOH was used. After this, we repeated these same steps, but with vinegar in our flask, instead of all water. This lab was really interesting, and it was a challenge to try and get the solution to turn pink without making it too dark. Here is a website that I found that explains how titrations work: How to Titrate

This is what it looked like when the NaOH reacted and started to form a pink color.

This is what the solution in the flask looked like after we were done titrating and it was staying light pink.

This is what the whole funnel/buret set-up looked like.

Acids and Bases

Properties of Acids: tastes sour, feels sticky, turns Litmus paper red, corrosive, less than 7 pH

Properties of Bases: tastes bitter, feels slippery, turns Litmus paper blue, caustic, greater than 7 pH

Arrhenius acids: produce hydrogen ions in solution (H+)

Arrhenius bases: produce hydroxide ions in solution (OH-)

Arrhenius acids and bases 

Water can be an acid or a base (amphoteric).

Bronsted-Lowery acids donate a proton.

Bronsted-Lowery bases accept a proton.

Acids produce conjugate bases and bases produce conjugate acids. 

Bronsted-Lowery acids and bases

Vitamin C Lab

In chemistry class, we recently completed a lab where we tested different fruity liquids for their concentration of Vitamin C. We predicted that the liquids would rank as following (1= highest concentration):

1. V8 Juice

2. Unsweet White Grapefruit Juice

3. Apple Juice

4. Pear Nectar

To carry out this experiment, we took 20 drops of each solution, put them into test tubes, combined them with 3 drops of starch, then added drops of iodine until the solution turned and stayed dark blue. Here are all of the pipettes of our test solutions:

Here is what one of the solutions looked like after it turned dark blue due to all of the Vitamin C being used up.

After we completed the lab, we concluded that our concentration predictions were pretty close to the actual results. Here is the actual order of Vitamin C concentration:

1. Unsweet White Grapefruit Juice

2. Apple Juice

3. V8 Juice

4. Pear Nectar

Here are a few links that deal with what we did during this lab:

How to test for Vitamin C

Acid-base reactions

Titrations

Murder Investigation Lab

On Wednesday in class, we did a lab about finding the molarity of aqueous solutions. We were given some details about a theoretical murder case, and the list of possible suspects, and we had to determine who the murderer was. In order to do this, first we combined 20mL of the unknown substance with 40mL of NaCl and the reaction produced a solid. Here is a website about precipitation reactions like this, just for some review: Precipitation Reactions. Next, we filtered the solution to get the solid out. Once we had it completely filtered and it had dried, we weighed the filter paper to find the mass of the solid. From there, we had everything we needed to find molarity and determine the murderer. Overall, this lab was really helpful because it allowed us to practice finding molarity with an actual real life problem, instead of just practice ones. I now have a much better understanding and capability of how to do this. Here is a link to the only information we were given for this lab: Murder Lab

The solution while it was being filtered through the paper and funnel.

The filter paper with the solid in it, after it had dried.

Molarity

This past week in class, we have been focusing on morality. The equation to find a substances molarity, divide the moles of the solute by the liters of the solution. Although, volume is temperature dependent, so molarity can change with temperature. Here is a website I found explaining the basics of calculating molarity: Finding molarity

We also learned about the concentration of ions in a solution and the Van't Hoff factor. This is the number of ions that a compound will contribute to a solution (i). The Van't Hoff factor for covalent compounds is always 1. Here is another website I found, this one explaining the factor: Van't Hoff factor.

Lastly, we learned about dilutions. To find molarity of volume in a dilution, use the formula M1V1= M2V2. In a dilution, the high concentration substance is the stock and the low concentration substance is the making. An aliquot is a small amount of a starting solution. Here is another website, this one explaining dilutions and their equation: Dilutions

Solution Composition

Solutions are made up of two distinct parts: the solute and the solvent. The solvent is whatever part is present in the largest quantity. The solute is whatever is dissolved into the solvent. Here are some examples of how to identify which is which in a solution:

-Chlorine tablets in a swimming pool: 

Solute: tablets

Solvent: H20

-Sugar and kool-aid mix in water:

Solute: sugar and kool-aid

Solvent: H20

-Cigarette smoke in air:

Solute: smoke particles

Solvent: air

-Sodium chloride in water:

Solute: NaCl

Solvent: H20

There is a limit to how much solute can be dissolved into any solvent. The solution can either be unsaturated, saturated, or supersaturated. Most solvents hold more solute at higher temperatures.