Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.
|Research Question:||What is the concentration of acid in our chemical reaction?|
|Age:||High school and up|
|Safety concerns:||Acids and bases can easily burn the skin or surfaces. Wear eye protection and gloves when working with these chemicals|
|Independent variable: Amount of base used
Dependent variable: Concentration of acid
|Control variable: pH indicator, amount of starting acid|
Chemical reactions are the cornerstone of all living things. One important chemical reaction is called an acid-base reaction. Here, an acid, or a substance that can donate hydrogen ions, reacts with a base, a substance that can accept hydrogen ions. When acids and bases are balanced in a reaction, they form water and a salt, called a neutral solution. To review acids and bases you can look at this lesson: Acids and Bases
But, don't despair! Acid-base reactions are incredibly important for living things. Thus, it is important to know the concentration of acids and bases in certain situations. To figure this out, scientists use an experiment called titration. During a titration, scientists use a known volume of an acidic sample and add base until the solution is neutral. For more information on titration, check out this video lesson: Titration of a Strong Acid or a Strong Base
During titration, scientists use a pH indicator to find the concentration of acid or base in a solution, which is what we will do today. For more information on pH indicators, you can read this lesson: Acid-Base Indicator: Definition & Concept
- 75mL hydrochloric acid
- 300mL - 1 Molar sodium hydroxide
- 50mL Buret
- 500 mL flask
- 25mL volumetric pipette
- 2 - 250mL beakers
- Safety goggles
1. First set up your titration equipment. Attach the clamp to the stand and clamp the buret in place, leaving room for the flask underneath.
Safety Tip!! Always wear goggles and gloves when handling acids or bases.
2. Measure out 25mL of acid and pour it into the flask using the funnel. Record this volume in your notebook.
3. Measure 100mL of base into one of the 250mL beakers.
4. Add 5mL of base to the buret, being careful to coat all the sides, and run it through to rinse the buret.
5. Repeat step 4.
6. Fill the buret with 50mL of base and let two drops flow through to clear any air bubbles into the beaker.
7. Put 2-3 drops of phenolphthalein in the flask
8. Replace the beaker with the flask underneath the buret. You're ready to start the experiment.
9. Carefully add one drop of base at a time to the flask. Phenolphthalein is clear when the pH is acidic and turns pink if the pH is basic.
10. Continue to add one drop at a time until the first moment the acid turns a very light pink. This is the equilibrium point where the amount of acid is equal to the amount of base present.
11. Now it's time to do the math. First, note how much base is left. Subtract this number from the starting volume (50mL) to get the volume of base used. Divide this number by 1000 to get the volume in liters (L).
12. Now multiple the volume of base used by the molarity of the base (known). This gives you the number of moles of base used.
13. In this experiment, the ratio of base to acid is 1:1, so for every mole of base used, one mole of acid is used. First, convert the volume of acid used (25mL) to liters by dividing by 1000. Next, divide the number of moles of acid by the volume used to get molarity. This is the concentration of your acid.
14. Thoroughly rinse your equipment with water and repeat steps 2-13 three more times to validate your data. Take an average of the three molarities of the acid calculated.
This experiment is very touchy and it might take even more than 3 tries to get repeatable results. The amount of base used must be exactly when the solution turns pink, not after.
Why did the solution turn pink?
What other substances do you think would work in this experiment?
How It Works
During the titration, we combined sodium hydroxide (NaOH) with hydrochloric acid (HCl) in the flask. When these two chemicals combine, the hydrogen ion wants to get with the hydroxide ion (a hydrogen atom bound to an oxygen atom) attached to the sodium. The acid, the hydrogen ion, combines with the base, the hydroxide ion, and form water, H2O. The sodium is left with a positive charge and the chloride ion from the hydrochloric acid is left with a negative charge. If you remember from physics, opposites attract. The sodium and chloride come together to make salt, NaCl, just like your table salt at home.
Since the hydrogen ions are combining with the hydroxide ions, the hydrogen ions are no longer floating around in solution. This causes the pH to increase, since we're losing hydrogen ions. This process continues until there is a perfectly balanced solution of acid and base, called the equivalence point, where there is an equal number of moles of acid and base.
The phenolphthalein is clear between pH 0 and 7, but turns a progressively brighter pink as the solution becomes basic. So, if you're looking for the equivalence point, you're looking for pH 7, or exactly when the phenolphthalein starts to turn pink.
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