By: Andrew Hoitt
CHEM 105 Lab
Lab # 11 – Acid - Base Titration
The purpose of this lab is to determine the molarity (M) of an unknown HCl solution. A NaOH solution will be made and its molarity calculated. A sample of the NaOH solution will be titrated against the unknown HCl solution to calculated the volume needed to neutralize it. With these volumes the unknown molarity can be calculated.
Solutions are made up of solvents and solutes. Materials known as acids when dissolved in water produce H+ ions. Acids are the proton donators. All of the inorganic acids are aqueous solutions. A base is a material that can neutralize acids. Bases when dissolved in water produce OH- ions. Bases are the proton acceptors.
The molarity (M) or the concentration of a solution is expressed by moles / Liter of solution. Titrating allows for the concentration or molarity of an unknown acid or base to be found. This can be done by titrating the acid solution against a base solution of which the molarity is known. The resulting reactants of an acid reacting with a base are water and a salt. This type of reaction is known as a neutralization reaction because the products of which are assumed to be neutral. Careful titration of an acid against a base until neutralization is reached allows for the assumption that moles of H+ = moles of OH-. The following relationship allows for the unknown molarity to be derived:
MA x VA = MB x VB
Where MA = molarity of the acid, MB = molarity of the base, VA = volume of the acid, and VB = volume of the base.
To determine the molarity of an unknown HCl solution, first denote this as MA. Next a NaOH solution with a known molarity should be denoted as MB. Then titrate a sample of this solution against the unknown HCl solution until neutralization is reached. The titration gives the two needed volumes, VA and VB, allowing for the molarity of the acid MA to be solved for.
- Acquire all equipment required to perform this lab:
← Small piece of paper towel
← Dial-a-gram balance
← Metal spatula
← Rubber gloves
← 3.00 g - 4.00 g NaOH pellets
← 100-mL volumetric flask
← 100 mL water
← Eye dropper
← 50 - 100 mL of unknown HCl solution (in fume hood)
← 100-mL graduated cylinder
← 5 clean Erlenmeyer flasks
← 40 drops phenolphthalein indicator
← Safety glasses
Start by getting a piece of paper towel large enough to cover the dial-a-gram balance dish. Weigh the piece of paper towel. Add between 3.00 g - 4.00 g of NaOH pellets onto the towel using the dial-a-gram balance. DO NOT TOUCH THE NaOH PELLETS! Use the metal spatula to acquire the proper amount of pellets. Carefully add the NaOH pellets to the empty volumetric flask. Discard the piece of paper towel. Add water to the flask with the NaOH pellets making sure not to fill past the 100-mL mark. Cover the flask and shake until all of the NaOH pellets have dissolved. With an eyedropper carefully add water to the volumetric flask until the water level is equal to the 100-mL line. Retrieve 50 - 100 mL HCl with unknown molarity from the brown bottle in the fume hood. Fill the burette with HCl solution. Be sure that all the air is expelled from the nozzle of the burette before starting the titration. Measure 20 mL of NaOH solution with the 100-mL graduated cylinder and pour this into a clean dry Erlenmeyer flask. Repeat this last step with the rest of the solution and Erlenmeyer flasks. Add 8 drops phenolphthalein indicator to each flask with 20 mL of NaOH solution. Record the initial volume of the burette and slowly add acid until the solution turns clear. This will happen very fast so going slow is essential to getting accurate results. Record the final volume of the burette. Repeat this process with the rest of the...