1. Lab #9: Determining the Molar Mass of a Diprotic Acid
December 2016

2. Purpose: To determine the molar mass of a diprotic acid Materials: Part A: 250 mL beaker, KOH, 250 mL volumetric flask Part B: burette, funnel, 10 mL pipette, phenolphthalein, unknown acid, KOH from Part A

3. Part A: Making KOH Solution
You will need to make 250 mL of 0.50M KOH
Determine the number of moles of KOH needed and then its mass
In an empty 250 mL beaker, mass the KOH. Add water to begin the dissolving and then pour it into a 250 mL volumetric flask. Add water to the fill line, invert to mix. Label and set aside for tomorrow.

4. Part B: Determining the molar mass of an unknown diprotic acid (H2A)
Dissolve 0.50 g of the unknown acid in 25 mL of water in an Erlenmeyer flask, add 2 drops of phenolphthalein and titrate with the 0.50M KOH solution that you made yesterday.
Repeat 2x by making fresh acid solution (0.50 g of acid in 25 mL of water and titrate) or until your results are reproducible.
Create the equation for the reaction that occurred during the titration process. Calculate the molar mass of the unknown acid using the average volume of KOH.
NOTE: the unknown acid comes in the hydrated form. The percent water is 28.58% which means that only 71.42% of the mass of acid that you used was the acid itself. You will need to adjust the mass of acid used before you begin your calculations.

5. What you need to submit…
A formal lab report – see website: grade 11 for how to write a lab report if you are unsure All data collected in an organized chart. All neutralization reactions and calculations including proper use of significant digits, units Your percent error