Lab #6: Determining the Molar Mass and Ka1 of a Diprotic Acid May 2016

Purpose: To determine the molar mass of a diprotic acid To determine the Ka1 of the diprotic acid Materials: Part A: 250 mL beaker, NaOH, 250 mL volumetric flask Part B: unknown acid, Erlenmeyer flask, water, pH paper Part C: burette, funnel, 10 mL pipette, phenolphthalein

Part A: Making NaOH Solution You will need to make 250 mL of 0.50M NaOH Determine the number of moles of NaOH needed and then its mass In an empty 250 mL beaker, mass the NaOH. 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. NOTE: I made the NaOH solution for you so you can skip this part.

Part B: Determning the pH of a Diprotic Acid 1) Mass out 0.50 g of the acid, dissolve in 250 mL of water in an volumetric flask 2) Using pH paper, determine the pH of the acid solution. NOTE: The measured pH was 2. You can use this value in your lab report.

Part C: Determining the molar mass and Ka1 of an unknown diprotic acid (H2A) Mass 0.50 g of the acid, dissolve it in 25 mL of water, add 2 drops of phenolphthalein and titrate with the 0.50M NaOH solution. 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. Calculate the molar mass of the unknown acid using the average volume of NaOH by first creating the balanced equation for the neutralization reaction. 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.

Part C: cont’d 4. Calculate the Ka1 of the unknown acid by: determining the concentration of the acid. You will need to use the calculated molar mass to do this. creating an ICE chart.

What you need to submit… 1) A formal lab report – see website: grade 11 for how to write a lab report if you are unsure 2) Your lab report should include ALL data collected in an organized chart under “Observations” 3) Your lab report should include all the neutralization reaction, the ionization reaction, the ICE chart and calculations including proper use of significant digits, units 4) Your percent error given that the unknown acid is oxalic acid, H2C2O4 The lab report will be due by Tues. May 31st.