The case of the Green Crystals

4-Part Mystery Lab
´â╝Purification (partly)
Oxalate: Redox Titration
Absorption spectrophotometry
Water: Gravimetric Analysis
Identification /Yield
Still trying to Save the World!
These are Amy’s beautiful green crystals from last week
Today we’ll see if our careful placement of the hot green
solution in the lab drawers gave us the brilliant crystals
we’re expecting.
Procedures for today
Washing the crystals
– Fill a 250 mL beaker with ice and place a squirt bottle
w/deionized water in it to chill
– Prepare a funnel with filter paper for gravity filtration
– Carefully transfer crystals to the filter paper in funnel
– Rinse crystals 2x with a small amount of cold water
from the squirt bottle (Not too much!)
• Filtrate goes in waste container
– Remove crystals from filter paper and blot dry with
paper towels; let air dry for 5-10 min before weighing.
– Weigh crystals w/top loading balance & record mass
– Place crystals in 250 mL beaker and cover w/wrap
Part B: Analysis for oxalate2•
Work in Groups of 2 for redox titration
Goal is to get ONE good titration per group MnO
Check out buret from stockroom
Set up ring stand/wire gauze/bunsen burner
Obtain ~35 mL of KMnO4 in your small beaker (not more!)
Rinse buret with a few mL of KMnO4 solution and place in
buret clamp (all rinses go in your waste beaker)
• Put remaining KMnO4 solution into buret and record
reading (top of meniscus). You only need about 20 mL to
be in the buret. Do not fill buret to the top!
Part B: Analysis for oxalate2• Today’s titration is a redox reaction with gas evolution
(aq) + 5 C2O42-(aq)+ 16H+(aq) →
(aq) + 10CO2(g) + 8H2O(l)
• Start with a known mass (0.20 – 0.23 g; use analytical balance)
of your crystals and place it in 250 mL Erlenmeyer flask
• Add 25 mL deionized water , then 5 mL 3M H2SO4
– Heat gently until steaming & crystals dissolve (turns yellow)
• Begin titrating while solution is still hot (Use paper towel to
hold flask). Swirl as you add the KMnO4 solution. The titration
will take between 9 and 15 mL, so take your time and go slow.
• You will see a dark purple spot that disappears on swirling
when you start titrating.
• The endpoint is a change from yellow to clear to light
pinkish/purple. Record final volume reading on data table
Are we done yet?
• Do data calculations before cleaning up – in case you need
to do a second titration
• Check with me after you’re finished with the calculations
• If the calculations are correct, write your results on the
board and when everyone is done, record class average.
• Calculations:
– Volume of KMnO4 solution reacted (Vfinal – Vinitial)
– Moles of KMnO4 (volume (L) x M)
– Moles of oxalate reacted (use mole ratio from equation)
– Mass of oxalate reacted (# moles x molar mass)
– Mass % oxalate in crystals (mass oxalate/mass of crystals) x 100
Getting closer: KxFey(C2O4)z • nH2O
• Today we will wash, dry and weigh our crystals (yield)
• Then we will determine the mass % oxalate in them via a
redox titration
• Thursday, we will prepare standard solutions and do an
analysis to determine the mass percent of iron in the
crystals using absorption spectrophotometry.
• We will also find mass percent of water in our crystals by
weighing a sample and then heating the sample in the
drying oven to remove the hydrated water.
Dire Warnings
• Use caution with the bunsen burners – be aware of all the
burners around you.
• Strong acid: 3 M H2SO4: bad for skin and clothes
• Strong Oxidizer: KMnO4 Irritating to skin and will stain!
• Pay attention to each step in the lab so you don’t
accidently discard your crystals or use too much of them
This is a 4 part lab and you need YOUR crystals each
• Collect all waste from filtration and titration in a waste
beaker and place in correct hazardous waste container in