Purity of a Hydrate Lab Chem 1A April 22, 2010 What is a Hydrate? Ionic salt with a fixed ratio of water molecules trapped its crystalline lattice. Can be dehydrated by heating Hydrate can be formed from the anhydrate (without H 2 O) by adding water and re-crystallizing Ni(NO 3 ) 2 6H 2 O Copper (II) sulfate pentahydrate Iron (II) sulfate heptahydrate Co(NO 3 ) 2 6H 2 O 1
Finding the formula for a Hydrate Dehydrating CuSO 4 xh 2 O 0.1122 anhydrous Mass of CuSO 4 xh 2 O mass of CuSO 4 = mass of water 0.1774 g CuSO 4 xh 2 O 0.1122 g CuSO 4 = 0.0652 g H 2 O Moles of water: 0.0652 g H 2 O x 1 mol /18.02 g = 0.00362 mol H 2 O Moles of CuSO 4 : 0.1155g CuSO 4 x 1 mol /159.62 = 0.0007235 mol CuSO 4 Mole ratio in formula: 0.00362 mol H 2 O/0.0007235 mol CuSO 4 = 5.00 Formula of hydrate: CuSO 4 5H 2 O Our Challenge: A Trial Run Student #29 was given a sample of CuSO 4 5H 2 O with an unknown amount of NaCl mixed in. The goal was determining the purity (mass %) of CuSO 4 5H 2 O in the mixture. They collected these data: Mass of Crucible: 28.2051 g Mass of Crucible + Sample: 30.5402 g Mass of Crucible + Residue After Heating: 30.0249 g The first calculations were simple: Mass of sample = 30.5402 g 28.2051 g = 2.3351 g Mass Lost = 30.0249 g 30.5402 g = 0.5153 g = mass of H 2 O Was matter destroyed? What/where is the lost mass? What info do we need to calculate the mass %? Mass of hydrate Mass of sample 2
What to do with the data? To get to mass %, we need to find the mass of CuSO 4 5H 2 O Mass A Moles A Moles B Mass B From mass of H 2 O, we calculate moles H 2 O: 0. 5153 g H 2 O x 1 mol /18.02 g = 0.028596 mol H 2 O From mol H 2 O we get mol CuSO 4 5H 2 O : 0.028596 mol H 2 O x 1 mol CuSO 4 5H 2 O/ 5 mol H 2 O = 0.0057192 mol CuSO 4 5H 2 O From mol of CuSO 4 5H 2 O, we calculate the mass of hydrate: 0.0057192 mol CuSO 4 5H 2 O x 249.72 g CuSO 4 5H 2 O /mole= 1.4282 g CuSO 4 5H 2 O Mass % CuSO 4 5H 2 O in sample = (1.4282 g/2.3351 g) x 100% = 61.16% Let s Look at those steps again Initial sample mass = g CuSO 4 5H 2 O + g NaCl Mass lost after heating = mass of H 2 O 2.3351 g 0.5153 g Mass A Moles A Moles B Mass B 0.5153 g H 1 mol H 2 O 1 mol CuSO 4 5H 2 O 249.72 g CuSO 4 5H 2 O 2 O x x x 18.02 g 5 mol H 2 O 1 mol CuSO 4 5H 2 O = 1.4282 g CuSO 4 5H 2 O Purity of the hydrate = Mass % CuSO 4 5H 2 O in sample = 1.4282 g CuSO 4 5H 2 O x 100% = 61.16% 2.3351 g total sample 3
An Alternative method We have mass of H 2 O; now find mass of CuSO 4 5H 2 O Use mass ratios = mass of one part of compound molar mass of compound Mass of H2O sample Mass of CuSO 4 5H 2 O sample = Mass of H2O compound Mass of CuSO 4 5H 2 O compound Solve for Mass of CuSO 4 5H 2 O sample : Mass CuSO 4 5H 2 O = 0.5153 g H 2 O x 249.72 g CuSO 4 5H 2 O 90.10 g H 2 O = 1.4282 g CuSO 4 5H 2 O today s hydrate Strontium Chloride Hexahydrate: SrCl 2 6H 2 O The salt emits a bright red colour in a flame SrCl 2 used to make red fireworks 4
Our Procedure Equipment and Reagents: Nickel crucible (stockroom) Unknown mixture of hydrate and NaCl Ring stand and small ring Analytical balance Clay triangle Bunsen burner Crucible tongs Preparation of crucible Getting started Heat the crucible on a clay triangle over a low, blue flame for ~ 3 min to remove moisture or contaminates. Allow crucible to cool on wire gauze on bench top (5-6 min). DO NOT place a hot crucible directly on the bench top. After crucible is cool, weigh it and record its mass. Sample preparation Get a sample of the SrCl2 (H2O)6/NaCl mixture from stockroom. Record the unknown code of the sample. Place about 1.5 grams of the mixture into the crucible. Do not use the entire contents of the sample. Using the same balance, weigh crucible and mixture precisely and record the mass 5
Turning the heat on Heat crucible and mixture over low, blue flame for 12 min Listen and watch the sample; avoid popping and spattering If sample starts to spatter, remove Bunsen burner briefly Increase height of inner blue cone gradually as the reaction proceeds (bottom adjustment on burner) Do not overheat no need to melt sample Cool to room temp (~ 6 min) and weigh crucible using same balance; record mass Heat the sample again for five minutes. Cool to room temperature, weigh crucible + residue w/same balance If the difference between the masses of crucible + residue after the 1 st and 2 nd heating is > 0.003 g, repeat; heating for 5 min/cool/re-weigh until mass of crucible + residue differs by < 0.003 g between consecutive heatings. it s Like Déjà vu all over again Do a second trial with a fresh sample of your unknown. For 2 nd trial, DO NOT CLEAN the CRUCIBLE! The final mass of crucible + residue from trial 1 is your Mass of crucible for the 2 nd trial. Add a fresh sample of your unknown mixture on top of residue. Reweigh to obtain Mass of crucible + mixture before heating. Heat as directed in trial one (heat/cool/re-weigh, etc). Discard residue into labeled waste container in hood If residue sticks to crucible loosen it by soaking crucible in hot water for a few minutes. Return the crucible and your unknown to stockroom. 6
Dire Warnings All sample spills or residues in crucibles go into the labeled waste container in the hood! Keep the balance area clean! Work alone and carefully Results graded on accuracy! Do not put hot crucibles directly on the lab bench use your wire gauze Be careful with the Bunsen burners Don t touch the ring clamps after heating they stay hot a long time! For the coming week Tuesday: Density of 7 Up lab worksheets (w/prelab) due PreLab for Solubility Rules, etc Lab due Thursday: Lab Quiz: Measurements/Sig Figs/Graphing Nomenclature Density of 7-Up Know information in the intro of worksheets and how to do any calculations described in the labs 7