Need help with #5 please see procedure In this lab, you will be assigned a white
ID: 577547 • Letter: N
Question
Need help with #5 please see procedure In this lab, you will be assigned a white solid mixture, identified only as "Unknown A" or "Unknown B", etc. Record which Unknown you are will receive different Unknowns with different compositions, so it is important that you focus your sample. Your grade will depend, in part, on how close your reported values (mass of each component and percent of each component) match with the correct values (which only the instructor knows). CAUTION: Use of the hotplate and drying ovens will make the glassw portions of this lab. Use tongs to handle the hot glassware! given in your notebook. Different students are very hot at several 1) Weigh -5 grams of your assigned unknown. Record the mass to the nearest hundredth of a gram (e.g. 5.0S g) and transfer the solid to a 150 mL beaker. 2) Add 25 mL. DI water to the mixture and gently stir the beaker for several minutes until all the NaCI has dissolved. 3) Set-up the gravity filtration apparatus as shown in Figure 1. Put your initials on the collection beaker for easy identification later. Before filtration add 2-3 boiling chips to the collection beaker and weigh the beaker with the chips. If you weigh the boiling chips and beaker before filtering the solution you will not need to remove the boiling chips once the NaCI has dried, you can just subtract their mass from the total mass of beaker, NaCl and chips. 4) Filter the solution being sure to transfer all of the solid to the filter paper. Use-5 mL of DI water to rinse the beaker and transfer this 'washing to the filter fiunnel. Wash again with 5 mL 34Explanation / Answer
The procedure given for the separation of sodium chloride, calcium carbonate and silicon dioxide uses water extensively for washings and all reagents added like HCl and sodium carbonate all are aqueous leaving quite large quantities of the high-boiling solvent water in the precipitates obtained by separation.
The water evaporation step in obtaining pure NaCl calls for complete evaporation of the water in NaCl solution and removal of the hot beaker from the oven or hotplate and weighing the solid only after it cools down. NaCl is quite a hygroscopic solid which means it tends to absorb water onto its crystal lattice to stabilize the structure and the very hot NaCl crystals will be completely devoid of water when right out of the oven and tend to rapidly absorb considerable amounts of water from moisture in the environment increasing the weight of NaCl than acutal. To avoid this, the hot beaker with NaCl should instead be cooled in a dessicator which has high moisture absorbing dessicant that will prevent the NaCl from getting any moisture to absorb thus not giving a positive error.
During the separation of calcium carbonate from its state as calcium chloride by sodium carbonate addition, an extra 1mL aliquot is required for complete precipitation. Here the positive error can arise solely from improper drying of the solid calcium carbonate obtained where the seemingly dry salt is acutally wet with a considerable quantity of water still in its amorphous lattice.
The same holds good for the final drying of silicon dioxide and so in these two cases, a greater than initial quantity can be avoided by proper drying of the solids and cooling them, preferrably in dessicators instead of out in the open.