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Old Oct 20th 2012, 04:43 PM   #1
ssekhon
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Chemistry- Solubility

These are discussion questions for a solubility lab we did in which we separated a mixture (sand, mostly SiO2, KNO3 and a blue impurity) into pure compounds using filtration.
To separate the sand, we dissolved the KNO3 and blue impurity in DI water. We then sepraeted the sand using filtration.
The remaining filtrate was heated to reduced the volume of water in it. After cooling the solution, it was placed in an ice bath where the KNO3 crystallized. The solid KNO3 was separated from the blue impurity by filtration.

Data:
Mass of test tube: 30.3115 g
Mass of test tube + sample: 45.0550 g
Mass of filter paper/Lg.watch glass: 47.5741 g
Mass of filter paper/Lg.watch glass + sand: 57.2976 g
Mass of filter paper/Sm.watch glass: 19.0828 g
Mass of filter paper/Sm.watch glass + KNO3: 22.2194 g
Results (percent recovered):
Sand: 65.951%
KNO3: 21.274%

Discussion Questions:
1. Was your % composition value for sand high or low relative to the accepted value? Identify the most likely reason for this error and discuss how the error made your value higher or lower than the actual value.

2. Use the CRC handbook or other valid reference to look up the solubilities for potassium nitrate, KNO3, at 100C and 0C. Use the solubility data for potassium nitrate at 0C to determine the mass of potassium nitrate that will theoretically dissolve in 15.00 mL of water at 0C.

3. Use the mass of your original mixture and the accepted value for the percent potassium nitrate to determine the actual mass of potassium nitrate present in your unknown.

4. What mass of potassium nitrate would you expect to crystallize at 0C for your sample knowing that some will stay dissolved even at 0C? (Your answer from #2 is the mass that will stay dissolved even at 0C)

5. Percent recovery is a term used to describe the amount actually recovered divided by the amount expected to be recovered (x 100%). Determine your percent recovery for KNO3 using the adjusted amount expected to crystallize.

6. Should you expect to obtain 100% recovery of KNO3 in this experiment? Identify possible procedural errors or reasons for both a low % recovery and a % recovery greater than 100%, discussing how those errors result in low% or high % recovery.

Last edited by ssekhon; Oct 20th 2012 at 05:00 PM.
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Old Oct 20th 2012, 05:13 PM   #2
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Check my answers please!!

So the accepted (actual) values for percent recovery were:
Sand: 66.67%
KNO3: 33.33%

1. My % composition for sand was low compared to the accepted value because there were lots of places such as during filtration and transfer of product where the sample was lost. Loss of sample, therefore resulted in a lower percent recovery.
2. Theoretical dissolve in 15.00 mL at 0 degrees C:
= (12.0g)/(100 mL)
= .120 g of KNO3 dissolves per mL
=(.120 g/mL) * (15.00 mL)
= 1.80 g
3. Accepted value= 33.33%= .3333
Mass of original mixture:
= (mass of test tube + sample) - (mass of test tube)
= 45.0550 g - 30.3115 g
= 14.7435 g
Actual mass of KNO3:
= (accepted value of KNO3) * (Mass of original mixture)
= .3333 * 14.7435 g
= 4.9140 g
4. Mass expected to crystallize at 0 degrees C:
= (actual mass of KNO3) - (Mass of KNO3 that will dissolve at 0 degrees C)
= 4.9140 g -1.80 g
=3.114 g
5. Amount actually recovered:
= (Mass of filter paper/Sm.watch glass + KNO3) - (Mass of filter paper/Sm.watch glass)
= 22.2194 g - 19.0828 g
= 3.1366 g
Percent recovery:
= [(Amount actually recovered) / (Amount expected to be recovered)] * 100%
= [3.1366 g / 3.114 g] * 100%
= 100.7%
6. No, we can't expect to obtain 100% recovery of KNO3 because in order to do so, we'd have to crystallize the substance multiple times (and even then we won't get everything) and also because we lose sample in transfer.
A low % recovery would result mostly from loss of sample between filtration and transfer.
A high & recovery would result from impurities and errors. Such as, the blue impurity not dissolving completely, or not all the KNO3 dissolved that should have. Also something could have fallen off and onto the sample when it was being heated in the oven or even when it was left out to dry in the funnel.

Last edited by ssekhon; Oct 20th 2012 at 05:17 PM.
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Old Oct 21st 2012, 07:05 AM   #3
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