Sunday, March 05, 2006
Honors: Solubility Curves
I keep having to correct a solubility curve error on the last exam so TAKE HEED!
In a question, when you are given an ACTUAL amount of solute dissolved in a given amount of solvent, DO NOT EVER SCALE the solute or solvent quantities UP or DOWN or CHANGE THOSE NUMBERS IN ANY WAY!!! If you do, YOU NO LONGER HAVE THE ACTUAL QUANTITIES THAT ARE RIGHT IN THE QUESTION. In every example that we did in class and in EVERY ONE of the DOZENS of worksheet examples, WITHOUT EXCEPTION, the ACTUAL amount of solute dissolved in THE ACTUAL AMOUNT of SOLVENT was NEVER ALTERED or SCALED in ANY WAY.
For example, if 10. g of sodium chloride are dissolved in 50. g of water at 80 degrees Celsius, then the solution is unsaturated. According to the solubility curve Table D, 40 g of NaCl would saturate 100 g of water so, proportionally, 20 g of NaCl would saturate 50. g of water. NOTICE THAT I DID NOT ADJUST THE ACTUAL AMOUNT OF NaCl or water !!! I simply scaled down the solubility curve to concur with the ACTUAL amount of SOLVENT-water = 50 g. So, since 10 g of NaCl are ACTUALLY dissolved and 20 g of NaCl would make a saturated solution, then the solution in UNSATURATED AND adding 10 more grams of NaCl would make the solution saturated.
The ONLY quantities that MAY be scaled are those on the experimentally determined SOLUBILITY CURVES. Those curves tell you the MAXIMUM amount of a given solute that can be dissolved in 100. grams of water ONLY!!! The curves do not reflect the TOTAL ACTUAL AMOUNT OF SOLUTE FOR ANY OTHER AMOUNT OF WATER; most questions do NOT give you a quantity of solute in exactly 100 grams of water (as I promised)!!!
Half of the students in each class are scaling the ACTUAL amount DOWN or UP to the amount of solute that COULD be dissolved in 100. gram of solvent. Furthermore, they DO NOT BOTHER TO SCALE THE AMOUNT BACK UP TO WHAT IS ACTUALLY IN THE QUESTION.
On the other hand, there are quite a few perfect or near perfect (write your UNITS!) scores so obviously some were well-versed and practiced on all of the given objectives.
A little worksheet practice BEFORE the test would have brought this scaling error to light. Obviously, some of you are just looking at the worksheet answers (or not) without doing the problems first. Unless you have the WRITTEN experience and reinforcement/repetition of the class material, you are NOT studying and your grades will be lower than they could be. This is the Math of Chemistry quarter and the math is below the 8th grade level; by regularly applying the math to these problem types, most students increase their averages significantly. Count on seeing this question type again on this week's test.
In a question, when you are given an ACTUAL amount of solute dissolved in a given amount of solvent, DO NOT EVER SCALE the solute or solvent quantities UP or DOWN or CHANGE THOSE NUMBERS IN ANY WAY!!! If you do, YOU NO LONGER HAVE THE ACTUAL QUANTITIES THAT ARE RIGHT IN THE QUESTION. In every example that we did in class and in EVERY ONE of the DOZENS of worksheet examples, WITHOUT EXCEPTION, the ACTUAL amount of solute dissolved in THE ACTUAL AMOUNT of SOLVENT was NEVER ALTERED or SCALED in ANY WAY.
For example, if 10. g of sodium chloride are dissolved in 50. g of water at 80 degrees Celsius, then the solution is unsaturated. According to the solubility curve Table D, 40 g of NaCl would saturate 100 g of water so, proportionally, 20 g of NaCl would saturate 50. g of water. NOTICE THAT I DID NOT ADJUST THE ACTUAL AMOUNT OF NaCl or water !!! I simply scaled down the solubility curve to concur with the ACTUAL amount of SOLVENT-water = 50 g. So, since 10 g of NaCl are ACTUALLY dissolved and 20 g of NaCl would make a saturated solution, then the solution in UNSATURATED AND adding 10 more grams of NaCl would make the solution saturated.
The ONLY quantities that MAY be scaled are those on the experimentally determined SOLUBILITY CURVES. Those curves tell you the MAXIMUM amount of a given solute that can be dissolved in 100. grams of water ONLY!!! The curves do not reflect the TOTAL ACTUAL AMOUNT OF SOLUTE FOR ANY OTHER AMOUNT OF WATER; most questions do NOT give you a quantity of solute in exactly 100 grams of water (as I promised)!!!
Half of the students in each class are scaling the ACTUAL amount DOWN or UP to the amount of solute that COULD be dissolved in 100. gram of solvent. Furthermore, they DO NOT BOTHER TO SCALE THE AMOUNT BACK UP TO WHAT IS ACTUALLY IN THE QUESTION.
On the other hand, there are quite a few perfect or near perfect (write your UNITS!) scores so obviously some were well-versed and practiced on all of the given objectives.
A little worksheet practice BEFORE the test would have brought this scaling error to light. Obviously, some of you are just looking at the worksheet answers (or not) without doing the problems first. Unless you have the WRITTEN experience and reinforcement/repetition of the class material, you are NOT studying and your grades will be lower than they could be. This is the Math of Chemistry quarter and the math is below the 8th grade level; by regularly applying the math to these problem types, most students increase their averages significantly. Count on seeing this question type again on this week's test.
# posted by C @ 9:14 PM 
