recrystallization question:
a student's correctly calculated value for the % recovery of purified unknown solid was 102%. assume that the student performed all steps of procedure correctly.
a) comment on the validity of the result (include reasons).
b) suggest possible explanations for such result
102% probably not a valid response.
My best guess is that the crystals were not dry.
a) Comment on the validity of the result:
The calculated value of 102% recovery for the purified unknown solid is not valid. In recrystallization, it is not possible to have a recovery rate greater than 100%. This indicates that there may be some error or mistake in the student's calculation.
Reasons for the result not being valid include:
1) Measurement error: There may have been inaccurate measurements during the experiment, leading to incorrect calculations of the recovery percentage.
2) Contamination: There's a possibility that impurities were present in the purified solid, which artificially increased the mass used for the calculation of recovery percentage.
3) Incomplete drying: If the student did not completely dry the solid before weighing, the increased mass due to residual moisture could have led to an inflated recovery percentage.
b) Possible explanations for the result:
1) Experimental error: The student may have made mistakes during the recrystallization procedure, such as incorrect solvent selection, inadequate cooling or filtration, or incomplete washing of the solid. These errors could have led to inaccurate recovery calculations.
2) Sample mix-up: There could have been a mix-up with the samples, where the student used more initial solid or less impure solid than he/she thought, leading to an artificially high recovery percentage calculation.
3) Measurement error: The weighing balance used to measure the solid could have been faulty, giving inaccurate readings and leading to an incorrect recovery percentage calculation.
4) Human error in calculations: The student might have made an error during the calculation of the recovery percentage, such as entering the wrong numbers, using the wrong formula, or misinterpreting the units.
It is crucial for the student to review the experiment procedure, double-check all measurements, and recalculate the recovery percentage to obtain a valid result. If the error persists or is significant, the student should seek guidance from the instructor or a subject expert for further analysis.