Questions and Answers
The melting point of a sample of compound X was found to be 121 - 125 °C. Which was it most likely to be?
In this lab, a melting range of _____ °C indicates that a compound is relatively pure.
2.00
In a melting point determination, the heating rate as the solid is melting should be _____ °C per minute.
1.00
Why is it necessary to use a fresh sample for a second melting point determination?
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Melting points can provide the following information about a sample. Which of these is correct?
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A solid sample melted at 177.0-181.0°C. Which was it most likely to be?
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What can one conclude about a sample with a melting point of 133 - 137°C?
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If the sample from question 1 is one of four possible compounds (133°, 135°, 137°, and 139°C), which is it most likely to be?
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How could you determine if two test tubes contain the same or different compounds using melting points?
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What effect would having a sample still moist with solvent have on its melting point?
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What two pieces of information can melting point determination provide?
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Study Notes
Melting Points Overview
- Melting point of compound X was found to be 121 - 125 °C; likely it is 125 °C.
- Impurities cause compounds to melt over a range rather than at a single temperature.
- A 4 °C melting range indicates the presence of impurities, lowering the melting point compared to pure compounds.
Purity Indicators
- A melting range of 2.00 °C or less indicates a relatively pure compound.
- Pure solids melt over a narrow temperature range.
Heating Rate
- The optimal heating rate for melting point determination is 1.00 °C per minute.
- Slow heating allows temperature equilibrium between heat source, sample, and thermometer.
Fresh Sample Requirement
- A fresh sample is necessary for a second melting point determination to prevent variations due to:
- Possible decomposition of the sample.
- Changes in crystal structure.
- Potential impurities from the prior test.
Melting Points as Identification Tools
- Melting points help determine:
- Identity of the compound by comparison with known values.
- Purity, as a narrow range indicates purity while a wide range suggests impurities.
Melting Point Case Studies
- A solid sample melting at 177.0-181.0°C likely corresponds to 183.5°C (the highest option).
- A sample with a melting point of 133-137°C is deemed impure due to exceeding a 2°C range.
Melting Point Between Compounds
- To determine if two compounds in test tubes are the same, use the mixed melting point technique:
- If different, the mixture will widen and lower the melting point.
- If the same, the melting point remains narrow and identical.
Impact of Solvent Moisture
- Taking a melting point measurement of a sample still moist with solvent introduces soluble impurities.
- This leads to a lower and broader melting point range compared to pure samples.
Melting Point Determination Benefits
- Melting point analysis provides two key insights:
- The identity of a compound.
- The relative purity of the compound.
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Description
Test your knowledge of melting points and compound identification with these flashcards. Learn how impurities affect melting point ranges and determine which compound is most likely to match the observed data. Enhance your understanding of thermal properties in chemistry!
