Which Cannot Determine Molar Mass?

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In summary, the conversation discusses a multiple choice question regarding the determination of molar mass. The options given are osmotic pressure, change in temperature of freezing point, vapor pressure measurements, specific heat, and density of solid. The participants agree that options a and b can be ruled out, and there is a debate between options c, d, and e. Option c is eliminated as it is a colligative property and the boiling point can be determined through vapor pressure. Option d works well for metals but not for other materials, while option e works well for crystalline solids but less accurately for other substances. It is also mentioned that options a, b, and c can be used if the molecule does not dissociate or associate,
  • #1
apchemstudent
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I had a multiple choice question like this:

Which one of these CANNOT determine the molar mass?

a) osmotic pressure

b)change in temperature of freezing point

c)vapor pressure measurements

d)specific heat

e)density of solid


I know its not a or b. However i cannot decide between c, d, or e...
I had d as well because you cannot determine the molar mass for everything. According to petit-dulong law, it only applies to metal elements. And even still, you cannot really determine the molar mass directly(can only estimate), where as osmotic pressure you can find molar mass directly and precise if values are correct.

please help thanks.
 
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  • #2
You can rule out (c) likewise. It's a colligative property. After all, the BP is the temperature at which VP = 1 atm.

Anyway, like you said, it's a bit of a toss-up between (d) and (e). I'll let you decide.

d) works approximately for metals using Dulong-Petit... pretty accurately (within ~3% accuracy) for most elemental metals (Hg is off by ~15%); less accurately for alloys. Doesn't work for very much else.

e) works very well for a crystalline solid, if you know its crystal structure. But this requires aqdditional information (ie : the crystal structure). Barely works for amorphous materials, though there's a rough correlation among some classes of organic compounds.
 
  • #3
You can use a,b,c if you are sure that the molecule will not dissociate/associate.
Infact they were used .Then people noticed that it gives stupid results with some molecules due to association/dissociation,they were used to find degree of dissociation.Look up vant Hoff factor and all that.
 

1. What is molar mass?

Molar mass is the mass of one mole of a substance, which is defined as the amount of the substance that contains the same number of particles as there are atoms in 12 grams of carbon-12.

2. Why is molar mass important?

Molar mass is important because it allows us to convert between the mass of a substance and the number of moles of that substance. It is also used to calculate other important quantities, such as the concentration of a solution.

3. How is molar mass calculated?

Molar mass is calculated by adding the atomic masses of all the atoms present in a single molecule of the substance. This can be found on the periodic table, where the atomic mass is listed below the element's symbol.

4. What are some examples of substances with known molar mass?

Some examples of substances with known molar mass are water (H2O), which has a molar mass of 18.015 g/mol, carbon dioxide (CO2), which has a molar mass of 44.01 g/mol, and glucose (C6H12O6), which has a molar mass of 180.16 g/mol.

5. Why are some substances unable to have their molar mass determined?

Some substances may have an unknown or variable composition, making it impossible to determine their molar mass. Other substances may not have a defined molecular structure, such as polymers or complex mixtures, making it difficult to calculate their molar mass.

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