What is Molecular weight: Definition and 31 Discussions

The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quantity relative molecular mass, as defined by IUPAC, is the ratio of the mass of a molecule to the unified atomic mass unit (also known as the dalton) and is unitless. The molecular mass and relative molecular mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in g/mol. The molar mass is usually the more appropriate figure when dealing with macroscopic (weigh-able) quantities of a substance.
The definition of molecular weight is most authoritatively synonymous with molecular mass; however, in common practice, it is also highly variable as are the units used in conjunction with it. Many common preparatory sources use g/mol and effectively define it as a synonym of molar mass, while more authoritative sources use Da or u and align its definition more closely with the molecular mass. Even when the molecular weight is used with the units Da or u, it is frequently as a weighted average similar to the molar mass but with different units. In molecular biology, the weight of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average.
The terms molecular mass, molecular weight, and molar mass are often used interchangeably in areas of science where distinguishing between them is unhelpful. In other areas of science, the distinction is crucial. The molecular mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. Prior to the 2019 redefinition of SI base units quantities expressed in daltons (Da or u) were by definition numerically equivalent to otherwise identical quantities expressed in the units g/mol and were thus strictly numerically interchangeable. After the 20 May 2019 redefinition of units, this relationship is only nearly equivalent.
The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the composition of elements in the molecule. The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry; however, methods based on viscosity and light-scattering are also used to determine molecular mass when crystallographic or mass spectrometric data are not available.

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  1. R

    Get the mass of an argon atom and its molecular weight from Cv

    I don't see how isolate m or M. That is, how to get rid of <v^2> which is how I wrote the average of v2
  2. P

    Molecular weight of lead(ii) nitrate Pb(NO_3)_2

    Hello everyone I am trying to solve the following question from the book Analytical Chemistry 7th edition by Gard D. Christian - Page 164 Example 5.17 My problem is when I calculate the molecular weight of Pb(NO_3)_2 I get 331.2 g/mol but in the book they get 283.2 g/mol . Could you explain...
  3. R

    How did 19th century scientists measure atomic and molecular weight?

    Found myself wondering about this recently, though I can't recall the context. When Mendeleev proposed the periodic table of elements, I believe that it was known that atomic weights of the known elements were multiples of hydrogen's atomic weight. Presumably also with substances like oxygen...
  4. D

    Calculate the molecular weight of Al2(SO4)3.14.3H2O

    Homework Statement Calculate the molecular weight of Al2(SO4)3.14.3H2O Homework EquationsThe Attempt at a Solution May i know how to do this ? Does it mean 14*3 = 42(H20) , where there re 84H , 42O atoms ?
  5. A

    Chemistry Polystyrene repeat unit molecular weight.

    Edit: thread moved from non HW forum. I'm trying to calculate the unit molecular weight of polystyrene. I'm given the structure, and I have to count all the elements in the structure. To calculate the weight, I keep counting 8C & 7H. The correct answer is 8C +8H. I'm having a hard time...
  6. hdp12

    Chemistry Determine Weight Percent given molecular weight....

    Homework Statement So for my homework assignment in 'Materials Science and Engineering', We were given the following problem. 1. Li and Ra both are both BCC metals. For this problem assume they form a complete solid solution (even though the very large difference in their atomic size tell us...
  7. E

    Chemistry Calculating weight needed for micromole of solution

    Homework Statement Here is the question: What weight of neutral red (molecular weight 289) must be dissolved in 1.00 ml to give a =40.0 µM (µmol l-1) solution? Give your answer in MICROGRAMS (µg). Homework Equations Not sure the only one I'm aware of is mole concentration (mol dm^-3) =...
  8. N

    Finding Molecular Weight from Freezing Point Depression

    A compound contains boron (40.3%), nitrogen (52.2%), and hydrogen (7.5%) - by mass. If 6.32g of this compound if dissolved in 100g of benzene, the solution freezes at 1.30 celcius. If the cryoscopic constant is 5.12 and freezing pt of benzene is 5.48, what is the molecular weight of compound...
  9. Y

    Is the Molecular Weight of Quantum Dots Inclusive of Organic Ligands?

    Hi All, I use commercial quantum dot from evident tech, and from the datasheet I can know its molecular weight (Mw), my question is, is this Mw also includes the organic ligand or only consider its core? what is the normal case? thanks, Yuliar
  10. E

    Abaqus Ultra High Molecular Weight Poly Ethylene (UHMWPe) Material Modelling

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  11. G

    Chemistry What is the Molecular Weight of a Gas Using the Ideal Gas Law?

    [b]1. A 0.0097561 g quantity of an unknown gas occupies 0.2 L at 500 K and 2 atm. What is the Molecular Weight (MW) of the gas ? [b]2. PV = nRT [b]3. mw=0.0097561*0.082*500/2*0.2 mw=1
  12. T

    Chemistry Molecular weight of unknown acid through titration

    Homework Statement The molecular weight of a monoprotic acid HX was to be determined. A sample of 15.126 grams of HX was dissolved in distilled water and the volume were brought to exactly 250mL in a volumetric flask. Several 50mL portions of this solution were titrated against NaOH solution...
  13. P

    Chemistry A compound with a molecular weight of 110.1 g

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  14. T

    Chemistry Calculating Average Molecular Weight and Equilibrium Constants for Gas Mixtures

    Homework Statement I'm trying to figure out how to compute the average molecular weight of a gas mixture (nitrogen dioxide and dinitrogen tetroxide). I think I'm suppose to use this equation: M(mix)=X(NO2)M(NO2)+ X(N2O4)M(N2O4) , where I would just need to calculate the mole fractions and the...
  15. K

    Whats the max molecular weight for a single polymer strand of styrne

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  16. K

    Molecular Weight Units for Mean Square Speed Calculation

    I need to calculate the mean square speed of molecules in an ideal gas. Mean square speed is equal to 3RT/M, where M is the molecular weight. But what units do I use for the molecular weight if I use 0.08206 L.atm/(mol.K) as R? Thanks a lot.
  17. L

    Chemistry Hydrocarbon molecular weight grade 11

    Homework Statement A gaseous hydrocarbon weighing 0.185g occupies a volume of 110.cm^3 at 26 celsius and 743mmHg. what is the molecular weight of this compound? Homework Equations PV=nRT The Attempt at a Solution i have no idea where to start ;(
  18. D

    Calculating the Molecular Weight of Glucose Polymers

    Glucose (C6H12O6) is a monomer that can form naturally occurring polymers. What would be the approximate molecular weight of a polymer made from 5 glucose monomers? a) 810 b) 828 c) 882 d) 900 i said D, but my friend in convincing me its B :S
  19. T

    Finding molecular weight from specific heat at constant volume

    The specific heat at constant volume of an unknown solid is measured at high temperature to be 319.5Jkg-1K-1. Estimate its molecular weight. Cv=3R Attempt. I used the above equation to get a mass of the solid to be 0.0128kg but I have no idea of what to do next!
  20. D

    Chemistry Exploring the Correlation Between Thermal Conductivity & Molecular Weight

    is there any corrolation between thermal conductivity coefficient and molecular wieght Mw according to the units [K] W/(m·K) it is independant of mass units (kg or g/mol etc) in my question i am asked to show with a graph, but i have no idea what graph they are talking about
  21. W

    Chemistry Why do molecular weight and BP/MP differ in formaldehyde and ethane?

    While both molecules have a very similar molecular weight, their boiling and melting points vary significantly. Explain the behavoir. i have look everywhere to try and figure out why the bp and mp are different from mw but i just can't get anywhere!
  22. D

    Chemistry Chemistry Moles, Molecular Weight

    Homework Statement I did an experiment and found the weight of CaC(2)O(4)H(2)0 to be 31.4966 subtracted by the weight of a crucible which is 31.2612. I put this for my weight of CaC204H20 (as I can't think of any other weight that it would be requested). I followed the next question...
  23. M

    Chemistry Calculate Molecular Weight of Polyethylene from Osmotic Pressure

    WOuld someone be able to give us a hand for this question, I am stumped. Polyethylene is a synthetic polymer or plastic with many uses. 2.06 g of a polyethylene sample was dissolved in enough benzene to make 100 mL of solution, and the osmotic pressure was found to be 1.86 torr at 25oC. What is...
  24. F

    Chemistry Experimental Molecular Weight higher than Literature Value

    Homework Statement I just purified alkaline phosphatase, and my calculated molecular weight is 100000 g/mol, while literature value is 89000 g/mol. According to how others in my lab did calculations, my calculations are performed correctly, so I am left to explain why my purification is...
  25. S

    Molecular Weight of Mer - Calculate PTFE Mer Weight

    How would i calculate the weight of a mer, for example a mer of PTFE.
  26. S

    Chemistry Please help me in finding this relative molecular weight

    Hi Guys! my question goes like this a 5% solution of urea is isotonics with 0.877% of Solute A. Calculate the relative molecular weight of solute A. now my dear friend, i know its a very simple question but unfortunately, just can't getting any interpretation. I know isotonics sol...
  27. M

    Molecular Weight for nucleotides

    When it says; "Molecular Weight: daltons (g/M)" What does daltons mean and what does (g/M) stand for? Does g/M stand for g/mol or what? How can they calculate Molecular Weight for nucleotides? Thanks for any inputs.
  28. M

    What is the Molecular Weight of Potassium Phosphate Buffer at pH 6.8 and 6.4?

    What is the molecular weight of potassium phosphate buffer pH 6.8 and pH 6.4? I tried to find the answer but no luck. Please help calculate. Thank you. :bugeye:
  29. Clausius2

    What is the average molecular weight of a binary gas mixture?

    Hi friends. I am wondering about the next calculation. Imagine there is a hole of some diameter, through which it is discharging a jet of an ideal gas of molecular weight W_1 (Kg/mol). Outside, there is an atmosphere at rest of molecular weight W_2 (Kg/mol). The external atmosphere has...
  30. D

    Radial Molecular Weight Distribution X Vortex

    Chemical Separation Using Ranque-Hilsch Vortex The full title for the post should be "Can molecules be separated in solution using a Ranque-Hilsch vortex?" I am a chemist, and my conventions re: aerosols/liquids may be wrong. My interest is in a system that might enrich a soluble analyte in...
  31. Loren Booda

    Gas with greatest molecular weight

    What room-temperature gas has the greatest molecular weight?