What is Molality: Definition and 30 Discussions

Molality is a measure of number of moles of solute in a solution corresponding to 1 kg or 1000 g of solvent. This contrasts with the definition of molarity which is based on a specified volume of solution.
A commonly used unit for molality in chemistry is mol/kg. A solution of concentration 1 mol/kg is also sometimes denoted as 1 molal. The unit mol/kg requires that molar mass be expressed in kg/mol, instead of the usual g/mol or kg/kmol.

View More On Wikipedia.org
  1. S

    Chemistry How to get molality without using a molecular formula?

    Homework Statement Freezing point depression question[/B] student determines molar mass of acetone using the method used in this experiment. She found that the eq temp of water and ice was 1 deg C. She then adds 11.1g of the sample to mixture and temp falls to -3 deg C. Mass of solution was...
  2. S

    Molality calculation.... Is it correct?

    Homework Statement Commercial grade HCl solutions are typically 39.0% (by mass) HCl in water. Determine the molality of this HCl solution, if it has a density of 1.20 g/mL. Homework Equations molality = moles solute / kg solvent The Attempt at a Solution 39.0g HCl (1mol/36.46g/mol) =...
  3. HelloCthulhu

    What is the molality of 1M NH4OH in 70.092ml of water?

    Homework Statement What is the molality of 1M NH4OH dissolved in 70.092ml of water? What is the total mass of the solution? What is the total volume? What is the concentration percent? What is the molar concentration? 1M NH4OH = 35.046g density of water = 1g/ml density of NH4OH = 0.88 g/ml...
  4. Wrichik Basu

    A problem in volumetric calculations in Chemistry

    Homework Statement :[/B] The density of a 2.05 molar solution of acetic acid is 1.02g/ml. The Molality of the solution is: (a) 1.14 mole/kg (b) 3.28 mole/kg (c) 2.28 mole/g (d) 0.44 mole/kg Homework Equations :[/B] The Attempt at a Solution :[/B] I tried it out very simply using unitary...
  5. A

    Delta T is making my solution come out negative

    Homework Statement Lauryl alcohol is obtained from coconut oil and is used to make detergents. A solution of 5.00 gram of lauryl alcohol in 0.100Kg of benzene freezes at 4.1 degrees Celsius. What is the molar mass of the lauryl alcohol? Kf for benzene = 5.12, Freezing point of benzene is 5.5...
  6. O

    Calculating mass of a solvent in a mixture, given volume and molality?

    Say for example you have take a sample of 500ml from a bulk saline solution (NaCl and H20). If you know that for every 100 grams of water there is 10 grams of NaCl, can you calculate the amount of NaCl in 500ml of the final mixture?
  7. S

    Calculating Molality: Solution Density and Molarity Relationship

    Question A 1.392 M sodium carbonate solution has a density of 1.1353 g/mL. Calculate the molality of the solution. Attempt 1.392 M Na2CO3 = 1.392 mol Na2CO3 per 1 L solution I can't really make sense out of this. What would be the next step?
  8. Q

    Calculating Molarity and Molality: Tips and Tricks | Chemistry Homework Help

    Homework Statement Screenshot of some prof's website. This is just ridiculous. http://i.minus.com/jnUa604Tz6bKX.png Homework Equations Molarity to molality. Molality is moles / kg of solvent. Molarity is moles / volume of solution. The Attempt at a Solution Pretty sure all the ones...
  9. Q

    Calculating Mole Fraction from Molality: Sodium Nitrate Solution Example

    Homework Statement The molality of an aqueous sodium nitrate solution is 2.14 m. What is the mole fraction of sodium nitrate? Homework Equations Molality = moles of solute / kg of solvent. Mole fraction of A = moles of A / total number of moles (in this case, moles of solute +...
  10. K

    Chemistry Calculate Mole Fraction, Density, & %w/w given Molarity and Molality?

    Homework Statement Consider the following aqueous solution at 25°C Cr2(SO4)3, 1.26 mol/L, 1.37 mol/kg Calculate the mole fraction, density and % w/w Homework Equations χ = n Cr2(SO4)3 / n Cr2(SO4)3 + n H2O Unit conversions ρ= g solvent / mL solution % w/w = mass solvent / 100 g...
  11. S

    Molality for compound with 2 particles

    Can someone explain why we take the number of particles into account after we have determined our molality? I feel like I'm just memorizing this and not understanding why you need to determine # of particles for molality in the problem below. The observed boiling point was elevated by 1.02...
  12. E

    Molality Calculation: How Much NaCl is Needed for a 0.35 Molal Solution?

    Homework Statement How many grams of NaCl must you add to 750g of water to make a 0.35 molal solution? Ans. 7.59g Homework Equations molality=\frac{mol_{solute}}{kg_{solvent}} MW of NaCl = 23 + 35.45 = 58.45 g/mole The Attempt at a Solution...
  13. B

    Molality and the von Hoff factor.

    I have seen in my textbook that most colligative properties are defined in terms of molality. This must mean that there is some relation between molality and the von Hoff factor, since von Hoff factor is directly related to the colligative properties. However, as far as I know, the only...
  14. B

    Chemical Thermodynamics: CO2 dissolves in water, find molality and pH.

    Homework Statement When carbon dioxide "dissolves" in water, essentially all of it reacts to form carbonic acid, H2CO3: CO2(s) + H2O(l) <--> H2CO3(aq). The carbonic acid can then dissociate into H+ and bicarbonate ions, H2CO3(aq) <--> H+(aq) + HCO3-(aq). Consider a body of otherwise pure...
  15. G

    Comparing Molality & Molarity: Which is More Concentrated?

    hello, i had been comparing the molarity and molality of a solution. i know molarity is moles of solute per L of solution, and molality is moles of solute per kg of solvents. but what i m interested to know is- which of the two is more concentrated- 1molal solution or 1 molar solution. my...
  16. T

    Chemistry Calculating Molality and Mole Fraction for a 10% (by wt) Solution of KNO3

    Homework Statement A 10% (by wt) solution of KNO3 would have what molality and what mole fraction Homework Equations mole fraction = moles of KNO3/total moles wt % = # g of solute/total g molality = # moles KNO3/kg H2O The Attempt at a Solution 1 g KNO3/10 g total 1 g KNO3/101.1...
  17. P

    Solve Molality to Mass Homework Problem

    Homework Statement Hi. This is a question based on a lab I did. We mixed an unknown solute in a solution of pure cyclohexane. Based on numbers we got from our lab, we were asked to find the molal concentration of the unknown. The freezing point of the unkown was found to be 1.5 C and the...
  18. G

    Determining the molality of a solution

    Homework Statement In a lab, 40.3 mL Mn2+ is mixed with 5 mL H3PO4. 0.4g of potassium periodate is added. The flask is heated and then diluted to 100mL. The final solution prepared is a permanganate solution. Calculate the final molarity of the solution. C Mn2+ = 0.06061 M (60.61...
  19. R

    Molality: Moles of Solute/kg of Solvent? Example

    Is molality equal to moles of solute / kg of solvent or is it equal to mole of particles of solute / kg of solvent? example: is the molality of 0.5 mol of NaCl dissolved in 1 kg of water 0.5 m or 1 m?
  20. G

    How to go from density to molality

    Homework Statement hey guys just wondering how to do this. Lithium tetrafluoroborate (LiBF4) is sold as a 1.00 M solution in acetonitrile. If the solution has a density of 0.852 g/mL, the molality of the solution is? i tired using denisty=m/v and concentration=n/v but always had 2...
  21. M

    Chemistry = Freezing Point & Molality

    Homework Statement You have a 10.4g mixture of sugar (C12H22O11) and table salt (NaCl). When this mixture is dissolved in 150g of water the freezing point is found to be –2.24 degrees centigrade. Calculate the percent by mass of sugar in the original mixture. Homework Equations delta...
  22. F

    Calculate Molality of 27% H2SO4 Solution

    Homework Statement Find the molality of a sulfuric acid solution which contains 27,0% by mass of H2SO4 and a density of 1,198 g/mL Homework Equations molality = moles of solvent / mass of H2SO4 (kg) The Attempt at a Solution If we have 1kg of the solution, then we have 0,270 kg of...
  23. S

    Having a massive brain fart SIMPLE molarity -> molality conversion

    Having a massive brain fart... SIMPLE "molarity -> molality" conversion... This isn't even a homework question per se, I'm just trying to understand how the question converted the molarity to the molality value, via the density given. I just got a different value of molality when trying to...
  24. M

    Molarity, Molality & Mole Fraction Calculations for Glycerol-Water Solution

    Homework Statement A solution containing equal masses of glycerol, C3H8O3, and water has a density of 1.10 g/mL. Calculate the (a) molarity; (b) mole fraction of glycerol; (c) molality of the solution. Homework Equations M=moles of solute/liters of solution mole fraction = n1/n(total)...
  25. N

    Molarity vs Molality: Understanding the Difference

    Just having some problems understanding why one is dependent on pressure and temperature and the other is not. If molarity is moles of solute/litres of solvent, why would this change with pressure and temperature? A litre of water will remain a litre of water at any given temperature and...
  26. K

    Molarity vs. Molality: Choosing the Right Measurement for Solutions

    When is it favourable to use molality instead of molarity?
  27. N

    Calculate Molality of Antifreeze Needed to Reduce Freezing Point of Water -1 atm

    Hi. How many grams of antifreeze must be dissolved in 300 g of water in order to reduce the freezing point of water to –13.02o C under 1 atm pressure (Kf: 1.86 o C/molal and C3H5(OH)3: 92.09 g/mol). I have not learned molality yet and I am completely confused as to where to start. I...
  28. S

    Molality creating problems for me

    Molality creating problems for me! calculate the molality and osmotica pressur of a 0.5% solution of urea (Mol Mass=60) in water at 0 degree celsius. R-0.0821 now my dear friends! i have calculated successfuly the osmotic pressure in this question which is osmotic pressure = 1.86...
  29. L

    Archived Calculating molality given density and molarity

    I haven't taken chemistry in over 3 years so I have forgotten a lot of the very basic concepts (even while understanding the higher level stuff) and am having problems with this simple thing! Please help :(. Question 1- At 20°C, a 2.32 M aqueous solution of ammonium chloride has a density of...
  30. B

    Molality: Calculate Aqueous Solution Density 1g/(cm^3)

    How do I find the molality of an aqueous solution containing 1g/L of a certain protein with molar mass 9.0 x 10^4 g/mol if the density of the solution is 1g/(cm^3)??