Molarity vs Molality: Understanding the Difference

In summary, molarity measures the concentration of a solution in terms of moles of solute per litre of solvent, and is dependent on temperature and pressure due to changes in density. Molality, on the other hand, measures the concentration of a solution in terms of moles of solute per kilogram of solvent, and is not affected by changes in temperature and pressure.
  • #1
nothing123
97
0
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 pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass? If that were in fact the case, wouldn't the molality be dependent on these factors too then (since 1L = 1 kg)?

Thanks.
 
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  • #2
Nothing123 wonders:
A litre of water will remain a litre of water at any given temperature and pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass?

That is wrong.
 
  • #3
nothing123 said:
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 pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass? If that were in fact the case, wouldn't the molality be dependent on these factors too then (since 1L = 1 kg)?

Thanks.

There are density changes with changes in temperature and pressure - are you not familiar with this phenomena?
 
  • #4
Hmm so as we increase the temperature of a solvent say water, we're giving kinetic energy to the molecules which allows them to spread out more and thus letting more solute dissolve. This would increase the molarity. However, why wouldn't the molality also increase? If more solute can dissolve in that same 1 kg of water at a higher temperature, how isn't molality also affected by temperature?
 
  • #5
You still don't get the most important thing. Density of the solution changes with temperature, thus 1L = 1kg holds only for 4 deg C. When you heat the solution its volume changes, thus molarity changes as well. But molality is constant, as it is per mass - and mass is unchanged.
 
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  • #6
Yes, I think I got it now. I was confusing the idea of solubility with molarity, which is simply a measurement of solubility. Thanks.
 
  • #7
nothing123 said:
molarity, which is simply a measurement of solubility.

No, it is a measurement of concentration.
 
  • #8
As we know
1 molal mean 1 mole of solute dissolve in 1 kg solvent...
1 molar mean 1 mole of Solute dissolve in 1 dm3 solution
then how could we effected 1 molar solution with increase in Temperature ?
 
  • #9
Imagine you have exactly 1L of 1M solution of NaCl at some temperature. Obviously it contains exactly 1 mole of of NaCl.

Now you heat it and the solution expands. Imagine you heated it enough so that the volume is 1.001L now (that's just 1mL more). This 1.001L of solutions still contains 1 mole of NaCl, but its molarity is now 1 mol/1.001L=0.999M.
 

What is the difference between molarity and molality?

Molarity and molality are both units used to express the concentration of a solution. However, molarity is defined as the number of moles of solute per liter of solution, while molality is defined as the number of moles of solute per kilogram of solvent. In other words, molarity takes into account the volume of the solution, while molality takes into account the mass of the solvent.

Which unit is more commonly used in scientific experiments?

Molarity is more commonly used in scientific experiments because it is easier to measure and calculate. Most laboratory equipment, such as beakers and graduated cylinders, are calibrated in volume measurements, making it more convenient to use molarity.

Why is molality preferred in certain situations?

Molality is preferred in situations where the temperature may vary significantly during the experiment. This is because molality is not affected by changes in temperature, unlike molarity which is dependent on the volume of the solution. Additionally, molality is more accurate for solutions with a high temperature coefficient, such as concentrated solutions.

How are molarity and molality related?

Molarity and molality are related by the density of the solution. The molarity can be calculated from the molality by multiplying it by the density of the solution. Similarly, the molality can be calculated from the molarity by dividing it by the density of the solution.

Which unit is used in the calculation of colligative properties?

Molality is used in the calculation of colligative properties, which are properties of a solution that depend on the number of particles in the solution, not their chemical identity. This is because molality takes into account the mass of the solvent, which is necessary for accurately determining the number of particles in a solution.

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