Calculate The Molalities of The Following Aqueous Solutions 1.22 M
'/%3E%3Ccircle cx='48' cy='39' r='19' fill='%23f2c9a5'/%3E%3Cpath d='M27 35c3-16 39-17 42 2-8-8-27-9-42-2z' fill='%23374151'/%3E%3Cpath d='M21 86c5-24 49-28 56 0z' fill='%232563eb'/%3E%3Ccircle cx='41' cy='41' r='2' fill='%23111827'/%3E%3Ccircle cx='55' cy='41' r='2' fill='%23111827'/%3E%3Cpath d='M42 52c4 3 9 3 13 0' stroke='%2392400e' stroke-width='3' fill='none' stroke-linecap='round'/%3E%3C/svg%3E)
Reviewed by Calculator Editorial Team
Molality is a measure of solution concentration that expresses the number of moles of solute per kilogram of solvent. This calculator helps you determine the molality of aqueous solutions, including the given 1.22 m solution.
What is molality?
Molality (m) is defined as the number of moles of solute dissolved in one kilogram of solvent. It's particularly useful in cryoscopic and ebulloscopic measurements because it's independent of temperature changes.
Molality formula:
m = moles of solute / mass of solvent (kg)
The unit for molality is moles per kilogram (mol/kg). For example, a 1.22 m solution means there are 1.22 moles of solute per kilogram of water.
How to calculate molality
To calculate molality, you need to know:
- The mass of the solute in grams
- The molar mass of the solute
- The mass of the solvent in kilograms
The calculation involves these steps:
- Convert the mass of solute to moles using its molar mass
- Divide the moles of solute by the mass of solvent in kilograms
Important: Molality is independent of temperature because it's based on mass, not volume. This makes it useful for applications where temperature changes might affect volume.
Molality vs. molarity
While both molality and molarity measure concentration, they differ in their bases:
| Property | Molality | Molarity |
|---|---|---|
| Definition | Moles of solute per kg of solvent | Moles of solute per liter of solution |
| Temperature dependence | Independent of temperature | Depends on temperature |
| Use in freezing point depression | Preferred | Less accurate |
For most laboratory work, molarity is more commonly used, but molality is preferred in cryoscopic and ebulloscopic measurements.
Example calculation
Let's calculate the molality of a solution made by dissolving 25.0 grams of sodium chloride (NaCl) in 500 grams of water.
- First, find the molar mass of NaCl: 22.99 (Na) + 35.45 (Cl) = 58.44 g/mol
- Calculate moles of NaCl: 25.0 g / 58.44 g/mol ≈ 0.428 mol
- Convert mass of water to kg: 500 g = 0.500 kg
- Calculate molality: 0.428 mol / 0.500 kg = 0.856 mol/kg
This solution has a molality of 0.856 m.
Practical applications
Molality is particularly useful in:
- Cryoscopic measurements (freezing point depression)
- Ebulloscopic measurements (boiling point elevation)
- Preparing solutions for analytical chemistry
- Understanding colligative properties of solutions
In the given example of 1.22 m solution, this concentration would be appropriate for applications requiring precise solvent-solute interactions.
FAQ
What is the difference between molality and molarity?
Molality is based on the mass of the solvent (kg), while molarity is based on the volume of the solution (liters). Molality is temperature-independent, while molarity changes with temperature.
Why is molality used in freezing point depression calculations?
Molality is preferred because it's independent of temperature changes, which is important when measuring how much a solvent's freezing point is lowered by a solute.
How do I convert molarity to molality?
You need to know the density of the solution at the given temperature to convert between molarity and molality. The formula is: m = M × (density of solution / density of pure solvent).