Introduction
There are several concentration units available in the literature to carry out different measurements. All these units are interconvertible. One of these units is molality which may sound similar to molarity. But the concept of molality is different from molarity. The molality of a solution is calculated to check the concentration of a solution. Our molality calculator is here to help you find the molality of a solution more quickly. Continue reading to know more about the concept of molality.
Concept of Molality
Molality is different from molarity. Although both are concentration units; there is some key difference between these two concepts. Molarity is the number of moles of solute present in a certain volume (liters) of the solution while Molality is the number of moles of solute added in a certain mass (kg) of solvent.
Another noticeable difference is that molarity includes the volume of solution i.e., solute + solvent collectively whereas, molality uses only the mass of solvent in kilograms. Hence we can say that molarity is the space that something takes up while molality is the mass how much something weighs.
Importance of Molality
Following are the benefits of molality over molarity
- It is easier to prepare a molal solution than a molar solution.
- Molality stays constant over a wide range of temperature and pressure while molarity changes with a change in temperature or pressure.
- There is a risk of error while filling the volumetric flask up to the mark (molar solution preparation) whereas in molality there is the least risk of error.
The formula of Molality
Molality is the number of moles of solute per kg of solvent. The SI unit of molality is mol kg-1 and the following is the formula of molality:
$$Molality\;=\;\frac{moles\;of\;solute\;}{mass\;(in\;kg)\;of\;solvent}$$
If the number of moles of solute is not given, then we can calculate the number of moles first by using the mass and molar mass of solute.
$$Moles\;of\;solute\;=\;\frac{mass}{molar\;mass}$$
Or you can just use our molality calculator to save time and energy. Just enter the number of moles of solute and mass of solvent then press the “calculate” button. The molality of the given solution would appear on your screen. Following are some examples to calculate the molality of a solution manually.
Example # 1
1.08 mole of KBr is dissolved in 0.910 kg of water. Calculate the molality of the solution.
Formula;
Molality = moles of solute / mass of solvent
Molality = 1.08 / 0.910
Molality = 1.19 mol/kg
Answer; the molality of the above solution is 1.19 mol/kg.
Example # 2
Calculate the molality of a solution containing 90 g of NaCl in 1000 ml of water.
First, we calculate the moles of NaCl present in the solution
Moles = mass / molar mass
Moles = 90 / 58.44
Moles = 1.5
The formula for molality
Molality = moles of NaCl / volume of water
As we know; 1000 ml = 1L then,
Molality = 1.5 mol / 1 L
Molality = 1.5 mol/kg
Answer; Molality of the above solution is 1.5 mol/kg.
