What Is Molality?
The concentration of a substance is a very important quantity in our daily lives. Household items like rubbing alcohol, mouthwash, bleach and various cleaning products can have various concentrations of their active ingredients. When we buy products, we sometimes take the concentration of the active ingredient into consideration, such as when buying an astringent: it is either 1% or 2% salicylic acid, so we make our choice based on how sensitive our skin is.
Chemists also have to be aware of the concentration of solutions they use during laboratory procedures. One of the ways concentration is expressed is through molality. Molality, symbolized by a small m, is the number of moles of solute per kilograms (kg) of solvent.
Formulas Needed for Problem Solving
The unit of molality is therefore expressed in mol/kg. The formula for molality is:
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In problem solving involving molality, we sometimes need to use additional formulas to get to the final answer.
One formula we need to be aware of is the formula for density:
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In molality, the unit is in moles of solute/kg of solvent. There are times when we are given the mass of the solvent in grams. To convert the mass from grams to kilograms, we use the conversion factor:
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Sometimes, we are given the number of gramsof solute. Molality is the moles of solute per kg of solvent. To convert the number of grams of solute to moles of solute, we do the following:
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To find the molar mass, we need to count the number of atoms for each element and get the sum of all the atomic masses of the atoms, from the periodic table, in the chemical compound.
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Problem Solving: Molality
Let us go over a few examples of common problems encountered when we want to find the molality of a substance.
Example 1
What is the molality of a solution containing 0.75 moles of NaOH in 500 mL of water? The density of water is 1 g/mL.
Solution
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Thank you very much for the answer
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