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Question

How can we find out molarity and molality of a solution? 


Solution

Molarity (M) is defined as the number of moles  of solute per liter of solution.
molarity = moles of solute/liters of solution

Molality (m) is defined as the number of moles  of solute per kilogram of solvent.
molality = moles of solute/kilograms of solvent

Molarity and molality are both ways to express concentration. Molarity is abbreviated as 'M' and is the moles of solute per liter of solution.
M = mol solute / L solution.
Molarity is usually used in stoichiometric calculations involving solutions.

Molality is abbreviated as 'm' and is moles of solute per kilogram of solvent. 
m = mol solute / kg solvent.

Molarity:

For example, if you have 0.5 moles of NaCl in 1.5 L of H2 O, the molarity is 0.5 / 1.5 L. M = 0.33 M.

Molality:
 For example

Calculate the molality of a solution prepared from 29.22 grams of NaCl in 2.00 kg of water.

Solution

Molarity is calculated using the formula:

m=moles of salute/kg of solvent

where molesSOLUTE is the number of moles of solute, in this case NaCl
and KgSolvent is the mass in kilograms of the solvent.

First, calculate the number of moles of NaCl.

Using a periodic table, the atomic masses are:
Na = 22.99 g/mol
Cl = 35.45 g/mol

Add them together to get the molecular mass of NaCl

molecular mass NaCl = 22.99 g/mol + 35.45 g/mol
molecular mass NaCl = 58.44 g/mol

Convert grams to moles of the NaCl

Moles =29.22/58.44

moles NaCl = 0.50 moles

Plug in this and the mass of the water into the molarity formula

m=0.5mole s/2kg

m = 0.25 moles/kg
or
m = 0.25 molal

 

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