What is Molarity?
The molarity (M) of a solution is defined as the number of moles of solute dissolved in one liter of solution.
To analyze the molarity of a solution, divide the moles of solute present in a solution by the volume of the solution stated in liters.
Equation used for finding the molarity of a solution is given below;
Molarity (M) = moles of solute / liters of solution = mol/L
When a molarity is reported, the unit used to represent is M and it is generally read as “molar”.
For instance, a solution labeled as 1.9 M NH 3 is read as “1.9 molar ammonia solution”.
One molar is referred to as the molarity of a solution where one gram of solute is dissolved in one liter of solution.
Molarity vs Molality
1) Molarity is referred to as concentration of a substance that is calculated as the number of moles of solute that is dissolved in one liter of solution while molality is defined as concentration of a substance that is calculated as the number of moles of solute found in one kg of solvent.
2) The molarity is represented by the symbol M, whereas molality is represented by a symbol m .
3) The formula for molarity is written as moles / liter whereas the formula for molality is written as moles / kg.
4) Molarity is generally affected by changes in temperature of the system while molality is not affected by changes in temperature of system.
5) Molarity is usually affected by changes in pressure whereas molality is not affected by changes in pressure of system.
6) Molarity may result in a vague and inexact concentration, whereas molality always results in an accurate and exact measurement of concentration of solution.
How to Calculate Molarity #1
A solution is prepared by dissolving 52.23 g of NH 4 Cl into enough water to make 600.0 mL of solution. Calculate its molarity.
Given; Mass – 52.23 g NH4Cl
Molar mass of NH4Cl = 53.50 g/mol
Volume of solution given = 600.0 mL = 0.600 L
Initially, the mass of the ammonium chloride given is converted to moles. Then the molarity is calculated of the solution by dividing by liters.
52.23 g NH4Cl x (1mol of NH 4 Cl / 53.50 g NH 4 Cl) = 0.9762 g
The given volume has to be converted to liters.
0.9762 / 0.600 = 1.621 M
The molarity is 1.621 M, this means that a litre of the solution would contain 1.621 mol NH4Cl.
How to Calculate Molarity #2
A chemist needs to make 4.00 L of a 0.350 M solution of potassium permanganate (KMnO4). What mass of KMnO4 does she need to make the above solution?
Given; molarity = 0.350 M
volume = 4.00 L
molar mass KMnO4 = 158.04 g/mol
Unknown ; mass KMnO4 = ? g
Moles of solute can be calculated by multiplying molarity of solution by liters. Then, moles has to be converted to grams.
Mol KMnO4 = 0.350 M KMnO4 x 4.00 liter = 1.4 mol KMnO4
1.4 mol KMnO4 x (158.04 g/mol/ 1 mol KMnO4) = 221.5 g KMnO4
When 221.5 g of potassium permanganate is dissolved into water to make 4.00 L of solution, and the molarity is 0.350 M.
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