Question

The molecular mass of $KOH$ is 56.What is the molarity of solution prepared by dissolving 84.0 g of KOH in 500 ml of solution.

Answer 1

Hint: In order to solve this question we need to understand the meaning of Molarity. Molarity or Molar concentration is defined as the number of moles of solute dissolved per litre of solution. Moles is the S.I unit for measurement of number of particles, atoms and ions. Molarity is the most common unit of concentration of solution. It is denoted by M. It has a unit of mole/litre.

Complete answer:
We calculate the Molarity by the following formula-
Molarity=Number of Moles of solute/Volume of solution (in litre)
A solution is made up of little components of solute dissolve in the solvent.
Number of Moles=Given Mass/Molecular Mass
The Relationship between 2 solutions with same number of Moles is given as-
${C_1}{V_1} = {C_2}{V_2}$
Where $C$ and $V$ are concentration and volume of solution respectively.
In this question we are given-Molecular Mass of $KOH$=56g/mole Volume=500ml
Given Mass=84g
$Number\;of\;moles=84g/56g/\;mole=1.5\;mole$
$Volume=500/1000=0.5\;litre$
 (We divide by 1000 to convert it into litres since 1litre=1000ml).
$Molarity=1.5 mole/0.5\;litre=3 mole/litre or 3 M $

Note:
Molarity can be used to calculate Volume of solvent and the amount of solute. This concept is used to describe concentration in solution. It is an important concept in studying reactions and reaction rates. It is a useful concentration method since Volume can be easily calculated. However the drawback of this method is that volume is Temperature dependent and hence affecting the calculations. There are several other concentration terms like Normality, Molality, and Mole Fraction etc.