Molarity

Mole Fraction

Molarity (M) determines the number of moles of solute per litre of solution which is denoted as moles/Liter and is amongst the most common units that are used for measuring the concentration of a solution.


Molarity is used for the calculation of the volume of the solvent or the amount of solute.

The relationship between two solutions having the same amount of moles of solute is represented by the formula c1V1 = c2V2, in which c is the concentration and V is the volume. For calculating the molarity of a solution, the number of moles of solute should be divided by the total litres of the solution that is produced. If the amount of solute is given in grams, you must first calculate the number of moles of solute by using the molar mass of the solute and then calculate the molarity by using the number of moles and the total volume. In this article, we will learn about the molarity definition, mole fraction, example of molarity, and how to calculate mass per cent to mole fraction.

Molarity Definition Chemistry

Molarity is defined as the number of moles of solute that are present in one litre of the solution. It is given by the equation:


Molarity =  \[\frac{no. of moles of solute}{volume of solution in litres}\]


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This is how you define molarity in chemistry.

 

Let us now look at how to define molality and mole fraction.

Molality 

Molality is defined as the moles of solute that are present in one kilogram of the solvent. It is given by the expression:



Molarity m =  \[\frac{no. of moles of solute}{mass of solution in kg}\]


Mole Fraction

The mole fraction also called as the molar fraction, is defined as the amount of the constituent, which is expressed as moles and represented by \[n_{i}\], divided by the total amount of all the constituents of a given mixture, which is also expressed in moles and is represented by \[n_{tot}\]. the mole fraction symbol is given as follows:


 \[x_{i}\]  = \[\frac{n_{i}}{n_{tot}}\]


The symbol of mole fraction indicates that the total sum of all the mole fractions of the mixture is equal to 1.


\[\sum_{i=1}^{N}\] \[n_{i}\] = \[n_{tot}\]  \[\sum_{i=1}^{N}\]  \[x_{i}\] = 1

Weight Percent or Mass Percent

The weight per cent or mass percent is said to be the ratio of the mass of solute to the mass of solution which is multiplied by 100 for calculating the mass percent. It is represented by (w/w %). You might have seen this symbol of the weight per cent on the backside of the medicines and tablets. It is amongst the most widely used units for representing the concentration.


The formula of the mass per cent is given as:


Mass percent = \[\frac{mass of solute}{mass of solution}\]  × 100

Molarity Examples

Let us now look at some solved examples of molarity to know in detail about what is molarity in chemistry.


Example 1:

Determine the molarity of NaOH solution which is prepared by dissolving its 4g amount in water to form a solution of 250 ml.


Solution:

According to the formula of molarity,


M =  \[\frac{w}{M}\]  × \[\frac{1000}{V ml}\]


= \[\frac{4g}{40g}\]  × \[\frac{1000}{250}\] = \[\frac{0.1mol}{0.250L}\]


= 0.4 mol/L or 0.4M


Example 2:

Determine the mass of NaOH dissolute for making a solution of 500 \[cm^{3}\], \[\frac{1}{2}\] M molar.


Solution:


As per the formula of molarity,


M = \[\frac{w}{M}\]  × \[\frac{1000}{Vml}\]


The molar mass of NaOH is 

23 + 16 + 1 = 40


Here, M = \[\frac{1}{2}\] and V = 500 ml



Therefore, 


\[\frac{1}{2}\] = \[\frac{w}{40}\]  × \[\frac{1000}{500}\]


Therefore, W = \[\frac{20}{2}\] = 10g


FAQ (Frequently Asked Questions)

1. What is the definition of molarity?

Molarity which is denoted by M indicates the number of moles of solute in one litre of solution denoted by moles/Liter. It is one of the most commonly used units that are used to measure the concentration of a solution. Molarity is used to calculate the volume of the solvent or the amount of the solute. The molarity of any given solution is a method for knowing the specific elements or compounds that are present in any given solution. To calculate molarity, you need to divide the moles of a solute by the number of litres of its solution. With the molarity calculation, you can easily identify the exact amount present of any given element or compound in a solution.

2. What do you mean by molarity, molality and normality?

Molarity:

Molarity, which is denoted by M, is defined as the number of moles of solute present in one litre of solution. It is given by the equation:


molarity = no. of moles of solute/volume of solution in litres


Molality:

Molality, denoted by m, is defined as the number of moles of solute present per kilogram of the solvent. The formula for molality is given by:


Molality m =  no. of moles of solute/volume of solution in kg


Normality:

Normality which is denoted by N refers to another ratio which relates the amount of solute present to the total volume of the given solution. Normality is defined as the number of equivalents present per litre of solution. It is given by the following formula:


Normality N = no. of equivalents/ 1 L solution


Normality and molarity are related to each other in the following way:


N = n x M, where, n refers to an integer.