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Chemistry > Solutions > Expressing Concentration of Solutions
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Expressing Concentration of Solutions

You know what is concentration, right? But, do you know how to express it? When there is too much salt in your lemonade, you say that it is “too salty”. But in chemistry, there is nothing like “too”. We have to define it. That is why we are going to study all about the concentration of solutions in this chapter. We will look at the various methods of calculating the concentration of solutions. So, let us start!

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Concentration of Solutions

We can calculate the concentration of solutions by various methods. Let’s study each method and determine the formulas for this method.

1) Mass/Weight Percentage or Percentage by Mass/Weight 

It is the amount of solute in grams present in 100 grams of the solution. Therefore, the formula will be:

concentration of solutions

concentration of solutions

concentration of solutions

The ratio mass of solute to the mass of the solvent is the mass fraction. Thus, the mass percentage of solute = Mass fraction × 100. 10% solution of sugar by mass means that 10 grams of sugar is present in 100 grams of the solution, i.e., we have dissolved 10 grams of sugar in 90 grams of water.

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2) Volume Percentage

It is the volume of solute in mL present in 100 mL solution. The formula will be:
concentration of solutions

10% solution of HCl by volume means that 10 mL of liquid HCl is present in 100 mL of the solution.

3) Mass by Volume Percentage 

It is the mass of solute present in 100 mL of solution. We can calculate the mass of the solute using the volume percentage. The formula would be:

concentration of solutions

A 10% mass by volume solution means that 10 gm solute is present in 100 mL of solution.

4) Molarity

The molarity of a solution gives the number of gram molecules of the solute present in one litre of the solution.

\small Molarity(M)=\frac{Number\ of\ moles\ of\ solute}{Volume\ of\ Solution\ in\ L}

For example, 1 mol L-1 solution of KCl means that 1 mol of KCl is dissolved in 1 L of water. Unit of molarity: mol L-1

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5) Molality

Molality of a solution is the number of moles of solute dissolved in 1 Kg of the solvent.

concentration of solutions

Thus, if one gram molecule of a solute is present in 1 kg of the solvent, the concentration of solutions is said to be one molal. The unit of molarity is mol kg-1. Molality is the most convenient method to express the concentration of solutions because it involves the mass of liquids rather than their volumes. It is also independent of the variation in temperature.

6) Normality   

The normality of a solution gives the number of gram equivalents of the solute present in one litre of the solution.

concentration of solutions

Thus, if one gram equivalent of a solute is present in one litre of the solution,  the concentration of solutions is said to be 1 normal.

  • 1N = Normal  = One gram equivalent of the solute per litre of solution   = Normality is 1
  • N/2 = Seminormal   = 0.5 g equivalent of the solute per litre of solution   = Normality is 0.5
  • N/10 = Decinormal   = 0.1 g equivalent of the solute per litre of solution   = Normality is 0.1
  • N/100 = Centinormal  = 0.01 g equivalent of the solute per litre of solution   = Normality is 0.01
  • N/1000 = Millinormal  = 0.001 g equivalent of the solute per litre of solution   = Normality is 0.001

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7) Mole Fraction 

The mole fraction of any component in a solution is the ratio of the number of moles of that component to the total number of moles of all components. The total mole fraction of all the components of any solution is 1. For a binary solution of A and B

Mole Fraction of A (XA) =  concentration of solutions

Mole Fraction of B (XB) =  concentration of solutions

And, XA+XB = 1

8) Parts per million (ppm)

When a solute is present in trace quantities, it is convenient to express the concentration of solutions in parts per million (ppm). The formula is as follows:

concentration of solutions

In case of mass, we may express it as: (Mass of solute/Mass of solution )× 106

In case of volume, we may express it as: (Volume of solute/Volume of solution) × 106

So, we can express the concentration of solutions in parts per million as mass to mass, volume to volume and mass to volume form. Atmospheric pollution in cities is also expressed in ppm by volume. It refers to the volume of the pollutant in 106 units of volume. 10 ppm of SO2 in the air means 10 mL of SO2 is present in 106 mL of air.

9) Formality

It is the number of mass in grams present per litre of solution. In case, formula mass is equal to molecular mass, formality is equal to molarity. Like molarity and normality, the formality is also dependent on temperature. It is used for ionic compounds in which there is no existence of a molecule. A mole of ionic compounds is called formal and molarity as the formality.

concentration of solutions

concentration of solutions

concentration of solutions

concentration of solutions

Where,

  • w = weight of solute,
  • f = formula weight of solute
  • V= volume of solution
  • nf = no. of gram formula weight

A Solved Example For You

Q: What is the weight percentage of urea solution in which 10 gm of urea is dissolved in 90 gm water.

Solution: Weight percentage of urea = (weight of urea/ weight of solution)\small \times 100

= 10/(90+10) \small \times 100 = 10% urea solution (w/W)

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