Understanding Concentration Methods in Solutions

Grade 11 ISC & CBSE Chemistry Guide

When preparing a solution in chemistry, we often need to describe how much solute is present in a given amount of solvent. This quantitative expression is called the concentration of a solution.

Different methods are used depending on the type of problem and experimental conditions.

MOLARITY (M)

Definition:

“The number of moles of solute dissolved per litre of solution is known as the molarity of the solution.”

It is denoted by M.

M=moles of solutevolume of solution in litresM = frac{text{moles of solute}}{text{volume of solution in litres}}

✔ Key Points

  • Volume is taken in litres of solution, not solvent.
  • Temperature affects molarity (because volume changes).

✔ Example

If 1 mole of NaCl is dissolved to make 1 L solution → 1 M NaCl solution

MOLALITY (m)

Definition:

“The number of moles of solute present in 1000 g of solvent is known as molality of the solution.”

It is denoted by m.

m=moles of solutemass of solvent in kgm = frac{text{moles of solute}}{text{mass of solvent in kg}}

✔ Key Points

  • Uses mass of solvent, not solution.
  • Independent of temperature → preferred in colligative properties.

✔ Example

1 mole glucose in 1 kg water → 1 m solution

MOLE FRACTION (χ)

Definition:

“The ratio of the number of moles of a constituent to the total number of moles of all the constituents present in the solution is the mole fraction of that constituent.”

It is denoted by χ (chi).

For component A:

χA=nAnA+nBchi_A = frac{n_A}{n_A + n_B}

For component B:

χB=nBnA+nBchi_B = frac{n_B}{n_A + n_B}

✔ Important Relations

For two components:

χA+χB=1chi_A + chi_B = 1

For three or more components:

χA+χB+χC+⋯=1chi_A + chi_B + chi_C + dots = 1

✔ Key Points

  • Unitless quantity.
  • Useful in vapour pressure & Raoult’s law calculations.

 PERCENTAGE METHODS

(i) Mass by Mass Percentage (W/W)

Amount of solute in grams dissolved in 100 g of solution.

%(w/w)=mass of solutemass of solution×100% (w/w) = frac{text{mass of solute}}{text{mass of solution}} times 100

✔ Used in solid–solid mixtures and alloys.

(ii) Mass by Volume Percentage (W/V)

Amount of solute in grams dissolved in 100 mL of solution.

%(w/v)=mass of solutevolume of solution×100% (w/v) = frac{text{mass of solute}}{text{volume of solution}} times 100

✔ Common in medical and biological solutions.

 STRENGTH OF SOLUTION

Definition:

“The amount of the solute (in grams) present in one litre of the solution is known as the strength of the solution.”

Strength=mass of solute (g)volume of solution (L)

Unit: g L⁻¹

✔ Relation with Molarity

Strength=M×Molar masstext{Strength} = M times text{Molar mass}

PARTS PER MILLION (ppm)

Used when the solute concentration is extremely small (trace amounts).

Definition:

The quantity of solute per million (10⁶) parts of the system.

ppm=amount of soluteamount of solution×106ppm = frac{text{amount of solute}}{text{amount of solution}} times 10^6

✔ In aqueous solutions:

ppm=mass of solute (mg)volume of solution (L)ppm = frac{text{mass of solute (mg)}}{text{volume of solution (L)}}

✔ Uses

  • Water pollution measurement
  • Mineral content in drinking water
  • Environmental chemistry

 IMPORTANT NOTE

1 dm3=1 L=1000 mL=1000 cm31 text{ dm}^3 = 1 text{ L} = 1000 text{ mL} = 1000 text{ cm}^3

 Quick Comparison

Method Based On Temperature Effect Unit
Molarity Volume of solution Affected mol L⁻¹
Molality Mass of solvent Not affected mol kg⁻¹
Mole Fraction Moles Not affected No unit
% w/w Mass Not affected %
% w/v Mass & volume Slight %
Strength g per litre Affected g L⁻¹
ppm Trace quantity Not significant ppm


Exam Tips & Common Mistakes

✔ Always convert volume to litres in molarity problems.

✔ Use mass of solvent (not solution) for molality.

✔ Mole fraction has no units.

✔ ppm is used for very dilute solutions.

✔ Strength is not the same as molarity.