Strength of a solution – Molarity and its formula

The amount of solute in grams present in a 1-litre solution determines the solution’s strength. Normality, molarity, and molality are the terms used to describe the strength of a solution. The strength of the solution can be characterised by the formula for molarity.

Mass of a solute in grams/Volume of a solution in litres = Strength of a solution.

(Solute’s mass in grams/volume of solution in litres) = Solution concentration

If the volume is measured in millilitres, multiply the total formula by 1000.

(Solute mass in grams/Solution volume in mL) X 1000 = Solution concentration

The concentration of the solution in grams per litre can be used to determine its strength. If there are 10 g of glucose (solute) in 1 L of solution, the solution’s strength or concentration is 10 g/L.

A solution is a homogenous mixture of two (or more) substances whose composition varies within defined bounds. This principle holds in the case of alcohol and water, as both chemicals are in the same phase.

What about sugar syrup, though? Which of the following has 60% sugar and 40% water? Is sugar going to be the solvent? Even though the replies are no, the sugar is still a solute. So, what is the proper definition for this situation? “Solvent is that component of the binary mixture that is in the same physical state as the solution.”

A binary solution consists of two components. The solvent is the component that is present in greater quantities, whereas the solute is the component that is present in minute quantities if the physical states of both components are the same.

Methods for calculating solution strength

• Mass percentage: The term “mass percentage” refers to the number of parts by mass of solute per hundred parts by mass of solution.

                  Mass percentage = weight of solute/weight of solution x 100

• Volume percentage: Several parts by volume of solute per hundred parts by volume of the solution is the volume percentage.

              Volume percentage = volume of solute/volume of solution x 100

• Molality The number of moles of solute dissolved in 1 kg of a solvent is known as the molality of a solution.

                   Molality m = moles of solute/wt. of solvent in kg

• Molarity: The molarity formula of a solution is the number of gram molecules of the solute present per litre. The concentration of a solution is stated to be one molar if one gram of a solute molecule is present in one litre of the solution.

Molarity formula m = no. of moles of solute/volume of solution (in litre)

• Normality: The number of gram equivalents of the solute present in one litre of a solution is known as the solution’s normalcy. The concentration of a solution is stated to be one normal if one gram equivalent of a solute is present in one litre of the solution.

Normality n = no. of gram equivalent of solute/volume of solution in (litre)

• Mole fraction: The mole fraction of any component in a solution is the ratio of that component’s moles to the total moles of all components present in the solution.

           Xsolute = n/n+N

          Xsolvent = n/n+N

           n = moles of a substance

              N = solvent in moles

Note: The sum of all the components in the solution’s mole fractions equals unity.

Illustration: HCl makes up 37 percent of concentrated hydrochloric acid (by mass). Its solution has a density of 1.18 g/ml. Calculate the solution’s molarity and molality.

Solution- Mass of solution = 100g

Since density equals mass divided by volume,

volume of solution = mass / density  = 100 g/ 1.18 g ml-1 =0.085 dm3 = 85 ml = 85 cm3

n(HCl) = 37 g/36.5 g mol-1 = 1.01 mol

HCl has a formula for molarity of 1.01 mol/0.085 dm3 = 12.0 mol dm3 = 12.0 M.

Solvent mass = (100- 37) = 63 g = 0.063 kg 

Molality of HCl = 1.01 mol/0.063 kg = 16.03 mol kg-1 = 16.03 m

Conclusion

The ratio of the amount of solute dissolved in the solution to the total volume of the solution is the strength of the solution.

A solution’s strength can be expressed in terms of ratio, fraction, or percentage.

The formula for molarity (m) is represented as moles of solute per kilogram of solvent. Units used for very dilute concentrations of solute include parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt). The kind and amount of intermolecular interactions are significant in solution composition.