What is the difference between activity and concentration in aqueos solution?

Answer 1

Activity is the mole fraction, multiplied by a coefficient, described by the equation:
a(i) = phi(i)*x(i)
where phi(i) is the activity coefficient, which must be measured for different substances, and x(i) is the mole fraction.


Concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the conept is limited to homogeneous solutions, where it refers to the amount of solute dissolved in a solvent.

At dilute conditions concentration = activity can be applied

Answer 2

Activities For Concentration

Answer 3

Lets take an example.

Define pH?

pH is the negative of the hydrogen ion activity

So pH = -log(a[H+]) where [H+] is the hydrogen ion concentration in mol/litre and a is the activity co-efficient.

At infinite dilution a tend to 1 and the usuala conenvention applies that activities = concentrations. The activity co-efficent was applied to better account for measured concentrations versus the theoretical values. The ions under consideration are in a mixture of electrolytes that do influence their behaviour.

Debye proposed the first theoretical treatment :-

a =kz^2I^0.5

where k is constant (approx 0.5 at 25oC), z is the charge and I is the ionic strength. Ionic strength is calculated as follows I= 0.5 sum(z^2C) where z is the charge on the ion and C is its concentration.

The practical consequence of this in terms of solubility is that it tends to increase the observed solubility.

Most chemistry problems tend to ignore activity (ie assume infinite dilution) as it tends to complicate the maths.

Answer 4

Concentration is the physical amount present in the solution. It is usually measured in grams or moles per unit volume. It is easily calculated.

Activity is the deviation from the theoretical ideal when determining how the actual concentration reacts in the solution. It is determined by experiments and is based upon the calculations of Activity Coefficients. The Activity is the Activity Coefficient times the actual Concentration of the reactant in solution.

The best example is the concentration of Hydrogen ions in very strong acid solutions. The Activity (apparent concentration) of the Hydrogen ions (as measured by reactions or a pH meter) is usually less than the actual Concentration of the Hydrogen ions present.

Answer 5

concentration = no. of moles/ volume of solution

activity = (activity coefficient) * concentration

for very dilute solutions we can take

concentration nearly equal to activity

Answer 6

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