Question

Active mass of 5 g $CaO$ is:
  (A) 2
  (B) 1
  (C) 5
  (D) 0

Answer 1

Hint: Active mass is another term for the concentration term often used in chemistry. It is not a measure of mass in a solution but it’s a substance's activity which is the measure of concentration.

Complete step by step answer:
  - As we know, molar concentration or molarity can be expressed as the number of moles of the solute per litre of the solution. Here we are asked about the active mass. It is the amount of mass of the sample which takes part in the reaction which is the effective concentration of the sample.
 - In other words, it can be explained as the actual number of entities taking part in a reaction at a particular instance. That is, it's the actual amount of substantial atoms participating in the reaction per unit time.
  - Active mass is also called as the chemical activity or reactive mass.
  - The given compound calcium oxide or quicklime is a solid and the value of the active mass of solids is always taken as one. Therefore, the active mass of calcium oxide is one.
Thus, the answer is option (B). That is the active mass of $CaO$ is 1.

Note: Whenever we get a question like this, always check the physical state of the given substance for the calculating active mass. Also do not confuse between quicklime and lime water. Calcium oxide ($CaO$) is the quicklime and it’s a solid whereas calcium hydroxide($Ca{{(OH)}_{2}}$) is the limewater. It should also be noted that concentration is not always equal to active mass.