Question

Active mass of \[5g\text{ }CaO\]is:
(A) 56
(B) 1
(C) 3.5
(D) 2

Answer 1

Hint: Active mass is the other name of the very common concentration term of chemistry, ‘molarity’ denoted as M or molar concentration which is expressed as number of moles of a solute per litre of the solution.
Formula used:
As we know, the formula of molarity is:
\[\begin{align}
  & molarity(M)\text{ = }\frac{mass\text{ }of\text{ }solute\text{ }present\text{ }in\text{ }the\text{ }solution\text{ }(g)}{molecular\text{ }mass\text{ }of\text{ }the\text{ }solute\text{ }(g/mol)\text{ }\times \text{ }volume\text{ }of\text{ }the\text{ }solution\text{ }(L)} \\
 & \text{ = }\frac{number\text{ }of\text{ }moles\text{ }of\text{ }solute\text{ }present\text{ }in\text{ }the\text{ }solution\text{ }(mol)}{\text{ }volume\text{ }of\text{ }the\text{ }solution\text{ }(L)} \\
\end{align}\]

Complete step by step answer:
- Active mass of \[5g\text{ }CaO\] will also be calculated with the same formula.
- Active mass means the amount of mass of the sample taking part in the reaction, which is the effective concentration of the sample.
- But here we have to take into account the calculation of \[CaO\] which is a solid.
So the active mass of \[CaO\] is 1g because it is always equal to 1 in solids.
Correct option is B.

Additional information: Common name of calcium oxide is quicklime or burnt lime, a widely used chemical compound. Appearance is white powder like.
- It is used to “lime” lake waters that have been acidified due to acid rain.
- Sir Humphry Davy discovered this compound.
Note: Always check for the physical state of the sample given in the question for calculation of active mass. And do not be confused with the term ‘active mass’ with something difficult to calculate. You may get confused between quicklime and lime water, but both are very different. Quicklime is \[CaO\]/ Calcium Oxide. Limewater is \[Ca(OH){}_{2}\] or Calcium Hydroxide.