Question

How many grams of ${{\text{K}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}$ is present in $1L$ of its $\dfrac{N}{{10}}$ solution in acid medium?
A.$4.9$
B.$49$
C.$0.49$
D.$3.9$

Answer 1

Hint: Molar Concentration: It is defined as the amount of substance per unit volume of solution.
Normality: It is defined as the gram equivalent weight per unit litre of solution. The unit of normality is: $eq/litre$.
Equivalent mass: It is defined as the mass of one equivalent.

Complete step by step solution:
First of all we will read about the concentration terms.
Solute: The compound which is mixed in a given substance, is known as solute.
Solvent: The compound which is given and in which solute is mixed, is known as solvent.
Molar Concentration: It is defined as the amount of substance per unit volume of solution.
Molarity: It is defined as the number of moles in the given volume of the solution. The unit of the molarity is as: $mol/litre$.
Molality: It is defined as the number of moles of solute per kilogram of solvent. The unit of molality is: $mol/kg$.
Normality: It is defined as the gram equivalent weight per unit litre of solution. The unit of normality is: $eq/litre$.
Equivalent mass: It is defined as the mass of one equivalent.
Ppm (parts per million): It is also expressed in milligrams per litre. It is defined as mass of substance per unit volume of water.
Valency: It is defined as the combining capacity of atoms or molecules.
Molar mass: It is defined as the mass of the one mole of a substance.
Molar mass of substance: It is defined as the sum of the molar mass of all the atoms in the substance. For example: molar mass of potassium dichromate is $2 \times 39 + 2 \times 52 + 7 \times 16 = 294$
To calculate equivalent weight we have to divide the molar mass by its valency in that compound.
So here the valency of chromium atoms is $x$ as the valency of potassium is fixed i.e. $ + 1$ and that of oxygen in this compound is $ - 2$. So valency of chromium atom is
$
  2 \times 1 + 2x + 7 \times ( - 2) = 0 \\
  x = 6 \\
 $.
So the equivalent weight of potassium dichromate will be as molar mass divided by its valency i.e. $\dfrac{{294}}{6} = 49$.
Now we are given with the normality i.e. $0.1N$ and volume i.e. one litre we have to calculate the number of grams of potassium dichromate.
So, according to the definition of the normality
$0.1{\text{N = }}\dfrac{{{\text{number of grams}}}}{{{\text{number of equivalent}}}}$
Hence number of grams of potassium dichromate $ = 0.1 \times 49 = 4.9$.

Hence option A is correct.

Note: Number of moles: It is defined as the ratio of mass of substance or molecule in the given solution to the molar mass of that substance or molecule in the solution.
Mole fraction: It is defined as the ratio of number of moles of the solute or solvent to the total number of moles in the solution.