Question

Active mass of 0.64 g $S{{O}_{2}}$ in 10 lit vessel is
A. ${{10}^{-2}}M$
B. ${{10}^{-3}}M$
C. ${{10}^{-1}}M$
D. 0.64 g

Answer 1

Hint:. To solve this question, we must know the meaning of the active mass of a compound. Active mass of a compound is nothing but the molarity of the given solution. So basically we need to find the molarity of the solution.

Complete step by step answer:
-Molarity is defined as the ratio of the moles of the solute to the volume of the solution and can be expressed mathematically as
$molarity = \dfrac{moles\text{ }of\text{ }solute}{volume\text{ of solution}} = \dfrac{mol}{L}$

-Now to find the molarity, we first need to find the moles of the solute. Only then we can find the molarity of the solution. Mole is the unit for measuring the amount of a substance. It is given by the formula
Moles of solute = $\dfrac{wt.\text{ in grams}}{molecular\text{ wt}\text{.}}$

-As the numerator and the denominator both have the same unit, the quantity ‘mole’ is unitless. For the given question, we can see that we are given the weight of $S{{O}_{2}}$ to be 0.64 grams. The molecular weight of $S{{O}_{2}}$ will be the sum of the molecular weights of S and both the oxygen atoms which can be shown as
Molecular weight of $S{{O}_{2}}$ = 32 + 16 + 16 = 64 grams.
So, the moles will be given as
Moles of solute = $\dfrac{wt.\text{ in grams}}{molecular\text{ wt}\text{.}}=\dfrac{0.64}{64}={{10}^{-2}}$

-So, $molarity = \dfrac{moles\text{ }of\text{ }solute}{volume\text{ of solution}} = \dfrac{{{10}^{-2}}}{10}={{10}^{-3}}$
So, the correct answer is “Option B”.

Note: Always take into account the unit of all the quantities present in the question. Remember that 1M= mol/L. No other unit defines 1M. Also, keep into account the relationship between the units of mass and the units of volume. 1L = 1000ml and 1ml = 1cc. Also, volume has to be taken of the solution and the moles are taken of the solute.