Question

The concentration of chloride ion, in molarity, in a solution containing $10.56g$ $BaC{l_2}.8{H_2}O$ per litre of solution is $(Ba = 137)$:
A.$0.06M$
B.$0.07M$
C.$0.18M$
D.$0.21M$

Answer 1

The molarity of $BaC{l_2}.8{H_2}O$ solution will indicate the number of moles of $BaC{l_2}.8{H_2}O$ per litre of solution. As, one mole of $BaC{l_2}.8{H_2}O$ gives $2$ moles of chloride ions, therefore, the concentration of chloride ions will be double the concentration of $BaC{l_2}.8{H_2}O$.

Complete step by step answer:
Molarity of a solution is defined as the number of moles of solute dissolved in one litre of the solution (solvent $+$ solute).
Number of moles is the ratio of given mass of a compound and its molecular mass.
Molecular mass or molar mass of $BaC{l_2}.8{H_2}O$
$= 137 + (2 \times 35.5) + (8 \times 18)$
$\Rightarrow 137 + 71 + 144$
$\Rightarrow 352g$
Number of moles of $BaC{l_2}.8{H_2}O$
$\Rightarrow$ $\dfrac{{10.56}}{{352}}$
$\Rightarrow 0.03$ moles
This means that one litre of the solution contains $0.03$ moles of $BaC{l_2}.8{H_2}O$. Therefore, the molarity of the $BaC{l_2}.8{H_2}O$ solution is $0.03M$.
As one mole of $BaC{l_2}.8{H_2}O$ gives $2$ moles of chloride ions, therefore, $0.03$ moles of $BaC{l_2}.8{H_2}O$ will give,
$\Rightarrow$ $2 \times 0.03$ moles
$\Rightarrow 0.06$ moles
This means that one litre of the solution contains $0.06$ moles of chloride ions.
Thus, the concentration of chloride ion in the solution is $0.06$ moles per litre of solution, that is, $0.06M$.

Hence option A is correct.

Note:
Besides molarity there are other units of concentration too. Molality and normality are some other units of concentration widely used.
-Molality is the number of moles of solute present per kilogram of the solvent. Here the volume of the solution is not taken into account.
-Normality is the number of gram equivalents of solute present per litre of the solution. Gram equivalent is the ratio of the given mass of the compound to the equivalent mass of the compound.