Question

${\text{2}}{\text{.46g}}$ of sodium hydroxide (molar mass${\text{ = 40}}$) are dissolved in water and the solutions is made to ${\text{100c}}{{\text{m}}^{\text{3}}}$ in a volumetric flask. Calculate the morality of the solution.
A. ${\text{A}}$ ${\text{0}}{\text{.515M}}$
B. \[{\text{B}}\] ${\text{0}}{\text{.615M}}$
C. \[{\text{C}}\] ${\text{0}}{\text{.655M}}$
D. \[{\text{D}}\] ${\text{0}}{\text{.555M}}$

Answer 1

Hint: Molar concentration also called Molarity is a measure of the concentration of chemical species of a particular solute in a solution. It is calculated by using formula:
$\dfrac{{{\text{No}}{\text{.of moles of solute}}}}{{{\text{volume of solution in litres or d}}{{\text{m}}^{\text{3}}}}}$ $\left\{ {\left. {{\text{1c}}{{\text{m}}^{\text{3}}}{\text{ = 1mL}}} \right\}} \right.$ $\left\{ {\left. {{\text{1d}}{{\text{m}}^{\text{3}}}{\text{ = 1L}}} \right\}} \right.$

Complete step by step answer: A solution is defined as a homogenous mixture of two or more chemically non-reacting substances, the relative amounts of which can be varied up to a certain limit. If a solution consists of only two components, it is called binary solution. The component present in a smaller amount is called the solute, while the other present in the larger amount is called the solvent. The concentration of a solution can be expressed in molarity.
Molarity of a solution is defined as the number of moles of solute present per litre of solution. It is represented by the symbol${\text{M}}$.
Molar mass of sodium hydroxide, ${\text{NaOH = 40gmo}}{{\text{l}}^{{\text{ - 1}}}}$
Therefore, ${\text{2}}{\text{.46g}}$ of sodium hydroxide${\text{ = }}\dfrac{{{\text{2}}{\text{.46}}}}{{{\text{40}}}}$ moles of sodium hydroxide ${\text{ = 0}}{\text{.0615}}$ moles of sodium hydroxide
                                           Or
Moles of NaOH ${\text{ = }}\dfrac{{{\text{Mass of NaOH}}}}{{{\text{Molar mass}}}}$
                             ${\text{ = }}$ $\dfrac{{{\text{2}}{\text{.46g}}}}{{{\text{40gmo}}{{\text{l}}^{{\text{ - 1}}}}}}$
                            ${\text{ = 0}}{\text{.0615}}$ Moles of sodium hydroxide
Volume of solution$ = 100$ ${\text{c}}{{\text{m}}^{\text{3}}}$
Molarity ${\text{ = }}\dfrac{{{\text{Moles of NaOH}}}}{{{\text{Volume of solution}}}}{ \times {1000}}$
                ${\text{ = }}\dfrac{{{\text{0}}{\text{.0615 mol}}}}{{{\text{100 ml}}}}{ \times {1000mL}}{{\text{L}}^{{\text{ - 1}}}}$
                ${\text{ = }}$$0.615$ ${\text{mol}}{{\text{L}}^{{\text{ - 1}}}}$
                          Or
                 ${\text{ = 0}}{\text{.615M}}$
So, the correct answer is “Option B”.



Note: Molarity is a unit of concentration measuring the number of moles of a solute per litre of solution. Since it is dependent on the volume it is also temperature dependent that is , it changes when the temperature changes.