Question

If \[4{\text{g}}\] of \[{\text{NaOH}}\] are contained in one deciliter of solution, its molarity would be:
A.\[4{\text{M}}\]
B.\[2{\text{M}}\]
C.\[1{\text{M}}\]
D.\[1.5{\text{M}}\]

Answer 1

Hint: To find molarity, we need to calculate moles of solute present in per litre of the solution. Moles of a substance is a SI unit of amount of substance, it can be calculated from mass, volume and number of a substance.

Complete step by step answer:
In order to find molarity of \[{\text{NaOH}}\] or sodium hydroxide, we need to first find moles of sodium hydroxide present in the given volume of solution. As the given mass of sodium hydroxide is \[4{\text{g}}\] and as molar mass of sodium hydroxide means mass of 1 mole sodium hydroxide molecules and that is \[40{\text{g}}\] , from this data we can easily calculate moles of sodium hydroxide and moles of a substance can be given as:
\[{\text{no of moles}} = \dfrac{{{\text{given mass}}}}{{{\text{molar mass}}}}\] ,
putting values of mass of sodium hydroxide given and molar mass of sodium hydroxide.
We get, number of moles of sodium hydroxide as: \[{\text{no of moles}} = \dfrac{4}{{40}} = 0.1{\text{mol}}\] .
Now volume of the solution can be determine from the given data in question as it is given that \[{\text{NaOH}}\] are contained in one deciliter of solution, it means volume of solution is \[0.1{\text{L}}\] .
Now molarity of the solution is defined as the number of moles of a solute present in \[1{\text{L}}\] or \[1000{\text{mL}}\] of the solution. In our case, solute is sodium hydroxide, thereby molarity of the solution can be given as:
\[{\text{Molarity}} = \dfrac{{{\text{moles of NaOH}}}}{{{\text{Volume of solution}}\left( {{\text{in L}}} \right)}} = \dfrac{{0.1}}{{0.1}} = 1{\text{M}}\] .

Thus, the correct option is C.

Note:
Molality of a solution is defined as the number of moles of solute present in \[1000{\text{g}}\] or \[1{\text{kg}}\] of a solvent. Mole fraction is the ratio of number of moles of solute or solvent present in the solution and the total number of moles present in the solution of the substance.