Question

Find the molarity of the solution in which 5g of NaOH is dissolved in the 500mL $ {{\text{H}}_2}{\text{O}} $?

Answer 1

Hint: The molarity of a solution gives us the concentration of a solution which can be determined by knowing the number of moles and the volume of the solution.

Formula used:
1. No. Of moles $ {\text{ = }}\dfrac{{{\text{Given mass}}}}{{{\text{Molar mass}}}} $
2. Molarity $ {\text{ = }}\dfrac{{{\text{No}}{\text{. of moles}}}}{{{\text{volume of solution in litre}}}} $

Complete answer:
Here we are given the mass of NaOH that is dissolved in the water solutions, by having this knowledge we can determine the number of moles of NaOH present in the solution, using the formula;
No. Of moles $ {\text{ = }}\dfrac{{{\text{Given mass}}}}{{{\text{Molar mass}}}} $
We are given the NaOH i.e., 5g, and to calculate molar mass we are just going to add up the respective molar mass of Sodium(Na), Oxygen(O) and hydrogen(H), given in the periodic table.
 $ \Rightarrow $ 22+16+1= 39 g,
Now using the formula we can calculate the number of moles,
 $ \Rightarrow $ No. of moles $ {\text{ = }}\dfrac{5}{{39}} $
 $ \Rightarrow $ No. of moles $ = \text{0.128} $
That means 0.128 moles of NaOH is dissolved in 500mL of water, now to calculate Molarity we must change the si unit of volume from mL to liter.
 $ \Rightarrow $ 500mL = 0.5 L
Simply putting up these values in the formula for Molarity we get,
Molarity $ {\text{ = }}\dfrac{{{\text{No}}{\text{. of moles}}}}{{{\text{volume of solution in litre}}}} $
 $ \Rightarrow $ Molarity = $ \dfrac{{0.128}}{{0.5}} $
 $ \Rightarrow $ Molarity = $ \text{0.256 M} $

Hence the molarity of the solution is 0.256 M

Note:
Before starting the calculation for Molarity, pay attention to the SI units of the given values, Molarity of a solution is determined by the volume of solution in liters, but in this question, the volume was given in mL Therefore, it is important to look out for S.I units.