Given, 20 gm NaOH dissolved in 500 ml solution. Find molarity of the solution.

Hint: To solve this question do consider the concepts of molarity. This includes the number of moles of the solute and volume of solution. Check if all this is given in the question and then proceed.

Complete answer:
Let us know about the concepts of molarity first of all.
Molarity-
It is the amount of a substance in a definite volume of solution. It is defined as the moles of solute in a litre of solution. Hence, also known as molar concentration of the solution.
Thus, molarity is the ratio of the number of moles of the solute to the volume of solution specifically in litres. It has the unit expressed in ‘M’ (moles per litre).
\[Molarity=\dfrac{n}{V}\]
where,
n = number of moles of solute
V = volume of solution
Number of moles of solute is expressed as ratio of given mass of solute to the molar mass of the same.
\[n=\dfrac{{{m}_{given}}}{{{m}_{molar}}}\]
Now, solving the given illustration we get,
Given that,
Given mass of NaOH = 20 gm
Volume of solution = 500 ml = 0.5 litres
Molar mass of NaOH = 40 gm/mol
Thus,
Number of moles of NaOH = $\dfrac{20gm}{40gm/mol}$ = 0.5 moles.
Now, molarity is given as,
Molarity = $\dfrac{0.5moles}{0.5litres}$= 1 M
Therefore, 20 gm NaOH dissolved in 500 ml of solution has 1M of molarity.

Note:
Do note that the molarity consists of volume of solution and not the volume of solvent.
Check the units.