What is the molarity of the solution made by adding $ 5ml $ of $ 0.1M $ of $ HCl $ to $ 45ml $ of the water is $ 0.01? $

Hint: Given that to the $ 5ml $ of $ 0.1M $ of $ HCl $ , from this calculate the number of moles of solute. To this $ 45ml $ of the water is added. Thus, total volume will be $ 50ml $ . By taking the number of moles of solute and volume as $ 50ml $ the molarity of the solution will be calculated.

Complete answer:
Hydrochloric acid is a strong acid with the molecular formula of $ HCl $ . generally concentrated hydrochloric acid is dangerous when it gets in contact with the body. For so many less reactions only, it can be used. In all the other cases, it will be diluted by adding water.
Molarity is known as molar concentration that can be defined as the number of moles of solute to volume of solution in litres, and can be written as $ M = \dfrac{n}{V} $
Given that $ 5ml $ of $ 0.1M $ of $ HCl $ is taken. By substituting these both values in the above formula.
The number of moles will be $ n = 0.1 \times 5 \times {10^{ - 3}} $
As $ 45ml $ of water is added to the above solution, the total volume will be $ 50ml $
Substitute the final volume and obtained moles in the above formula,
 $ M = \dfrac{{0.1 \times 5 \times {{10}^{ - 3}}}}{{50 \times {{10}^{ - 3}}}} $
Further simplification of the above values, molarity will be $ 0.01M $
Thus, the molarity of the solution made by adding $ 5ml $ of $ 0.1M $ of $ HCl $ to $ 45ml $ of the water is $ 0.01 $ .

Note:
While calculating the molarity, the volume should be taken in litres only, as the definition of molarity states that the number of moles of solute dissolved in volume of solution in litres. Conversion from millilitres to litres must be done if the volume is given in millilitres.