Question

The molarity of the solution obtained by mixing 750 mL of 0.5M HCl with 250 mL of 2M HCl will be:
A. 100 M
B. 1.75 M
C. 0.975 M
D. 0.875 M

Answer 1

Hint: For this problem, we have to calculate the total number of moles by adding the number of moles of 0.5M HCl and 2M HCl and we will add the total volume of both molecules. After which we can calculate the total molarity of the molecules.

Complete step by step answer:
- In the given question, we have to calculate the total molarity of the solution of the 0.5M HCl and 2M HCl.
- As we know that molarity is the ratio of the number of moles of the solution to the total volume of the solution in litres that is:
$\text{Molarity = }\dfrac{\text{Moles }\times \,\text{1000}}{\text{Volume (ml)}}$

- Now, it is given in the question that the first solution of hydrochloric acid whose molarity is 0.5M and has a volume of 750 mL, the number of moles will be:
$\text{Molarity = }\dfrac{\text{Moles }\times \,\text{1000}}{\text{Volume (ml)}}$ or we can write it as:
$\text{Moles = }\dfrac{\text{Molarity }\times \text{ Volume}}{1000}$, so by putting the values we will get
$\text{Moles = }\dfrac{\text{0}\text{.5 }\times \text{ 750}}{1000}\text{ = 0}\text{.375 moles}$
- Similarly, we will calculate the number of moles of 2M hydrochloric acid that is:
$\text{Moles = }\dfrac{\text{2 }\times \text{ 250}}{1000}\text{ = 0}\text{.5 moles}$
- So, the total number of moles will be $\text{0}\text{.375 + 0}\text{.5 = 0}\text{.875moles}$and the total volume will be 750 + 250 = 1000 mL or 1 L.

- Now, the total molarity of the solution will be:
$\text{Molarity = }\dfrac{0.875}{1}\,\text{ = 0}\text{.875M}$
So, the correct answer is “Option D”.

Note: Molarity and molality are the two entities to measure the concentration of the solution but molality is generally used to express colligative property because it is independent of the volume which can change under different conditions of temperature.