Question

20 ml of 0.5 M HCl is mixed with 30 ml of 0.3 M HCL, the molarity of the resulting mixture is:
A. 0.8 M
B. 0.53 M
C. 0.38 M
D. 0.83 M

Answer 1

The product of molarity and volume of a particular solution always remains constant. This property of molarity can be used to find the molarity of the resulting solution.

Complete answer:
In order to answer the question, we need to learn about molarity and its applications. Before that, let us know about solute and solvent. Solute is the substance that is present in lower concentration in a solution and solvent is the substance that is present in excess. Now when we add a solute to a solvent, we need to estimate how much solute is to be added. So, molarity is a type of unit which helps us to measure the quantity or concentration of solute that is dissolved in a solvent. Now molarity is defined as the moles of solute that are present in one litre of the solution. Number of moles is the ratio of the given mass to the molar mass of the solute. So, more the number of moles of the solute dissolved, more is the molarity. Mathematically, the formula of molarity can be expressed as:

$$Molarity=\dfrac{given\,mass}{molar\,mass}\times \dfrac{1000}{Volume\,of\,sol(mL)}$$ Now, it is to be remembered that the product of molarity and volume of a solution always remains constant. That means, if we decrease volume, molarity will increase and vice versa. So, we have: $${{M}_{1}}{{V}_{1}}={{M}_{2}}{{V}_{2}}$$ Where $ {{M}_{1}},{{V}_{1}}\,and\,{{M}_{2}},{{V}_{2}}$ refers to the molarity and volume of initial and final solution respectively. When two solutions are mixed, the above formula can be modified as: $${{M}_{1}}{{V}_{1}}+{{M}_{2}}{{V}_{2}}={{M}_{sol}}({{V}_{1}}+{{V}_{2}})$$ Now, we will substitute the values of molarity and volume of the initial solutions to get the final molarity. So, on substituting we get $ (0.5\times 20)+(0.3\times 30)={{M}_{sol}}\times 50$ . On solving the equation we get the value of final molarity as 0.38 M. **Hence we get option ‘C. 0.38 M’ as the correct answer.** **Note:** It is to be noted that when temperature is changed, the volume of solution changes, as $ PV=nRT$ , so the molarity also gets changed in that case.