why do you have to take that extra step and cant use the formula M1xV1=M2+V2?

Question

Someone posted this but i still dont understand why you cant use the M1xV1=M2xV2 formula :(

Question #1

If 23.67 mL of a solution of Ba(OH)₂ requires 15.05 mL of a 1.082 M solution of HNO₃ for complete titration, what is the molarity of Ba(OH)₂ solution? answer in units of M 

 

"First, write the balanced equation:

2 HNO₃ + Ba(OH)₂ → Ba⁺²_aq + NO^3-_aq + 2 H₂O

 

Figure moles of nitric acid used: 15.05 mL HNO₃ x (1 L / 1000 mL) x (1.082 mole/L) = 0.016 moles HNO₃

 

Use the stoichiometric ratio in the eqn: 0.016 moles HNO₃ x ( 1 mole Ba(OH)₂/ 2 moles HNO₃) =

       8.142 x 10^-3 moles Ba(OH)₂

 

Figure the concentration of the Ba(OH)₂: (8.142 x 10^-3 moles Ba(OH)₂ / 23.67 mL) x (1000 mL/L) =                     0.344 M  Ba(OH)₂  

 

check: 23.67 mL x (1 L / 1000 mL) x (0.344 mole/L) = 8.143 x 10_-3 moles 

 

So, the answer is that the concentration of the Ba(OH)₂ soln is 0.344 M"

Answer 1

Right. So what’s the problem/question? Are you asking why you cant use M1V1 = M2V2.? If you are, the answer is because the stoichiometry isn’t 1:1 as you already pointed out. It is 2 HNO3 : 1 Ba(OH)2.