Question

In the extraction of Fe(III) into diethyl ether, it is desired that the final acid concentration should be 1.0 M to 25 `mL` of the iron solution prepared in 1.0 M `HCl`. What volume of 12.0 M `HCl` should be added so that the final strength becomes 7.0 M?

Answer 1

`30mL`

Explanation:

The question is not clear enough

I proceed as I understand.
Initial volume of acid is
`V_1=25mL`

Initial strength of acid is
`S_1=1M`

Let xmL of 12M HCl to be added with 25mL 1M HCl to have (25+x)mL 7M final solution

So `V_2=xmL and S_2=12ML`

Again `V_3=25+x mL and S_3=7M`

So `V_1S_1+V_2S_2=V_3S_3`
`=>25*1+x*12=(25+x)*7`
`=>5x=150`
`=>x=30mL`