[sufler]

Hello.
There is a task in my exercise book which tells me to calculate the mass ratio of two solutions mixed together: 96% and 15% sulfuric acid, in order to prepare a 50% solution of the acid. I sort of had an idea how to go about solving this problem and it even agreed with the explanation presented later in the book, which is:

To take into account that the amount of dissoluted substance in the final solution will equate to the sum of amounts of the substance in the mixed solutions. Following this way of thinking, we get the equation: m₁ · c_p1 + m_p2 · c₂ = (m₁ + m₂)c_px, where m stands for the masses of substance in the solutions 1 and 2 and cm_p stand for the concentration percentage. The equation can be easily transformed into the ratio: m₁/m₂ = c_px - c_p2 / c_p1 - c_px with which we can immediately calculate the mass proportion.

But then I got the second thought. Why are we putting into the equation the masses of the substance in the intial solutions? Shouldn't we deal with the total mass of the solution instead, like the sum of the substance mass and the solvent?

[Borek]

Just summing masses of the solutions won't tell you anything about the final concentration of the solute in the mixture.

Plus, question doesn't ask about final mass, but about mass ratio.

If you were told to find masses of solutions required to prepare given mass of 50% solution, sum of masses would be an additional equation that you would use.