You need a 25% alcohol solution. On hand, you have a 375 mL of a 60% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

Question

You need a 25% alcohol solution. On hand, you have a 375 mL of a 60% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

Answer 1

If you add X amount of water to the solution, the total amount of solution is 375+X. And that must be 25%. 

The amount of solute in the current amount of solution is .6(375)=225. That amount won't change by adding water. So we set up the equation so that the amount of solute divided by the new total amount of solution equal 25%.

 

225/(375+X)=.25

cross multiply and solve

225/.25=(375+X)

900-375=X

X=525

 

then check 

225/(375+525)=.25