Question

You need a 30% alcohol solution. On hand, you have a 90 mL of a 45% alcohol mixture. How much pure water will you need to add to obtain the desired solution?

You will need
_____ mL of pure water
to obtain
______ mL of the desired 30% solution.

Answer 1

45ml of pure water to obtain 135ml of the desired 30% solution

Explanation:

45% of 90 = 40.5

So, 40.5ml of alcohol in 90ml

We want 30% and therefore need a ratio of 3:7

40.5÷3=13.5

so one part of our ratio is 13.5

we then times this by 7

13.5 x 7 = 94.5

so, 94.5ml of water

to work out how much we already have, we should do 90ml- 40.5ml = 49.5ml

and then 94.5- 49.5 = 45ml

We need 45ml of water and the total mo of the desired solution will be 90+45=135ml