Question

"What volume of water must be added to 100ml of 15 M of H2SO4 to dilute to 5.0M?"

I've tried using lots of different methods but keep having the same answer and don't know if the answer book is wrong or if it's me. Thanks!

Answer 1

you are diluting by a factor of three (15/5). That means you need one part original, 2 parts water added.
One part=100ml of 15M
two parts water=200ml
result: 3parts (300ml) 5M H2SO4

Answer 2

First find the total volume by
V1C1 = V2C2
So, 0.1 x 15 = V2 x 5
We get V2 = 0.3L = 300mL

Now to find the amount of water required subtract the initial volume(V1) from the total volume V2

We get 300-100 = 200mL or 0.2L

Important: Don't ever add water in acid, always add acid in water. :D