What volume of 0.75M potassium hydroxide solution is needed to make 350 mL of a 0.15 M solution?
For this problem we have to use the dilution equation and solve for the first volume:
M1 x V1 = M2 x V2
0.75M x V1 = 0.15M x 0.35L
V1 = 0.0525 / 0.75
V1 = 0.07L potassium hydroxide
**You are correct that the original volume is 70mL, but remember, you should always convert this to liters. Also, you can check to make sure this is correct by plugging the answer into the original equation and, if both side are equal, you are correct.
Approximately what volume of water is also needed?
So, you can answer this by asking yourself: what is the final volume and what is the amount of chemical I am putting in the solution?
Here your final volume is 0.35L and you are putting in 0.07L potassium hydroxide, so the amount if water would be the final volume minus the potassium hydroxide you are adding.
0.35 - 0.07 = 0.28
Answer: 0.28L of water are needed.
Here is a nice instructional video on how to do this: https://www.youtube.com/watch?v=cukudun0WWQ
Also, I would try doing this yourself again before turning it in just to make sure I did not make any mistakes.