Question

what is the concentration of the following solutions? : 25 ml C2H5OH is diluted to a volume of 150ml with water

Answer 1

I am not certain if I understand the question, but here is my shot at it:
You put 25 mL \[C _{2}H _{5}OH\] into 150 mL water. The sum of both would be 175 mL after the mixup. Then you calculate (25 mL /175 mL) and you should get 0.143, or 14.3%.

Answer 2

An helpful source for information related to concentrations such as these;
http://chemistry.about.com/od/lecturenotesl3/a/concentration.htm

Answer 3

^the TOTAL new volume is 150mL, not that 150mL of water was added to the solute. Do the same math with 25mL/150mL, not 25mL/175mL