How to prepare 90% ethanol?

There are generally two kinds of ethanol in the market, one is anhydrous ethanol (above 99.0%) and the other is 95% ethanol, which is usually called pentanol. Here 100% or 95% generally refers to the mass percentage.

I don't know if you mean 90% by mass or by volume.

Take anhydrous ethanol as an example:

1, volume percentage. For example, if you want to prepare1000 ml of 90% ethanol and have anhydrous ethanol on hand, assuming that Yml anhydrous ethanol is needed to prepare it, then Y= 1000*90%. The water quantity is1000-y.

2. Mass percentage. For example, if you want to prepare 1000 ml of 90% ethanol, you have anhydrous ethanol on hand, and the density of anhydrous ethanol is a, assuming that Yml anhydrous ethanol needs to be prepared, then y =1000 * 90%/a. The volume of water is1000-y.

Those two upstairs are right. Generally, distilled water is used for configuration.