You have a 95% EtOH stock solution and want a 70% EtOH solution for sterilization purposes. If you want a final volume of 250 mL, how much 95% EtOH must you add, and how much water must you add

65.8mL of water and 184.2mL of the 95% EtOH stock solution must be added.

Explanation:

To solve this question we need to find hoy many times the 95% EtOH must be diluted to obtain a 70% EtOH solution. That is:

95% / 70% = 1.357 times the solution must be diluted.

If we want to make 250mL of the diluted solution, the amount of 95% that must be added is:

250mL / 1.357 times =

184.2mL of the 95% solution must be added

And of water:

250mL - 184.2mL = 65.8mL of water

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