# Basic molarity and dilution formulas

LT72884
ok, for the life of me i can not remember how to do the BASIC math for my issue. I mean, i can do calc but i have forgotten basic math haha.

here is my question. If i have 91% grain alcohol, how do i figure out how much water to add to it to drop the percentage to 70%. Dimensional analysis is i think what its called haha

thanks

Homework Helper
Gold Member
Is this by volume, v/v type? Assuming yes, then if volume of water to add is v, and ignoring dimensional analysis (although would be useful), and if you have Q amount of volume of your given 91% alcohol,

0.91/(Q+v)=0.71

Be aware, the question does not necessarily involve molarity.

Be aware, the question could involve densities or volume contraction changes for the different concentrations of alcohol in water.

LT72884
Is this by volume, v/v type? Assuming yes, then if volume of water to add is v, and ignoring dimensional analysis (although would be useful), and if you have Q amount of volume of your given 91% alcohol,

0.91/(Q+v)=0.71

Be aware, the question does not necessarily involve molarity.

Be aware, the question could involve densities or volume contraction changes for the different concentrations of alcohol in water.

EDIT: the above will work with grain alcohol, but what about rubbing alcohol?

thank you! I don't know why my brain does what it does. Literally, i can do integration by parts, or with partial fractions, then some one asks me to help them dilute rubbing alcohol and grain alcohol and i am like "umm, ill get back with you." its just a basic ratio. i was going to attack it with some ideas from thermodynamics class with density, specific gravity etc etc, but figured there is a easier ratio that i am not seeing..

Homework Helper
Gold Member
EDIT: the above will work with grain alcohol, but what about rubbing alcohol?
Yes, of course. With jet fuel as well.

Fig Neutron
I was just asked to do this calculator yesterday. Originally I looked at it from a molality standpoint, but in the end ratio and proportions was much easier.

Homework Helper
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