# Basic molarity and dilution formulas

• LT72884
In summary, if you have 91% grain alcohol, and want to reduce the percentage to 70%, you would need to add water until the alcohol concentration was 70%.
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

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.

symbolipoint said:
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..

LT72884 said:
EDIT: the above will work with grain alcohol, but what about rubbing alcohol?
Yes, of course. With jet fuel as well.

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.

I was just asked to do this calculator calculation yesterday. Originally I looked at it from a molality standpoint, but in the end ratio and proportions was much easier.
If you look again at your original question, not a hint of moles came as part of the description or the question. All of it was concentration according to percents. If you try to put focus on moles or molarity OR molality when none is indicated, you simply make your question unnecessarily complicated. Sometimes, MOLALITY is involved in colligative properties, but again your question seems to not be of this kind.

Fig Neutron

## 1. What is molarity and why is it important in science?

Molarity is a measure of the concentration of a solution, specifically the number of moles of a solute per liter of solution. It is important in science because it allows us to accurately measure and compare the amount of a substance in a solution, which is crucial in experiments and other scientific calculations.

## 2. How do you calculate molarity?

Molarity is calculated by dividing the number of moles of solute by the volume of the solution in liters. The formula is: Molarity (M) = moles of solute (mol) / volume of solution (L).

## 3. What is the formula for dilution?

The formula for dilution is: M1V1 = M2V2, where M1 is the initial molarity, V1 is the initial volume, M2 is the final molarity, and V2 is the final volume. This formula is used to determine the volume of a stock solution needed to make a desired concentration of a solution.

## 4. How can you prepare a solution with a specific molarity?

To prepare a solution with a specific molarity, you will need to know the molar mass of the solute, the desired molarity, and the volume of the solution you want to make. Use the formula M1V1 = M2V2 to calculate the volume of the stock solution needed, and then add the appropriate amount of solvent to reach the desired volume.

## 5. How do you convert between molarity and other units of concentration?

To convert from molarity to other units of concentration, you will need to know the volume of the solution. To convert from molarity to moles per liter (mol/L), simply multiply the molarity by the volume. To convert from molarity to grams per liter (g/L), multiply the molarity by the molar mass of the solute and then multiply by the volume.

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