[JennLyn6]

how many mililiters of ethanol are needed to make 75% v/v solution using deionized water as solvent?





Edit: Editied title for indexing. Mitch

[Mitch]

not enough information.

[JennLyn6]

what more information do you need?...that is the question i recieved on my homework...thank you for looking at my problem

[AWK]

75 ml of ethanol and 25 ml of water

AWK

[JennLyn6]

thank you for your help but how did you get 75mL of ethanol and 25mL of water?

[AWK]

Use graduating measuring cylinder or appropriate calibrated pipette (note - there is no 75 ml pipette, instead use 3 x 25 ml)

AWK

[jdurg]

Actually, to be analy specific, this problem is bit of a tough one.  25 mL of water mixed with 75 mL of absolute ethanol won't result in 100 mL of solution.  It's a classic demo to mix pure ethanol with water in a sealed container as the volume of the liquid decreases when they are mixed.  The ethanol is able to fit around the water molecules quite tightly resulting in the overall solution taking up less space than the two separate liquids.  Therefore, a small correction needs to be made to take into account that lowered volume.  I just can't recall it off the top of my head.

[AWK]

Jdurg is quite right concerning contraction of volume for water and alcohol solution, but we discuss here about V/V percent concentration, and not about contraction.
Definition of percent concentration is:
100 x [mass(or volume or mole)of solute]/[sum of masses(or volumes or moles) of solute and solvent]

or 100 x mass fraction (or volume fraction or mole fraction)

AWK

[Donaldson Tan]

Are u suggesting that we dont take in account of the contraction? although the final volume isn't 100ml, but as long we use 25ml water and 75ml ethanol, we will still obtain 75% V/V aq. ethanol? I am confused

[AWK]

Yes, for V/V percentage calculation we should take sum of volumes before mixing. For mass and mole percentage both sums before mixing and after mixing are the same, for volumes may be not.

See also a nice formula put on the High School Forum by GregPawin under subject: percent composition question.
AWK

[gregpawin]

Yes the mixing of ethanol and water can be a tricky one.  Due to unequal attractions and repulsions between the ethanol and water, the volume can expand or contract.  Usually, this is governed by Raoult's Law where the vapor pressure is equal to the pure vapor pressure times the mole fraction.  

P₁=X*P₀
Here you get a divergence from an ideal solution behavior, where ideally, the solute molecules and solvent molecules have the same kind of interactions with each other.  I believe they're called azeotropes when they diverge from ideal solution behavior.  When this happens, it sometimes makes it impossible to separate something by distillation.

[jdurg]

Yes the mixing of ethanol and water can be a tricky one.  Due to unequal attractions and repulsions between the ethanol and water, the volume can expand or contract.  Usually, this is governed by Raoult's Law where the vapor pressure is equal to the pure vapor pressure times the mole fraction.  

P₁=X*P₀
Here you get a divergence from an ideal solution behavior, where ideally, the solute molecules and solvent molecules have the same kind of interactions with each other.  I believe they're called azeotropes when they diverge from ideal solution behavior.  When this happens, it sometimes makes it impossible to separate something by distillation.

Azeotropes are mixtures which boil at a lower temperature than either of the two components, therefore making distillation impossible.  At a solution of 95% ethanol and 5% water, the boiling point of the mixture is lower than either the boiling point of water or ethanol.  To generate pure ethanol, a dehydrating agent needs to be used, or benzene can be added which forms a different azeotrope allowing the ethanol to boil off pure.  

[AWK]

Though it not concern the main subject (V/V percentage) I should mension that azeotropes can boil also at higher temperature than either of the two components (eg HCl + water) which is missing in previous message of  jdurg.

AWK

[jdurg]

Though it not concern the main subject (V/V percentage) I should mension that azeotropes can boil also at higher temperature than either of the two components (eg HCl + water) which is missing in previous message of  jdurg.

AWK

Ah yes.  I can't believe I forgot that.  Well, if someone tried to cheat and just copy the answer from this message board, then they got their just desserts.  

[katmar]

My apologies for posting here so long after the thread was last active, but the information given here is wrong and because it will remain accessible to web searches for a long time yet I believe it is important to give the correct information.

jdurg was on the right track by introducing the subject of contraction on mixing.  This fact is CRITICAL to being able to calculate the volumetric concentration correctly. By definition, the ethanol volumetric concentration is the volume of pure ethanol (before mixing) divided by the volume of the mixture (after mixing).

To go back to the original question - there was in fact not enough information given to answer the question uniquely.  If the question was stated as "how many milliliters of ethanol are needed to make 100ml of 75% v/v solution using deionized water as solvent?" then it could be answered.  The answer would be 75 ml of ethanol.  The interesting question is then "how many millilitres of water must be added to 75 ml of ethanol to make 100 ml of 75% v/v solution".  This is a non-trivial question because of the contraction that occurs on mixing.

And to further complicate matters the degree of contraction varies with both temperature and concentration.  To fully specify the original question it should include the reference temperature.  Usually the reference temperature will be 20C or 60F (especially in the USA).  If the mixing were to be done at 20C you would find that to make a 75% v/v mixture starting with 75 ml of pure ethanol you would have to add 28.125 ml of water, and because of the contraction you would finish up with 100 ml of mixture.

Reference books like the CRC Handbook of Chemistry and Physics or Perry's Chemical Engineers' Handbook include tables to convert from v/v to mass/mass %, and from these tables you can verify the numbers I have given here.