Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: woodend on 09/12/2004 06:34:07

Title: Weight of Vodka
Post by: woodend on 09/12/2004 06:34:07

I am getting odd results measuring the weight of a litre of vodka. It is 40% by volume alcohol. 400ml of grain alcohol has a specific gravity of 0.79 and should weigh 916gms. The remaing 600ml (99.9% water) should weigh 600gms giving a total of 916gms. I consistently get about 947gms. Scales are accurate and I get similar results with cheaper and with imported vodka. Anybody help?

Title: Re: Weight of Vodka
Post by: chris on 09/12/2004 13:23:50

Maybe it is slightly stronger than 40% alcohol w/v ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Weight of Vodka
Post by: woodend on 10/12/2004 01:45:11

To get these results the vodka would need to be weaker. I have since discovered that volumes of aqueous fluids are not necessarly additive. To dilute 400ml of alcohol to 1000ml requires more than 600ml of water... perhaps 630ml.

Title: Re: Weight of Vodka
Post by: chris on 10/12/2004 11:04:30

Oh, that's interesting, why's that ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Weight of Vodka
Post by: Ylide on 12/12/2004 04:49:40

Stupid miskeying passwords.  I lost a really in-depth post on this so here's the more concise version:  [xx(]

When mixing two liquids of differing molecule size you're going to get a change in total volume that deviates from the theoretical.  This is mostly because of differences in molecule size but partially from the differences in intermolcular forces.  

Imagine you have two containers of marbles, 1L containing 2cm diamter marbles and 1L containing 1cm marbles.  Mixing them into a 2L container will not fill the entire container because the smaller marbles will fill the spaces between the bigger ones.  The greater the difference between the particle size, the greater the difference in total mixing volume.  

The reason you're not getting a correct measurement with your calculation in your post is because you're assuming that there is 400ml of ethanol and 600ml of water in 1L of 40% v/v vodka.  I don't recall the exact mixing ratio (easily found experimentally) but you're going to be closer to 660/440 water/ethanol respectively.  All the 40% means is that the total volume of ethanol is 2/3 the volume of water.  



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Title: Re: Weight of Vodka
Post by: chris on 13/12/2004 12:26:35

Wow, beatifully explained. If that was the concise version I can't imagine what the in depth answer was like !

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Weight of Vodka
Post by: Ylide on 15/12/2004 02:10:33

I went into great detail on the properties of intermolecular forces of liquids.  Since that was only partially the reason for this phenomenon, I decided not to retype it.

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