Percent, but percent by what?

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Offline Bill.D.Katt.

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Percent, but percent by what?
« on: 14/11/2010 00:47:28 »
Many common household chemicals show information indicating the percentage of the chemical in solution, along with other "inert" ingredients (I'm assuming H2O). My question: is this percent by mass, or by volume? Things such as bleach (NaClO), are measured by mass right? And things such as HCl and H2O2 are by volume?

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Offline chris

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Percent, but percent by what?
« Reply #1 on: 14/11/2010 10:27:12 »
Hi Bill

when you make a "percent" solution you are adding the amount of the substance in grams to a total volume of water of 100ml (i.e. the final volume of the solution, after the substance has mixed in, is 100ml).

So if you were making a 5% solution of glucose you would be dissolving 5g of glucose in a final volume of water of 100ml. For a liquid it's the same. So for a 5% ethanol (alcohol) solution you would dissolve 5g of ethanol in a final volume of water of 100ml.

Other variations are possible too: when the police breathalyse someone they say that person has "x number of mg percent" in their blood; in this instance they are using millimgrams of dissolved ethanol per hundred ml of blood.

Chris
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Offline Bored chemist

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Percent, but percent by what?
« Reply #2 on: 14/11/2010 18:48:35 »
They generally mean % mass/ volume but not always. Alcohol solutions are generally expressed as %v/v for example.
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