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### Topic: Mass percent of alcohol in the blood?  (Read 26825 times)

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#### marco9905

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##### Mass percent of alcohol in the blood?
« on: April 01, 2008, 02:18:26 AM »
Can any one help me with this question as i am completly confused.

A personâ€™s blood alcohol (C2H5OH) level can be determined by
titrating a sample of blood plasma with a potassium dichromate
solution. The balanced equation is:
16H+(aq) + 2Cr2O72- (aq) + C2H5OH(aq) → 4Cr3+(aq) + 2CO2(g) + 11H2O(l)
If 35.46 mL of 0.05961 M Cr2O7 2− is required to titrate 28.00 g of
plasma, what is the mass percent of alcohol in the blood?

#### Guitarmaniac86

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##### Re: Mass percent of alcohol in the blood?
« Reply #1 on: April 01, 2008, 03:30:18 AM »
I will do it step wise for you to show you a method of working these things through.

The first step is to work out how many moles of dichromate you have. This is worked out using the following formula:

Moles = (C x V) / 1000 where C is concentration in Mol dm-3 and V is in cm3 or ml.

Therefore the number of moles of dichromate is:

Moles = (0.05961 x 35.46)/1000

Moles = 2.11 x10-3 moles of dichromate.

The next step is to look at the balanced reaction. It takes two moles of dichromate to oxidise one mole of the alcohol therefore the moles of alcohol is half the value of the moles of dichromate therefore:

Moles = 2.11 x10-3 / 2

Moles = 1.055 x10-3 moles of alcohol.

The next stage is to convert the moles of alcohol into mass. This is achieved using:

Moles = Mass / RMM

Which when converted is:

Mass = Moles x RMM

Therefore:

Mass = 1.055 x10-3 x 46.0668 (the RMM of ethanol)

Mass = 0.0486g

To work out the % mass of alcohol use the following:

% mass = (Mass alcohol/ mass of plasma) x 100

Therefore

% mass = (0.0486 / 28. 00) x 100

% mass = 0.17%

Hope that being worked through helps. This is a general method of working through most titration problems.

#### marco9905

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##### Re: Mass percent of alcohol in the blood?
« Reply #2 on: April 01, 2008, 03:51:54 AM »
Thanks alot for that, it was a great help. Espically going through it step by step.

#### Borek

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##### Re: Mass percent of alcohol in the blood?
« Reply #3 on: April 01, 2008, 04:01:48 AM »
I will do it step wise for you to show you a method of working these things through.

Please read forum rules. I am more then sure than marco9905 was able to do the question on his own, just with a little bit guidance.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Guitarmaniac86

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##### Re: Mass percent of alcohol in the blood?
« Reply #4 on: April 01, 2008, 04:56:43 AM »
I will do it step wise for you to show you a method of working these things through.

Please read forum rules. I am more then sure than marco9905 was able to do the question on his own, just with a little bit guidance.

Ah ok, I see. I ll deffo change my method of answering from now on. I thought I was obeying the rules by not giving just an answer but a method too.

#### Borek

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##### Re: Mass percent of alcohol in the blood?
« Reply #5 on: April 01, 2008, 05:08:53 AM »
I thought I was obeying the rules by not giving just an answer but a method too.

Sometimes everything else fails and there is no other way as to list step by step procedure, but in most cases hinting is enough

In this particular case first part of calculations (finding mass of alcohol in the sample) can be very easily done with EBAS, see the following video posted on facebook: https://www.facebook.com/watch/?v=520731452082952
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info