Friday, May 27, 2016

Blood Alcohol Concentration

Drinking alcohol impairs your driving ability, so legal limits are set, not for the amount of alcohol you consume, but on the effects of alcohol in your system. In Australia we measure the amount of alcohol in your blood (BAC or Blood Alcohol Concentration) and have set a "maximum legal limit" of 0.05% for most drivers (BAC for learner, provisional and professional drivers is less).
A Blood Alcohol Concentration, BAC, of 0.05% is a weight per volume percentage, w/v%, that is, a BAC of 0.05% is 0.05 grams of alcohol in every 100 mL of blood.
Your Blood Alcohol Concentration, BAC, is effected by the alcoholic beverages you consume. The concentration of alcohol varies between different types of alcoholic beverages, and is given as w/v% concentration as shown in the table below:

Alcoholic BeverageTypical Alcohol Concentration
light beer2.7%
ordinary beer4.9%
port or sherry20%

So drinking 100 mL of wine will increase your blood alcohol concentration more than drinking 100 mL of beer.

But drinking alcoholic beverages is not the only way that alcohol can enter your blood stream. Alcohol is also present as an ingredient in other things you might ingest such as in some mouthwashes where it is used for its anti-bacterial properties, for example, original formula Listerine mouthwash contains 26.9% alcohol. Alcohol is also found as the solvent in many over-the-counter and prescription medications, for example, Benedryl has an alcohol concentration of 14% while Benedryl Decongestant has an alcohol concentration of 5%.

The amount of alcohol you ingest will be one factor in determining your blood alcohol concentration, but another factor is the rate at which your body removes alcohol from your blood, and this differs between individuals. There is no way to determine your blood alcohol concentration short of chemical analysis. Since it is not practical for every driver to do this before they set off, guidelines are produced based on the number of "standard drinks" that an "average person" can consume in an hour before he will be "over the legal limit". It should be noted that some people will need to drink less than the guidelines while there are some people who can consume more.
In Australia, a "standard drink" is one containing 10 g of alcohol.
So, in order for you estimate the number of "standard drinks" you have ingested, you need to know the alcohol concentration in your drink and the volume of the drink you drank. Which is not something you are likely to do while standing at the bar ordering your drink!
For this reason, Government authorities issue the guidelines showing the "standard drink" as volumes of different types of alcoholic beverages:

Alcoholic BeverageTypical Alcohol ConcentrationStandard Drink
light beer2.7%1 schooner, 425 mL
ordinary beer4.9%1 middy, 285 mL
wine12%1 glass, 100 mL
spirits40%1 nip, 30 mL
port or sherry20%1 glass, 60 mL

In order to maintain a blood alcohol concentration under 0.05%, an "average" person can consume 2 standard drinks in the first hour, followed by 1 drink each following hour. If your legal limit is 0.02%, then even just 1 drink can put you over the legal BAC for driving! And, ofcourse, if your "legal limit" is 0%, you cannot ingest any alcohol at all before driving. Be aware that an average person takes about 1 hour to remove the alcohol in 1 standard drink from their blood stream, so it is quite possible for you to be over the "legal limit" hours after you begin drinking!

Further Reading:
Weight/Volume Percentage Calculations:
Molarity (mol L-1 concentration) :

Suggested Study Questions:
  1. Determine the mass of alcohol in 100 mL of each of the following drinks:
    • light beer
    • ordinary beer
    • wine
    • spririts
    • port
  2. Determine the mass of alcohol in 250 mL of each of the following drinks:
    • light beer
    • ordinary beer
    • wine
    • spririts
    • port
  3. Calculate the mass of alcohol present in:
    • 425 mL of light beer
    • 285 mL of ordinary beer
    • 100 mL of wine
    • 30 mL spririts
    • 60 mL port
  4. Assuming the recommended dose of Benedryl, or Benedryl Decongestant, is 20 mL, what mass of alcohol is present in each dose?
  5. How much of each cough mixture above do you need to consume in order to ingest the same amount of alcohol as present in a standard drink?
  6. Explain why the volume of a "standard drink" differs for different types of alcoholic beverage.
  7. In one hour, Phyl the Physicist drinks 1 schooner of ordinary beer and 1 glass of wine at Science Expo.
    • What mass of alcohol has Phyl consumed?
    • How many "standard drinks" has Phyl consumed?
  8. Also at the Science Expo is Bobby the Biologist who mixes herself a Manhattan; 1 nip of vermouth and 2 nips of whiskey, and a dash of bitters, served in a chilled glass with ice and a cherry garnish.
    • What is the minimum mass of alcohol Bobby will consume when she drinks this?
    • Approximately how many standard drinks is this equivalent to?
  9. Sam the Science student has a cold. She takes a 20 mL dose of Bendryl before going to the Expo. In the same hour, Sam drinks 1 middy of light beer at the Expo.
    • What mass of alcohol has Sam consumed?
    • How many "standard drinks" has Sam consumed?
  10. Chris the Chemist is the "designated driver" for his team members at the Science Expo. So Chris drinks a 100 mL cup of mulled wine, which is red wine that has been heated with various spices. Chris thinks the alcohol content of his drink must be less than that of 1 "standard drink". Do you agree? Explain your answer.
  11. Assuming "alcohol" refers only to ethanol (ethyl alcohol), convert the following concentrations in w/v% to concentrations of ethanol in mol L-1 (molarity):
    • light beer 2.7%
    • ordinary beer 4.9%
    • wine 12%
    • spririts 40%
    • port 20%
  12. Ethanol is a liquid at room temperature and pressure. Why do you think alcohol concentrations are given in units of grams per 100 mL? Explain your answer

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