How do you weigh a gas like carbon dioxide?

When people measure carbon dioxide, a lot of the time it is measured in weight. They'll say that your car releases so many pounds of carb.....
28 May 2006

WEIGHING_SCALES

5kg shown on the dial of a scale.

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Question

When people measure carbon dioxide, a lot of the time it is measured in weight. They'll say that your car releases so many pounds of carbon dioxide. But how do you weigh a gas?

Answer

Everything here on Earth is made of atoms and molecules and they must all "weigh" something...

Since we know how much each of these individual atoms and molecules weighs, it's very simple to say that, because we know how much gas came out of the car and how much of each gas was in it, we can work out how much the carbon dioxide weighs. That's the simple argument.

Now, to put a bit more complexity into the argument, chemists have a very clever measurement called a "mole". This is a convenient measure by which you can compare directly how much of something you've got. One mole of any chemical substance contains about 6 followed by 23 zeros atoms (Avogadron's number - 6 x 10^23). So, if you have one mole of carbon dioxide, you know you've got 6 followed by 23 zeros molecules of carbon dioxide.

We know how much one molecule of carbon dioxide weighs, and so we know how much one mole of carbon dioxide weighs: one mole of carbon dioxide weighs 44 grams.

The average person is said to produce through their lifestyle about 4.5 tonnes of carbon dioxide every year. So you could say that four and a half tonnes is about 4400 kilograms of carbon dioxide, or 4.4 million grams of carbon dioxide. I told you that one mole of carbon dioxide weighs 44 grams, so if you divide 4.4 million grams by 44, that means you must have ten to the power of five moles of carbon dioxide that you've released through the year.

We know that one mole of gas takes up 24 litres at room temperature and pressure. So if you times 24 by that, that means the average person produces through their lifestyle, 2.5 million litres of carbon dioxide gas every single year. That's half the size of a large swimming pool, which is a considerable amount of gas!

Comments

1 gallon of diesel fuel weighs around 7 lbs. Burning said gallon produces 22 lbs of CO2. I was taught that matter can neither be created or destroyed. It just changes state and phase. So how does 7 lbs become 22 lbs with this avenue of reason.

Diesel is a hydrocarbon, comprising carbon chains coupled to hydrogen atoms.

When the fuel burns, each of the carbon atoms (atomic mass 12) links up with 2 oxygen atoms (atomic mass 16) to make a molecule of CO2 (molecular mass 44).

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