Short answer- About 50 pounds of CO₂ per year, with 1 ton total over a reasonable life time (40 years).

Longer answer- CO₂ is not actually in a tree. Trees, like all living things, are made up of carbon (along with other elements). Humans obtain their carbon from the food we eat. Trees, on the other hand, obtain their carbon from CO₂ in the atmosphere. So the tree itself is not made of CO₂, but rather carbon that was taken from Co₂. Knowing this, we can do a little math and make a few assumptions. About half of a tree's dry weight is carbon, and about 72.5% of a trees total weight is dry weight (trees absorb water from the ground). These are reasonable averages, but the numbers can vary based on species of tree. So to figure out the amount of carbon in a tree, do the following- [(total weight) x 0.725] x 0.5.

Now you have how much carbon is in the tree, and we know all of this carbon came from CO₂. CO₂ has two oxygens for every carbon, so it is heavier than plain carbon, ~3.67 times heavier. So now take how much carbon is in the tree, and multiply it by 3.67 and that is the equivalent amount carbon dioxide it contains!

Ex- Let's look at an extreme case- some giant sequoias can weigh as much as 2000 tons! So this would contain [2000 x 0.725] * 0.5 = 725 tons of carbon. Then 72 x 3.67 = 2,660 tons of CO₂ in 1 tree! (admittedly one of the largest trees around).

There is very little amount of gaseous carbon dioxide in a tree. However, through photosynthesis, CO₂ is transformed into organic matter. In fact, the wood fiber comes from the CO₂ the tree removed from the air and water it absorbed from the soil.

Carbon in trees is not in the form of carbon dioxide. It is in the form of sugars, which are bound into cellulose, and into complex carbon-hydrogen compounds called lignins, in the wood. When burned, the compounds in a tree are combined with oxygen in the air to form carbon dioxide and water vapor.

The percentage of carbon by weight in sugar is about 40%, so if you have one ton of wood, approximately 400 kg of that wood is carbon. The percentage of carbon in carbon dioxide in weight is about 35%. This means that if you burn one ton of wood, you would get slightly more than a ton of carbon dioxide.

To be pedantic, there isn't much CO₂ in a tree. The CO₂ that plants take in is used during photosynthesis to make other products. But since plants use carbon to build all of their solid parts, one can find the amount of CO₂ taken out of the atmosphere to make a tree.

Some trees are bigger than others, so it will have to be calculated for each tree. With some general assumptions, this isn't too difficult though. As given here*, about 65% of the weight of a living tree is solid (i.e, not water), and 50% of that solid material is carbon. This is just carbon (C) though, not CO₂. To convert weight of C to the equivalent weight of CO₂ which was consumed, we need to consider the entire molecule. Each molecule of CO₂ has 2 oxygen (O) atoms. The atomic masses of carbon and oxygen are 12 and 16, respectively, so the molecular mass of CO₂ is 12+2*(16)=44. Then take 1 g C * (44 g CO₂) / (12 g C) = 3.67 g CO₂ / 1 g C. That is, every gram (or pound, or other unit of weight or mass) of carbon in the tree corresponds to 3.67 g (or lbs, etc.) taken up from the atmosphere.

So, for a tree that weighs 2 tons while alive, the weight of CO₂ which were taken up to grow that large is approximately: (2 tons green weight) * (0.65 tons solid) / (1 ton green) * (0.5 tons C) / (1 ton solid) * (3.67 tons CO₂) / (1 ton C) = 2.39 tons CO₂.

*Calculation on page 4. Note that this page also includes a factor of 1.2 to account for subterranean biomass. I'm not sure why since the calculation uses mass of the tree, not height. My guess at this point is that "green mass" refers only to the above-ground portion of the tree.