Density of an object that floats.

[qwertysaur]

For this weeks lab in chemistry class, I need to describe a way to find the density of an irregular piece of wood. How could I do this?

[Vivisteiner]

Measure the displacement it causes when dropped in water (the volume of water displaced). Then measure the mass of the wood.

Divide the mass of the wood by the volume and voila, you have density.

[qwertysaur]

but not all of the wood displaces water.

[Tavrobel]

Density is mass over volume, as you should know.

You should have the mass as given, or calculated (probably a balance). As for volume, measure the amount of water displaced, and multiply that with the percentage of the object submerged in water. If your density is > 1.00 g/cm³ (or mL), then you know you've done it wrong. Alternatively, you could just check what your friends have. One of them is bound to be right.

Although, if I were you, I would just figure out the chemical composition of the wood, and do a bunch of stochiometry. But that approach isn't for everyone, since I doubt that you've had AP Chem.

[Vivisteiner]

but not all of the wood displaces water.
What do you mean?

Just dunk one hundred percent of it under water. Measure the displaced volume of water. Then divide the mass of the wood by that volume. Its really not that hard.

(The water absorbed by wood should be negligible - If you want, you can cover it with something and work out the volume displaced by the cover and then minus that from the total displaced volume. But thats only if you have to be very accurate)

[qwertysaur]

Thank you, I think I will use Tavrobels meathod though, since Vivisteiner's meathod has too many sources for error.

[Little Blue]

If its an irregular object, I doubt there's any easy way to estimate the submerged percentage, that method would be easiest with something displaying some kind of symmetry.

You could just poke it down with a pin or something. As long as the volume of the poky thing is negligible compared to the volume of the chunk of wood, it should have very little impact, plus it would give something to discuss in a conclusion.

Alternatively, weigh it down with weights of a known volume, then subtract the volume of the weights from the volume of displaced water and there you go. I would prefer that method to poking or estimating.

[Vivisteiner]

@Qwerty: Lol, my method is exactly the same as Tavrobel's except I didnt go into detail.

Unless if you mean you're going to do the stochiometry.

What I was just saying is to make the percentage Tavrobel mentioned as 100% otherwise it just complicates the matter pointlessly. Just get a lot of water and submerge it all in that or do LB's method. Obviously youve gotta measure stuff accurately using mass balances and perhaps a burette or measuring cylinder type thing.

[blackmage_nuke]

The water thing wouldnt work if there was an empty chamber inside the object with no entrance

[Vivisteiner]

^Yes it would, wouldnt it?

Why wouldnt it work?

[Little Blue]

You'd have a sealed air cavity inside that water would'nt flow into. Therefore more water would be displaced than there should be, which gives an oversized volume measurement, and would result in an underestimate of the density.

[Vivisteiner]

^Yeah, but the density of the overall thing would still be accurate.

But I see what you mean - If you want to find out the density of that type of wood and not just the density of the object, then it wouldnt work. Its like a steel boat. The steel is more dense than water, but the overall density of the boat is less dense than water.

[Little Blue]

Yup. I guess it depends on the aims of qwertyxsora's experiment then, but I'd imagine it'll probably be assumed to be a solid lump of wood with a uniform mass distribution inside.

[qwertysaur]

I was referring to the stochiometry in Tavrobels awnser, sorry if I confused you a little. It's a lab that is supposed to connect the chemistry and botany classes. But I don't take Botany so I don't get to do the Botany half of the lab. The Chem half of the lab is determining several properties of different wood samples. My friend who takes Botany says that they then take those findings and determine why those properties were favored for natural selection.

[Tavrobel]

You know there's like, 3000 different types of wood, right? Unless your friend has the type as a given.

No euphemism intended.

[qwertysaur]

No, it is a structured lab. Were are comparing wood from four different species of tree, and then we get a sample from an unknown tree and must find which tree it is the most similar to. Then the botany half I was told that they will investigate why specific traits developed, and which known sample was the closest to the unknown, and then they create a cladogram with the five wood samples.

[Woodinator]

Well if you want to know how much water is abosrbed by the wood, take the mass before it is submerged and after...subtract the masses and you have the mass of water and then since at standart temp and pressure pure water is 1g/ ml you could multiply that out to find ml of water in the wood...

If i were you though, I'd just do simple water displacement though, you're looking at small error values that are inevitable any way you do it, you're probably just over-thinking :P

[Chemical]

I remember doing this...

MASS:
When an object floats, the mass of the water it displaces is equal to its own mass.
Note: The density of water is 1 g/cm3.

Mass of Water
= D x V
=1 g/cm3 x Volume displaced (1mL = 1 cm3)
= Mass of object


VOLUME:
When an object SINKS, the volume of the water it displaces is equal to its own volume. (When two objects are equal in size, this is also the volume of the object that floats)

You have M and V, substitute to find D