[cubejunkies]

So I'm doing a summer homework assignment for AP Chem and I took normal honors chem last year so I know terminology and things like that, but this question in the summer homework assignment for AP Chem that I've been working on in the review-ish section of the book is not making any sense. I have no idea where to start with this problem. The problem is as follows:

"A student is given a crucible and asked to prove whether it is made of pure platinum. She first weighs the crucible in air and then weighs it suspended in water (density = 0.9986 g/mL). The readings are 860.2 g and 820.2 g, respectively. Based on these measurements and given that the density of platinum is 21.45 g/cm^3, what should her conclusion be? (Hint: An object suspended in a fluid is bouyed up by the mass of the fluid displaced by the object. Neglect the bouyance of air.)"

WTF I have no idea where to start, and this seems like more of a physics question idk, but it just through me off, and its a chemistry course, so yeah...

Sorry if this is a really stupid question

Anthony

[Jorriss]

I'm not sure if this helps, but here's a hint.

An objects mass is actually more than it appears. An objects true mass is it's apparent mass (what you measure on a balance or by some other means) plus the mass of the displaced fluid.

So, the mass of the object measured in the air was the mass of the object, measured, + the mass of the displaced air.

And the mass of the object measured in the water was the mass of the object, measured, + the mass of the ...?


There's also a helpful assumption you can make about the density of air...

[Borek]

Yes, this is more like a physics question. So what? Some things are taught in physics course, some things are taught in the chemistry course, but whenever you face a real problem you never think in terms of "this is physics" or "this is chemistry" but in terms "what of what I know is applicable?".

Do you know Archimedes principle?

[opti384]

I think you should consider the given hint. First, let's assume the volume of the crucible as x.

As the crucible submerges into the water, it will push the same volume of water. The volume x of water is replace by the volume x of the crucible. I'm quite sure that this is the Archimedes principle.

Then we can find the volume of the crucible because we know the mass of the water displaced by the crucible which will be 40g.

Finally, with the volume and the mass of the crucible you can check out if the crucible is platinum or not.

[Donaldson Tan]

"A student is given a crucible and asked to prove whether it is made of pure platinum. She first weighs the crucible in air and then weighs it suspended in water (density = 0.9986 g/mL). The readings are 860.2 g and 820.2 g, respectively. Based on these measurements and given that the density of platinum is 21.45 g/cm^3, what should her conclusion be? (Hint: An object suspended in a fluid is bouyed up by the mass of the fluid displaced by the object. Neglect the bouyance of air.)"

Let v be the volume of the crucible. (unit: ml)
Let ρ be the density of the crucible. (unit: g/ml)

Mass of crucible = measurement of crucible's weight in air
=> ρ*v = 860.2g

Apparent mass of crucible in water = Weight of crucible - Weight of water displaced by crucible
820.2 = 860.2 - v*0.9986
v = (860.2 - 820.2) / 0.9986 = 40.05
ρ = 860.2 / 40.05 = 21.47 g/ml