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### Re: Density Lab B

Posted: **January 29th, 2019, 11:43 am**

by **John Richardsim**

Srama wrote:If I want to follow Guideline 3.B which states that answer should be in metric and approximate significant figures, Can anyone tell me how would you express the final answer for Boyles law problem.

Also, Is the answer expected to be adjusted to scientific notation for full credits?

For Eg:A container holds 2L of helium. When the pressure is reduced to 2.5 atmospheres and the volume in the container increases to 12L, what was the initial pressure exerted on the container?

Don't forget what 3.B. says before that: "Unless otherwise requested." Hopefully what units the answer should be in would be stated directly in the problem or on an answer sheet. If not, I'd recommend asking the proctor directly. If that doesn't work, I'd go ahead and just convert it to an SI or SI-derived unit (in the example you gave, you would convert the pressure in atmospheres to pascals (N/m^2)).

As for scientific notation, there doesn't seem to be anything in the rules addressing it, so you would only have to do it if explicitly instructed to do so on a test. There are a couple of cases, however, in which it would be a good idea to automatically put the answer in scientific notation even if not required to:

- If giving a very large or very small numerical answer (Example: The initial pressure would be 15 atmospheres, or 1.5x10^6 pascals, rather than writing 1,500,000 pascals)
- If you need to do so to express the correct number of sig figs (Example: if you need to write 300 with two significant figures, one typical way of doing this is instead writing it in scientific notation as 3.0 x10^2)

### Re: Density Lab B

Posted: **January 29th, 2019, 1:03 pm**

by **Srama**

Much appreciate your response.

I specifically choose the above example, since the lowest sig fig is "1". That case, how do you express your final answer?

2. for density, answers could be in any metric units ? gm/cm3 ? or should it be in kg/m3

3. Pascal or Kilopascal should be good for pressure, right?

I understand "unless specified " clause.

### Re: Density Lab B

Posted: **January 29th, 2019, 2:45 pm**

by **John Richardsim**

Srama wrote:Much appreciate your response.

I specifically choose the above example, since the lowest sig fig is "1". That case, how do you express your final answer?

2. for density, answers could be in any metric units ? gm/cm3 ? or should it be in kg/m3

3. Pascal or Kilopascal should be good for pressure, right?

I understand "unless specified " clause.

1. Oh, yeah, you're right. The answer will have only 1 sig fig. In this case, we will need use the rounding rules in the SO sig fig policy:

https://www.soinc.org/sites/default/fil ... 2-5-12.pdf (SO's rules for rounding 5 either up or down are kinda weird).

So because the first nonsignificant digit is a 5, we need to round the number to the left to make it an even number. So 1.5x10^6 Pa ---> 2x10^6 Pa. (Again, since the rounding rules for 5 are strange, be aware that had the calculated answer been 2.5x10^6 Pa, it would also have rounded to 2x10^6 Pa.)

2. Yeah, I'd think they could be any metric units.

3. Both should be good.

### Re: Density Lab B

Posted: **January 29th, 2019, 2:55 pm**

by **Srama**

Awesome!! Thanks a lot

### Re: Density Lab B

Posted: **February 11th, 2019, 1:00 pm**

by **RockRoll92**

Alright, I may be doing this at states because no one else will. Is there anywhere in particular I should start? LIke looking things up or looking at practice tests and basing things off there? The rules are pretty vague, for density of solid, liquids, and gasses for example should we know the density of each material at room temperature?

### Re: Density Lab B

Posted: **February 11th, 2019, 1:39 pm**

by **Crimesolver**

Rock&Roll92 wrote:Alright, I may be doing this at states because no one else will. Is there anywhere in particular I should start? LIke looking things up or looking at practice tests and basing things off there? The rules are pretty vague, for density of solid, liquids, and gasses for example should we know the density of each material at room temperature?

I help my friend a lot for this event, so here the basics you should probably understand. You definitely need ideal gas laws, so find those equations. You also need Archimedes' principle, the basis for buoyancy. For the density of solids, liquids, and gases, all you really need to worry about is just getting those measurements. Most solids, liquids, and gases are taken at STP, standard temp. and pressure, but things like ice and steam can't be since they are temperature and pressure dependent. My friend has trouble understanding everything, which results in her not being able to apply all this, so make sure you understand each and every and law and equation. There's a bunch more you probably need to know, but you'll eventually figure it out through practice test.

### Re: Density Lab B

Posted: **February 19th, 2019, 10:00 am**

by **coopsadoodles**

for those of u who went to cornell, what did u place in density and what were your raw scores? just curious i got 11th with a 92.5. (we screwed up the units for the lab lol)

### Re: Density Lab B

Posted: **February 24th, 2019, 12:22 pm**

by **PolarFrost**

How do you solve the question on test exchange?

A rubber duck is placed in a liquid and floats, but it is not water. The buoyant force on the duck is measured to be

0.423 N, the acceleration due to gravity is 9.8 m/s2

, and the volume of liquid that the duck displaced is

approximately 30 cm3

. What is the density of the liquid the duck was placed in, in kg/m3

?

It says answer is 1440 kg/m^3 but we don't know why?

PLease Help

### Re: Density Lab B

Posted: **February 25th, 2019, 5:19 pm**

by **Chin**

Buoyant force = Volume of water displaced X density of liquid X g

0.423 = 30 X 10-6 (converted to m3) X d X 9.8

d = 1440Kg / m3

### Re: Density Lab B

Posted: **February 25th, 2019, 5:22 pm**

by **Chin**

Re: Density Lab B

by Chin ยป 1 minute ago

Buoyant force = Volume of water displaced X density of liquid X g

0.423 = 30 X 10-6 (converted to m3) X d X 9.8

d = 1440Kg / m3