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### Re: Density Lab B

Posted: **January 13th, 2020, 6:24 pm**

by **theprimegrinder**

Umm....I guess the wiki is a good place to start if u just began....then expand further based on practice tests u take...For example, the wiki will cover most things on the rules, like gas laws, but if u don' t understand it, search the topic and watch some videos on it. And for hands on portions, there are some good possibilities on this forum on previous pages

Other info u need to know which is not on the rules, is anything associated with STP(standard temp. and pressure) any unit conversions, whether it be units of mass & volume(g, kg, mL, L) or pressure (atm, torr, mm Hg) and molarity/molality.

U will also need a deep understanding of Ideal Gas Law Applications and Archimedes Principle/Bouyant Force.

Im not an expert, not anywhere near lol, but just trying to help. Hope it does!

EDIT: ASK ppl who have been in SciOly for ur school & have been doing the event longer for tips.

EDIT 2: also know how to calculate sig figs

### Re: Density Lab B

Posted: **January 30th, 2020, 6:45 am**

by **azboy1910**

Hi,

I've been looking for a density lab test for quite some time now. I have done all the tests on scioly.org and sciencenc.com. The only test I have not done yet is the one posted in this forum. If you do have a test to share in this forum, if you wouldn't mind, please do.

Thank you so much!

- azboy1910

### Re: Density Lab B

Posted: **February 3rd, 2020, 7:04 pm**

by **theprimegrinder**

https://www.sciencenc.com/resources/mid ... l/density/
This link has two tests

And there is a pretty basic test on the wiki

### Re: Density Lab B

Posted: **February 14th, 2020, 3:55 pm**

by **azboy1910**

Thanks for the resources, but I already took both of the tests on that website.

It's ok though, because I already found a bunch of helpful tests for Density Lab.

Thank you for trying to help though. Really appreciate it.

### Re: Density Lab B

Posted: **February 15th, 2020, 9:26 am**

by **theprimegrinder**

loll.....sorry!

### Re: Density Lab B

Posted: **February 19th, 2020, 3:22 pm**

by **astronomybuff**

So I was taking a practice test, and there were a couple questions I had trouble with. This is my first time doing Density, so I'm a noob. Please don't just tell me the answer. If you can, show a step by step process or a formula.

A spherical ball with density 0.70 kg/L has a radius of 10 cm. If the ball is placed on the surface of water, how much of it will become submerged?

If a person's body has a density of 995 kg/m^3, what percentage of the body will be submerged gently in a) freshwater, and b) saltwater with a density of 1027 kg/m^3.

Aluminum metal reacts with HCl to produce aluminum chloride and hydrogen gas. How many grams of aluminum metal must be added to an excess of HCl to produce 33.6 liters of hydrogen gas, if the gas is at STP?

Thanks in advance!!

### Re: Density Lab B

Posted: **February 19th, 2020, 5:03 pm**

by **NewSciolyer**

So I was taking a practice test, and there were a couple questions I had trouble with. This is my first time doing Density, so I'm a noob. Please don't just tell me the answer. If you can, show a step by step process or a formula.

A spherical ball with density 0.70 kg/L has a radius of 10 cm. If the ball is placed on the surface of water, how much of it will become submerged?

If a person's body has a density of 995 kg/m^3, what percentage of the body will be submerged gently in a) freshwater, and b) saltwater with a density of 1027 kg/m^3.

Aluminum metal reacts with HCl to produce aluminum chloride and hydrogen gas. How many grams of aluminum metal must be added to an excess of HCl to produce 33.6 liters of hydrogen gas, if the gas is at STP?

I am not a huge expert, but I'll do what I can.

For the spherical ball question, the object will sink until the amount of water displaced is equal to the object's weight, hence Archimedes principle. Since the ball is less dense than water, which is 1 kg/L, the volume of the water displaced would be less than the object's volume. You would take the ball's density 0.70 kg/L and divde it by water's density 1 kg/L and you get .7, meaning 70% of the object will be submerged.

For the second question, the same principle applies. Water has a density of 1000 kg/m^3, so 995 kg/m^3 divided by 1000 kg/m^3 and you get .995, which is 95% of the body will be submerged in freshwater. In saltwater, we use the same principles again, so 995 kg/m^3 divided by 1027 kg/m^3 and you get approxiately 0.9688, which is 96.88% submerged in saltwater.

Sorry if I dissapoint you, but I only have a vague understanding of the 3rd question. You have to understand the chemical reaction taking place using a periodic table. I'm not the best person to ask that kind of question, and I hope someone will help you and help me understand it more as well. Hope at least some of it helps.

Cheers

### Re: Density Lab B

Posted: **February 20th, 2020, 11:39 am**

by **astronomybuff**

NewSciolyer wrote: â†‘February 19th, 2020, 5:03 pm

So I was taking a practice test, and there were a couple questions I had trouble with. This is my first time doing Density, so I'm a noob. Please don't just tell me the answer. If you can, show a step by step process or a formula.

A spherical ball with density 0.70 kg/L has a radius of 10 cm. If the ball is placed on the surface of water, how much of it will become submerged?

If a person's body has a density of 995 kg/m^3, what percentage of the body will be submerged gently in a) freshwater, and b) saltwater with a density of 1027 kg/m^3.

Aluminum metal reacts with HCl to produce aluminum chloride and hydrogen gas. How many grams of aluminum metal must be added to an excess of HCl to produce 33.6 liters of hydrogen gas, if the gas is at STP?

I am not a huge expert, but I'll do what I can.

For the spherical ball question, the object will sink until the amount of water displaced is equal to the object's weight, hence Archimedes principle. Since the ball is less dense than water, which is 1 kg/L, the volume of the water displaced would be less than the object's volume. You would take the ball's density 0.70 kg/L and divde it by water's density 1 kg/L and you get .7, meaning 70% of the object will be submerged.

For the second question, the same principle applies. Water has a density of 1000 kg/m^3, so 995 kg/m^3 divided by 1000 kg/m^3 and you get .995, which is 95% of the body will be submerged in freshwater. In saltwater, we use the same principles again, so 995 kg/m^3 divided by 1027 kg/m^3 and you get approxiately 0.9688, which is 96.88% submerged in saltwater.

Sorry if I dissapoint you, but I only have a vague understanding of the 3rd question. You have to understand the chemical reaction taking place using a periodic table. I'm not the best person to ask that kind of question, and I hope someone will help you and help me understand it more as well. Hope at least some of it helps.

Cheers

First off, thank you so much for responding! I realized I was a big dummy on number 2. I was overthinking it. For number 1, the correct answer is 2.93 L, but I understand how you got 70%. If you know how to get that, please tell me. It's fine that you don't really understand number 3.

Once again, thanks for answering!

If anybody else knows, could they please explain?

### Re: Density Lab B

Posted: **February 20th, 2020, 11:55 am**

by **knightmoves**

astronomybuff wrote: â†‘February 20th, 2020, 11:39 am

First off, thank you so much for responding! I realized I was a big dummy on number 2. I was overthinking it. For number 1, the correct answer is 2.93 L, but I understand how you got 70%. If you know how to get that, please tell me.

It's the same answer. Your sphere has radius 10cm (=1dm) and so has volume (4/3)*pi dm^3 = (4/3)*pi L.

70% of 4/3 pi is 2.93.

But note that 2.93 L is not the correct answer. The question gave the radius of the sphere and the density to probably 2 significant figures (might be 1 for the radius, but let's say 2). The correct answer is therefore given to 2 significant figures, and is 2.9 L.

### Re: Density Lab B

Posted: **February 20th, 2020, 12:05 pm**

by **knightmoves**

astronomybuff wrote: â†‘February 19th, 2020, 3:22 pm

Aluminum metal reacts with HCl to produce aluminum chloride and hydrogen gas. How many grams of aluminum metal must be added to an excess of HCl to produce 33.6 liters of hydrogen gas, if the gas is at STP?

Hopefully you were given more information than that - you shouldn't be expected to know chemistry for Density Lab.

When aluminum reacts with hydrochloric acid, the balanced equation is

2Al+6HClâ†’2AlCl_3+3H_2 (underscores for subscripts)

1 mol of an ideal gas occupies 22.4 L volume at STP if you're pre-1982, and 22.7 L post-1982 (IUPAC changed the definition of STP in 1982).

So 33.6L of H_2 gas is probably 1.5 mol H_2 gas, because the question setter probably used the pre-1982 definition, because that makes round numbers.

You had an excess of HCl, so you look at the equation above and see 2 units of Al produces 3 units of H_2 gas, so 1 mol of Al will produce 1.5 mol H_2 gas.

The atomic mass of aluminum is 26.981539 u, so one mol of Al has mass 27.0g (3 sig figs, because we have 3 sig figs everywhere in our data).