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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: lokobreed on February 02, 2011, 05:33:04 PM

Title: Density Questions - please help.
Post by: lokobreed on February 02, 2011, 05:33:04 PM: I need help with these questions they are concerning density. This is general chemistry
. If you could help me with how to do these problems.

A rectangular block of lead has a mass of 175.3 g calculate the height of the block if the width is 2.25 cm and that length is 3.15 cm

An iron ore weighing 83.245 g was placed in a flask having a volume of 54.25 ml. The flask with the iron ore was then filled with water and weighed. The iron ore and water weighed 124.481 g. The density of water is 0.9976 G/ml. Calculate the density of the iron ore.

Thank you
Title: Re: Density Questions - please help.
Post by: lokobreed on February 02, 2011, 06:39:46 PM: For the first one I am completely lost.
For the iron ore when I take 124 0.481 g divided by 0.99726 G ML and subtract that from 70.530473 and come up with the answer of the density for the iron ore as 1.18?
Title: Re: Density Questions - please help.
Post by: rabolisk on February 02, 2011, 06:58:18 PM: Density is mass/volume. It relates mass of a substance to the volume of a substance. In other words, density tells you how much volume a given mass of substance will occupy. E.g. You can figure out how much volume 1 g of water occupies if you know it's density.

A formula for a volume of a solid is length x width x height (which you should know from geometry).

Where did you get 70.53 from?? You are not doing this right.
Title: Re: Density Questions - please help.
Post by: lokobreed on February 02, 2011, 07:41:23 PM: yes i feel lost right now ;/ and studying intensely.

ok, so for the block of lead if i found the volume of the block of lead to be 15.5 because the mass is 175.3 g and the density (from given in the problem) is 11.3 g/cm3 then the height would be 2.19 cm correct?

For the iron ore i got the 70.53 from dividing the weight of the ore be density of water... which i see is wrong completely. any tips on this ?
Title: Re: Density Questions - please help.
Post by: rabolisk on February 02, 2011, 08:41:55 PM: Correct.

You have iron ore with known mass but unknown volume and unknown density. You have water with known density but unknown mass and unknown volume. You HHS a flask whose volume is known.

In this problem, both mass and volume are additive. That is, mass(ore + water) = mass(ore) + mass(water). Same idea with volume. Once you figure out the role of volume(flask) you should be able to solve this.
Title: Re: Density Questions - please help.
Post by: lokobreed on February 02, 2011, 09:14:49 PM: thank you so much for the *delete me* I am still confused on this problem... I am thinkinh I need to find out the volume difference with the ore and without and then divide that to find the density of the ore?

If this is correcT?
Title: Re: Density Questions - please help.
Post by: rabolisk on February 02, 2011, 09:19:16 PM: Before you can find the density of ore, you need to find the volume of ore, which is volume of flask - volume of water. To find the volume of water, you need to use more information that is given in the problem.
Title: Re: Density Questions - please help.
Post by: lokobreed on February 02, 2011, 10:33:47 PM: Thanks again!

Ok I think I figured it out.

Density of ore = 83.245g/V of ore

Volume of flask - Volume of water is 54.25 (Volume of Flask) - 41.34 (Volume of Water)

I got the volume of water by doing setting the problem up as .9976 (density of water) = 41.236 (124.481 - 83.245)/ V
V = 41.34

So then the volume of flask - volume of water is 12.91 so the density of the ore is 83.245/12.91 = 6.448
Title: Re: Density Questions - please help.
Post by: rabolisk on February 02, 2011, 10:59:19 PM: That's right. Now can I get some mole snacks?? ;)
Title: Re: Density Questions - please help.
Post by: lokobreed on February 03, 2011, 12:49:24 AM: thank you so much!

Mole snacks given :)
Title: Re: Density Questions - please help.
Post by: lokobreed on February 04, 2011, 07:35:01 PM: Can anyone please help with this problem.
A container has a mass of 78.91 g when empty and 103.782 g one filled with water. The density of water is 1.0000 g/cm3. Calculate the volume of the container. When filled with an unknown liquid, the container had a mass of 88.42 g. Calculate the density of the unknown liquid.
I have attempted this problem but continue to become confused on how to set this up.
Title: Re: Density Questions - please help.
Post by: rabolisk on February 04, 2011, 09:02:19 PM: This is very similar to the one with the ore. One hint, if you were given a cup, and you filled it all the way with x volume of water, then what is the volume of the cup?
Title: Re: Density Questions - please help.
Post by: lokobreed on February 05, 2011, 01:03:39 PM: as you can tell chemistry is not my strong point but I am determined to learn it.

Ok well I would need to know the cups mass correct because V=M/D

So for this problem to caluclate the volume of the container would I take the Mass of the container with liquid minus the mass of the container with out the liquid - 103.72 - 78.91 and divide it by density of water 1.000 g/cm3?

and to find the density of the unkown liquid would I use the mass of the container and divide it by the volume found above?
Title: Re: Density Questions - please help.
Post by: rabolisk on February 05, 2011, 02:07:19 PM: Yes on the volume of the container, but no on the second one. To find density of x, you need mass of x and volume of x. Mass of the container is not the same thing as mass of the unknown liquid.
Title: Re: Density Questions - please help.
Post by: lokobreed on February 05, 2011, 02:25:58 PM: im giving you more mole snacks! lol

Ok so to find the volume i would do with liquid - w/o liquid divided by the density of liquid? correct?
Since Mass of X is given in the problem as 88.42, how do i find the volume of X ?

Thanks again
Title: Re: Density Questions - please help.
Post by: rabolisk on February 05, 2011, 02:55:10 PM: The mass of the unknown liquid is not 88.42g, although you will need that value (88.42) to find the mass of the unknown liquid. For the volume of the unknown liquid, again, if you have a cup with a certain volume, and you fill it with some liquid, then their volumes have to be the same. After all, in the real world, this is how you would measure volume of a liquid, using a container with a known volume (like a bucket that can hold a gallon, or a graduated cylinder with marks for volume).
Title: Re: Density Questions - please help.
Post by: lokobreed on February 05, 2011, 03:12:09 PM: ok so would i take mass of container with liquid 88.42 - mass of empty container 78.91?

im sorry as similaar as these equations are im so confused on this one
Title: Re: Density Questions - please help.
Post by: rabolisk on February 05, 2011, 03:17:20 PM: That's right. Also, volume of container = volume of unknown liquid (justify this to yourself).
Title: Re: Density Questions - please help.
Post by: lokobreed on February 05, 2011, 03:29:17 PM: ok so since the density of water is 1.0000 g/cm^3 then the volume of the container of a mass of 78.91 g when empty would be 78.91cm^3 right?

So the mass of container fill with liquid would have a volume of 9.51 g/cm^3?
Title: Re: Density Questions - please help.
Post by: rabolisk on February 05, 2011, 04:03:13 PM: No. I'm going to solve this problem for you so you can see what I'm doing.

A container has a mass of 78.91 g when empty and 103.782 g one filled with water. The density of water is 1.0000 g/cm3. Calculate the volume of the container. When filled with an unknown liquid, the container had a mass of 88.42 g. Calculate the density of the unknown liquid.

Part 1
Volume of the container = volume of water (when water fills up the container)
Volume of water = Mass_H₂O / density_H₂O
Mass_container + mass_H₂O = total mass
Mass_H₂O = total mass - mass_container
Volume_H₂O = (total mass - mass_container) / density_H₂O
Volume_container = volume_H₂O = (103.782 - 78.91g)/(1.0000g/cm³) = 24.872 cm³ = 24.87 cm³ (4 sig figs)

From now on, I'm going to use the Greek letter rho (ρ) to denote density, m to denote mass, and V to denote volume.

Part 2
Total m = m_container + m_liquid
V_liquid = V_container = 24.872 cm³
ρ_liquid = m_liquid / V_liquid = (total m - m_container) / (V_liquid) = (88.42 - 78.91g)/(24.872cm³) = 0.3824 g/cm³ (4 sig figs)

Note that I used variables (e.g. V_container) rather than the actual values until the very end. This minimizes any calculation error, and rounding error.