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Topic: Density Questions - please help.  (Read 10486 times)

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Offline rabolisk

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Re: Density Questions - please help.
« Reply #15 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).

Offline lokobreed

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Re: Density Questions - please help.
« Reply #16 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

Offline rabolisk

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Re: Density Questions - please help.
« Reply #17 on: February 05, 2011, 03:17:20 PM »
That's right. Also, volume of container = volume of unknown liquid (justify this to yourself).

Offline lokobreed

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Re: Density Questions - please help.
« Reply #18 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?

Offline rabolisk

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Re: Density Questions - please help.
« Reply #19 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 = MassH2O / densityH2O
Masscontainer + massH2O = total mass
MassH2O = total mass - masscontainer
VolumeH2O  = (total mass - masscontainer) / densityH2O
Volumecontainer = volumeH2O = (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 = mcontainer + mliquid
Vliquid = Vcontainer = 24.872 cm3
ρliquid = mliquid / Vliquid = (total m - mcontainer) / (Vliquid) = (88.42 - 78.91g)/(24.872cm3) = 0.3824 g/cm3 (4 sig figs)

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

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