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### Topic: General Energy Calculations  (Read 24874 times)

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#### jamesbrown

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##### Re: General Energy Calculations
« Reply #30 on: April 06, 2016, 05:37:12 PM »
Oh sorry man I thought you were saying that you couldn't do it because there wasn't enough information. You should have been more clear.
so density=mass/volume
so mass = volume*density
the volume is 100
the density is 0.2
so the mass = 20
is this correct?
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#### Borek

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##### Re: General Energy Calculations
« Reply #31 on: April 06, 2016, 06:14:07 PM »
Oh sorry man I thought you were saying that you couldn't do it because there wasn't enough information. You should have been more clear.

My question was perfectly clear from the very beginning.

Quote
the density is 0.2

No, 0.2 is not the density.
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#### jamesbrown

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##### Re: General Energy Calculations
« Reply #32 on: April 06, 2016, 06:21:36 PM »
So are we trying to find the density or the mass? if its the mass could you please help me out and tell me what the density is ?
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A: It went OK.

#### Burner

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##### Re: General Energy Calculations
« Reply #33 on: April 06, 2016, 09:12:23 PM »
Quote
then 100cm3  of 0.20 mol/dm3 copper sulphate solution was put in a calorimeter and 2.0g of magnesium power added. The temperature of the solution rose by 25.1 °C.

Number of moles of magnesium= 2/24.3 = 0.0823 mol
Number of moles of copper sulphate= 0.1*0.2= 0.02 mol

Magnesium is in excess.

If you know volume, and density, can you calculate mass of the solution?

@Borek

So we can just calculate the mass of the solution, neglecting the mass of undissolved magnesium and formed copper?
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#### Borek

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##### Re: General Energy Calculations
« Reply #34 on: April 07, 2016, 02:49:43 AM »
So are we trying to find the density or the mass? if its the mass could you please help me out and tell me what the density is ?

We are trying to find the mass, and you correctly stated what the density of the solution is at least twice in your earlier posts.

By definition, density is a mass of 1 mL of the substance. What is the mass of 1 mL of water?

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#### Borek

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##### Re: General Energy Calculations
« Reply #35 on: April 07, 2016, 02:51:54 AM »
So we can just calculate the mass of the solution, neglecting the mass of undissolved magnesium and formed copper?

To be precise we should take into account everything we put into the calorimeter (and you are perfectly right that magnesium mass needs to be taken into account), I am just trying to guide jamesbrown through the first step of the process.

Note that you are making the problem harder than it is. Because of mass conservation it doesn't matter what reactions took place, mass of the mixture after the reaction is exactly the same as it was before the reaction.
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#### jamesbrown

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##### Re: General Energy Calculations
« Reply #36 on: April 07, 2016, 06:27:55 AM »
What is the mass of 1 mL of water?
Is it 100g or 0.1kg?
Q: Did you hear oxygen went on a date with potassium?
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#### Borek

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##### Re: General Energy Calculations
« Reply #37 on: April 07, 2016, 09:20:29 AM »
What is the mass of 1 mL of water?
Is it 100g or 0.1kg?

Neither. And 100g and 0.1 kg is exactly the same number.

the mass of 1cm3 of water is 1g

And why do you guess, if you have already posted a correct number?
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#### jamesbrown

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##### Re: General Energy Calculations
« Reply #38 on: April 07, 2016, 09:49:59 AM »
Okay then btw I did know they were the same I just wasn't sure what unit you wanted it in?
Whta the next step then?
Q: Did you hear oxygen went on a date with potassium?
A: It went OK.

#### Burner

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##### Re: General Energy Calculations
« Reply #39 on: April 07, 2016, 11:14:19 AM »
So we can just calculate the mass of the solution, neglecting the mass of undissolved magnesium and formed copper?

To be precise we should take into account everything we put into the calorimeter (and you are perfectly right that magnesium mass needs to be taken into account), I am just trying to guide jamesbrown through the first step of the process.

Note that you are making the problem harder than it is. Because of mass conservation it doesn't matter what reactions took place, mass of the mixture after the reaction is exactly the same as it was before the reaction.

Thanks.

Okay then btw I did know they were the same I just wasn't sure what unit you wanted it in?
Whta the next step then?

You know that 1mL of water is 1g. So what is the mass of the 100mL solution(assume the solution's density is same as water)?
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

#### jamesbrown

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##### Re: General Energy Calculations
« Reply #40 on: April 07, 2016, 12:14:13 PM »
1ml of water is 1g so 100ml of the solution would have to be 100g
Q: Did you hear oxygen went on a date with potassium?
A: It went OK.

#### Burner

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##### Re: General Energy Calculations
« Reply #41 on: April 08, 2016, 01:55:35 AM »
Ok, you have mass of solution to be 100g. Now, what is the mass of the solids(magnesium and copper) formed? With these two numbers you can find out the total mass.

Hint:
Because of mass conservation it doesn't matter what reactions took place, mass of the mixture after the reaction is exactly the same as it was before the reaction.
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

#### jamesbrown

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##### Re: General Energy Calculations
« Reply #42 on: April 08, 2016, 10:46:47 AM »
Well it says 2g of magnesium was added. and im not sure but I think 100g was copper?
Q: Did you hear oxygen went on a date with potassium?
A: It went OK.

#### Burner

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##### Re: General Energy Calculations
« Reply #43 on: April 08, 2016, 11:24:47 AM »
100g is the mass of the solution not copper.

Borek have said that mass of the mixture after the reaction is exactly the same as it was before the reaction, so you only need to calculate the initial mass of the mixture, which is the mass of magnesium added and the mass of the solution. (Sorry for confusing you again with the mass of copper).
Year 1 science student in HKUST and a Chemistry geek.
If I make any mistakes in the forum, please don't hesitate to correct me as I want to learn.

#### jamesbrown

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##### Re: General Energy Calculations
« Reply #44 on: April 08, 2016, 11:32:39 AM »
Its all good so the overall initial mass is 102g right?
Q: Did you hear oxygen went on a date with potassium?
A: It went OK.