Titrations questions and answers help Watch

ApplyYourself
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I've got a two part question below with my answer and the mark scheme answer below, I'd like it if somebody could explain the mark scheme answers as they've confused me.

Question 1

A volumetric flask contains a very precise amount of water. The amount the flask contains is 500 ml of water.

a) A student wants to make up a solution of sodium carbonate of concentration 0.1 mol/dm3. What mass of sodium carbonate needs to be added to the flask?

My answer to part a)

Moles = mass X concentration
Moles = 0.1 X 0.5 = 0.05 moles of sodium carbonate
Mass = moles X Mr
Mass = 0.05 X 106 = 5.3 grams of sodium carbonate needed.

Mark scheme answer to part a)

1 mole of sodium carbonate weighs 106 grams
106/10 = 10.6
10.6/2 = 5.3 grams of sodium carbonate needed.

Question 1 part b

If a sample of 10 cm3 is taken from the flask, how many moles of sodium carbonate will be in the sample?

My answer to part b)

Moles = concentration X volume
We know the concentration of sodium carbonate is 0.1 mol/dm3
Moles = 0.1 X 0.01 = 0.001 moles of sodium carbonate in the sample.

Mark scheme answer to part b)

Mass of sodium carbonate = 5.3 grams
Moles of sodium carbonate = 0.05 moles
Now divide moles by 50 so that 0.05/50 = 0.001 moles of sodium carbonate in the sample.



So my answers are right, however I've reached them in a different way from the mark scheme, can someone explain the mark scheme answers to me as it's left me totally confused :confused:
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Reality Check
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(Original post by ApplyYourself)
I've got a two part question below with my answer and the mark scheme answer below, I'd like it if somebody could explain the mark scheme answers as they've confused me.

Question 1

A volumetric flask contains a very precise amount of water. The amount the flask contains is 500 ml of water.

a) A student wants to make up a solution of sodium carbonate of concentration 0.1 mol/dm3. What mass of sodium carbonate needs to be added to the flask?

My answer to part a)

Moles = mass X concentration
Moles = 0.1 X 0.5 = 0.05 moles of sodium carbonate
Mass = moles X Mr
Mass = 0.05 X 106 = 5.3 grams of sodium carbonate needed.

Mark scheme answer to part a)

1 mole of sodium carbonate weighs 106 grams
106/10 = 10.6
10.6/2 = 5.3 grams of sodium carbonate needed.

Question 1 part b

If a sample of 10 cm3 is taken from the flask, how many moles of sodium carbonate will be in the sample?

My answer to part b)

Moles = concentration X volume
We know the concentration of sodium carbonate is 0.1 mol/dm3
Moles = 0.1 X 0.01 = 0.001 moles of sodium carbonate in the sample.

Mark scheme answer to part b)

Mass of sodium carbonate = 5.3 grams
Moles of sodium carbonate = 0.05 moles
Now divide moles by 50 so that 0.05/50 = 0.001 moles of sodium carbonate in the sample.



So my answers are right, however I've reached them in a different way from the mark scheme, can someone explain the mark scheme answers to me as it's left me totally confused :confused:
Mark scheme answers:

(a) The Mr of sodium carbonate is calculated from the elements in the formula *NB you haven't got this in your working or answer, so you would lose a mark here.
You're then asked how much you need for a 0.1M solution. Given that is a tenth of a 1M soltion, you can multiply the amount needed for 1M (106) by 0.1, which leads to 10.6g.
But, you've only got 500mL of water. Which is half what you need. You're asked what mass you need to make a 0.1M solution, which is 0.1M in a 1L = 1000mL. If you've only got 500mL, you only need half the amount - 10.6/2 = 5.3g.

Does that make sense?
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ApplyYourself
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(Original post by Reality Check)
Mark scheme answers:

(a) The Mr of sodium carbonate is calculated from the elements in the formula *NB you haven't got this in your working or answer, so you would lose a mark here.
You're then asked how much you need for a 0.1M solution. Given that is a tenth of a 1M soltion, you can multiply the amount needed for 1M (106) by 0.1, which leads to 10.6g.
But, you've only got 500mL of water. Which is half what you need. You're asked what mass you need to make a 0.1M solution, which is 0.1M in a 1L = 1000mL. If you've only got 500mL, you only need half the amount - 10.6/2 = 5.3g.

Does that make sense?
Wow you're fast, brilliant that clears it up for part a), any chance you could look at b)?

Oh and I did show how the Mr is calculated in my answer forgot to type it here.
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ApplyYourself
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(Original post by Reality Check)
Mark scheme answers:

(a) The Mr of sodium carbonate is calculated from the elements in the formula *NB you haven't got this in your working or answer, so you would lose a mark here.
You're then asked how much you need for a 0.1M solution. Given that is a tenth of a 1M soltion, you can multiply the amount needed for 1M (106) by 0.1, which leads to 10.6g.
But, you've only got 500mL of water. Which is half what you need. You're asked what mass you need to make a 0.1M solution, which is 0.1M in a 1L = 1000mL. If you've only got 500mL, you only need half the amount - 10.6/2 = 5.3g.

Does that make sense?
I get it (I think).. It's 1/50 of the original amount hence divide the original answer by 50?
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MexicanKeith
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(Original post by ApplyYourself)
Wow you're fast, brilliant that clears it up for part a), any chance you could look at b)?

Oh and I did show how the Mr is calculated in my answer forgot to type it here.
For part B
The mark scheme simply uses the number of moles that you worked out for the whole 500cm^3 solution (ie 0.05moles)
And then uses the fact that you are taking 10cm^3, which is 1/50th of the total solution so contains 1/50 * number of moles
1/50*0.05 = 0.001 moles
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ApplyYourself
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(Original post by MexicanKeith)
For part B
The mark scheme simply uses the number of moles of that you worked out for the whole 500cm^3 solution (ie 0.05moles)
And then uses the fact that you are taking 10cm^3, which is 1/50th of the total solution so contains 1/50 * number of moles
1/50*0.05 = 0.001 moles
Great I get it

Thanks to both of you, problem solved!!
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Reality Check
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[QUOTE=ApplyYourself;68614654]

Question 1 part b

If a sample of 10 cm3 is taken from the flask, how many moles of sodium carbonate will be in the sample?

My answer to part b)

Moles = concentration X volume
We know the concentration of sodium carbonate is 0.1 mol/dm3
Moles = 0.1 X 0.01 = 0.001 moles of sodium carbonate in the sample.

Mark scheme answer to part b)

Mass of sodium carbonate = 5.3 grams
Moles of sodium carbonate = 0.05 moles
Now divide moles by 50 so that 0.05/50 = 0.001 moles of sodium carbonate in the sample.
/QUOTE]

Oh, ok.

From your answer at (a), you've established that you've got 5.3g of sodium carbonate. As you know the Mr is 106g per mole, you can say you've got 5.3/106 =0.05 mol in the flask. You then take a 10mL aliquot (=sample) from the flask and need to calculate how many moles there are in that. 10mL of 500mL is 10/500 = 0.02 or a fiftieth. So you multiply the moles in the flask (0.05) by the proportion in the sample (0.02) to reach 0.001mol in the sample.
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Reality Check
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(Original post by ApplyYourself)
I get it (I think).. It's 1/50 of the original amount hence divide the original answer by 50?
Yes, you got it as I was answering you. Does my working make sense to you?
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ApplyYourself
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(Original post by Reality Check)
Yes, you got it as I was answering you. Does my working make sense to you?
It does :bhangra: Thanks

The mark scheme should have had a few lines to explain though
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