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Topic: Chemistry past paper questions  (Read 2368 times)

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Offline jazzyjeff26

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Chemistry past paper questions
« on: July 05, 2016, 04:17:50 AM »
have a couple of questions that I am struggling with, any help would be appreciated!

1. How many milligrams of calcium are in 100mL standard solution, when 10mL of this solution gave an equivalence point at 8.1mL of a 0.010 M EDTA solution?

2. An 801 mg sample containing sulfate was dissolved in water and then treated with an excess of barium chloride. The precipitate was washed and dried at 800°C to remove all traces of water. The precipitate was found to weigh 377 mg. What was the percentage sulfate in the sample?

Thank you :)

Offline Arkcon

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Re: Chemistry past paper questions
« Reply #1 on: July 05, 2016, 05:44:59 AM »
Greetings, jazzyjeff26:,and welcome to the Chemical Forums.  According to the Forum Rules{click}, we'd like to see the beginnings of your attempt.  Can you strt to show us what work you have done for these disparate questions?
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline jazzyjeff26

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Re: Chemistry past paper questions
« Reply #2 on: July 05, 2016, 06:09:54 AM »
Hi Arkon.
I start by dealing with all in mmol/mg/ml. and mole ration of Ca:EDTA is 1:1. so mmol of EDTA is (0.010mmol/ml x 8.1ml) = 0.081mmol.
mmol of Ca in 10ml is 0.081, so in 100ml 0.81mmol.
mass if ca in 100ml then is (0.81mmol x 40.08 mg/mmol) = 32.5mg.
Assuming this is actually correct, I don't feel I am following a proper 'applies to all formula' and would be confused if the question was posed differently.

for the second question: I think the temperature is throwing me off...is it relevant to the question? I think I would follow the lines of dividing the final mass by the molar mass and converting to percentage but I don't know where to fit both these weights in (I am sure both are relevant). I realise that is not a very good start...!

Offline AWK

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Re: Chemistry past paper questions
« Reply #3 on: July 05, 2016, 06:29:07 AM »
1. Two significant figures
2. Temperature is a part of analytical procedure. This inform you that barium sulfate is anhydrous and free from ammonium salts which are used in procedure and eventual filter paper is completely burned.
AWK

Offline jazzyjeff26

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Re: Chemistry past paper questions
« Reply #4 on: July 05, 2016, 09:04:23 AM »
So the first one is good other than significant figures? I still don't know how to do the second one...

Offline AWK

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Re: Chemistry past paper questions
« Reply #5 on: July 05, 2016, 12:35:50 PM »
2. This is simple stoichiometry followed by using of percentage definition.
AWK

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