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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: jazzyjeff26 on July 05, 2016, 04:17:50 AM

Title: Chemistry past paper questions
Post by: jazzyjeff26 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 :)

Title: Re: Chemistry past paper questions
Post by: Arkcon on July 05, 2016, 05:44:59 AM

Greetings, jazzyjeff26:,and welcome to the Chemical Forums.  According to the Forum Rules{click} (http://www.chemicalforums.com/index.php?topic=65859.0), 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?

Title: Re: Chemistry past paper questions
Post by: jazzyjeff26 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...!

Title: Re: Chemistry past paper questions
Post by: AWK 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.

Title: Re: Chemistry past paper questions
Post by: jazzyjeff26 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...

Title: Re: Chemistry past paper questions
Post by: AWK on July 05, 2016, 12:35:50 PM

2. This is simple stoichiometry followed by using of percentage definition.