Help! Salters Chemistry Coursework

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LS.
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I'm given a sample of acid of concentration between 0.05 and 0.15 mol dm (-3) and asked to find its concentration

I'm gonna be titrating it with sodium carbonate - does the concentration of the sodium carbonate need to be roughly equal to the sample of acid - and if so, why?

Also, how do I find the amount of grams needed to make up a particular concentration?

Thanks very much
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chrisbphd
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(Original post by LS.)
I'm given a sample of acid of concentration between 0.05 and 0.15 mol dm (-3) and asked to find its concentration

I'm gonna be titrating it with sodium carbonate - does the concentration of the sodium carbonate need to be roughly equal to the sample of acid - and if so, why?

Also, how do I find the amount of grams needed to make up a particular concentration?

Thanks very much
You should endeavour to keep the concentration of sodium carbonate roughly equal to that of the acid because then you ought to have a rough idea of what the titre will be. If you have a 1:1 reaction, which I assume you do (does it say its a monoprotic or diprotic acid? - it should tell you) the titre should be roughly equal to the amount of the substance of known concentration you use, assuming the concs of both substances are roughly equal.

Now, I just have to pop out for a minute (its urgent) so I'll detail the mass stuff in half an hour or so.

CJB
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chrisbphd
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OK sprry for the delay

To get the concentration in grams per litre you just multiply the number of moles per litre (mol dm^-3) by the RMM of the substance,

so a 0.5mol dm^-3 solution of NaOH would require 0.5 x RMM = 0.5 x 40 = 20g of the solid in one litre of water. If you are using smaller amounts of water, just scale down accordingly, so a 250cm^3 0.5 M solution of NaOH would require 20 x (250/1000) grams = 5g.

So, a general formula for this:

Suppose you require a concentration of n moles per litre, and you require a volume of V. The RMM of the solid is r...

mass required = [(V in cm^3)/1000]nr

Easy!
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LS.
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Thanks for replying so quickly CJB

The acid I'm dealing with is sulphuric acid (yup, a 1:1 ratio). You're post has been incredibly useful to me, thank you so much for taking the time to write it ^_^

Also, would I need to include the variables that would affect experiment? If so, What kind of things should I include?
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chrisbphd
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You should include the variables that will affect the experiment...is your equipment clean, how accurately can you measure things, how many times will you repeat your results, how good is the indicator (do you guarantee to always stop exactly at the endpoint?).

PS - One tip...make sure you always use distilled water, it's surprising the amount of difference it can make to your results!
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LS.
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Wow, thanks for being so helpful, all this information is really helping me.

Another couple of questions...

- would I need to include any kind of hypothesis/prediction?

- Is the main focus of the coursework on the method used, and what makes it reliable?

- Do i need to include the chemistry of how an indicator works?

Also, in the introduction to the coursework, it mentions the situation that arises to cause an unknown quantity of acid, do I need to mention the situation in my coursework?
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