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Chemistry exam question

stud2014

Hie guys,

I have an exam tomorrow morning and desperately need help on the following question:

Describe how you would make up 250cm3 of a solution of sodium carbonate with an accurately known concentration between 0.05 and 0.06 mol dm-3

Please, please somebody help. This question does mot make any sense to me at all!

Reply 1

lozsmith

It sounds like there is a part missing to me, it really doesn't make sense.

Reply 2

Borek

Calculate mass of the sodium carbonate for both given concentrations. Then weigh a sample dried sodium carbonate* that is between these two masses - but weigh it exactly. Why this way? Because it is easier to exactly weigh a sample that is between 5 and 6 grams (random numbers, not related to what you will need!), than to weigh exactly some exact sample - like 5.0000 g. As long as you know your sample mass is between 5 and 6 grams and you know exactly how much (be it 5.0235 or 5.7653, or 5.3567, or whatever) you will know exactly how much you have.

*sodium carbonate is almost always hydrated, and exact composition of the hydrate depends on many factors, to make sure you know the composition of the solid you need to keep it for several hours in a drier, this way you end with the anhydrous salt.

Reply 3

shiva300

Original post by stud2014

mol x mr = mass
conc x vol = Mol

so 0.55 x 0.25dm3 = 0.173 mol

0.173 x 106 = 18.338g

sorry this might be wrong!
but you'd add water to 18.338g of sodium carbonate to make it up to 250cm3

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