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help on volumetric calculations

hi, we were given these calculations to do, but i couldn't quite understand them. can some one please help me with these. thanks...

Volumetric Calculations...

A. Calculate the Concentration (moldm-3) of the given solution from the mass weighed out in the specified volume.

1. 10.6 g of Na2CO3 dissolved in 250 cm3 of solution.

B. This question contains the results from simple titrations. Calculate the the concentration (moldm-3) of the substance in brackets.

1. 25cm3 of Na2CO3 solution reacting with 50cm3 of 0.2 moldm-3 HCl solution ---- (Na2CO3)

2. 25 cm3 of 0.2 moldm-3 KOH solution reacting with 40cm3 H3PO4 solution ---- (H3PO4).

The questions below are different to the ones above...

1. 18g of some impure sulphric acid was diluted with deionised water to a volume of 1000cm3. 10cm3 of this diluted solution required 28.8 cm3 of 0.1 moldm-3 sodium hydroxide solution for neutralisation. Calculate the % purity by mass of the sulphric acid.

Thanks, please explain how to do these questions properly. Thanks a lot bye.

Reply 1

Number6

Just my luck - i type out a load of stuff and it gets moved :mad:

oh well - here i go again :smile:

...

A. Find the moles present in 10.6g of NaCO3.
Use this answer in the equation n=c x v but watch the units.

B. Write out a full balanced equation for each reaction, then write what you know beneath each reactant/product so you know what's going on. Calculate the moles of anything you can, then use the molar ratios from the equation to find the moles of the thing you want to know the conc of, then rearrange the above equation for conc.

C. Not sure what the first part's getting at but I'm going to assume they mean 18g/dm3 of impure H2SO4.
Write out the balanced equation with information below. Work out the moles of what you can. Use the molar ratios to work out the moles of the other reactant. Then use c = n/v to work out the conc. The sulphuric acid is impure so you can't just use the 18g from the q and convert that to mol/dm3. Anyway, you know the moles of sulphuric acid in the mixture so rearrange n=m/M to find the mass, then do a percentage. I think that's right anyway - it's been a long half term :grins:

Hope this helps - feel free to PM me :smile:

Reply 2

sohaibahmad

thanks a lot for your help, it really helps. bye