calculating percent by weight?

Question

I want to determine the concentration of sodium carbonate and sodium hydrogen carbonate by titrating an unknown sample with HCl. I only have the mass of the sample and the volume of hcl used for titrating it with phenolphthalein and methyl orange. only with these data at hand, how will i be able to obtain the percent by weight?

Answer 1

1) First, as you know the volume of HCl used for titrating the sample, but surely you know also the concentration of HCl (we assume molarity) you apply:

moles HCl = C*V1

where: C=concentration of HCl
............V1 = volume of HCl to titrate Carbonate with phenolphtaleina
............V2 = volume of HCl to titrate Bicarbonate with Methyl Orange (recall that this second volume is obtained reading the final volume and susbtracting the first V1)

2) In any equilibria reaction the moles titrated are equal to moles of titrating agent hence:

moles of HCl in volume 1 = moles Na2CO3
moles of HCl in volume 2= moles of NaHCO3

3) Convert moles to grams using Molecular mass of each compound

4) If you know the mass of sample in order to compute the % by weight apply:

%(w/w) Na2CO3 = mass Na2CO3/mass of sample
%(w/w) NaHCO3 = mass NaHCO3/mass of sample

That´s it!

Good luck!

Answer 2

Please give the whole data!

First of all the HCl should be standardized.
Knowning HCl concentration proceed.

First use phenolphthalein as indicator.
Its colour change occurs during the first neutralization ONLY.
pink to colourless chang eoccurs
At the end point all Na2CO3 is converted to NaHCO3
From this reading calculate concentration of Na2CO3

Then to this solution add methyl orange.
titrate as usual till color change is obtained
End point corresponds to completion of reaction of NaHCO3 with HCl
Determine NaHCO3 from this reading.
Subtract Na2CO3 conc from it to get actual NaHCO3