Back titration

Pat24098 SG

Hi,

Can anyone please help?

May I know what is wrong when I get the mass of CaCO3 in dry powder higher than the mass of original sample that I put in?

I reacted 1.068g of samples with 50 ml of 1M HCl solution. The titre value of 0.1M NaOH was 273.1 ml.

Hence, I calculated the amount of CaCO3 in my sample as 1.1345g, which is higher than the original sample.

May I know what is wrong here?

Thanks and desperately waiting for reply.

Rgds,

Pat

UltimaOnline SG

Did you take into consideration that the carbonate(IV) ion is diprotic? (ie. Each CO3 2- ion accepts 2 H+ ions).

HCl and NaOH (which reacted with the excess HCl) are both monoprotic, of course.

Check your sequence of steps :

Find mol of NaOH required for the back titration. Hence find mol of HCl that reacted with the CaCO3 (ie. total HCl - HCl that reacted with NaOH).

Then, divide this value by 2 (since carbonate(IV) is diprotic) to find mol of CaCO3 present initially. Next, multiply this value by the molar mass (or Mr) of CaCO3 to find the actual sample mass of CaCO3 present initially.

Divide this value by the 1.068g impure sample, to find percentage (by mass) purity of the CaCO3.

If you still get the same answer, it means the question is flawed, eg. typo error in the question. The titre value of 273.1 ml for the NaOH seems ridiculously large (imagine the size of the burette required). It should more reasonably be 27.31 ml.