March 29, 2024, 02:20:24 AM
Forum Rules: Read This Before Posting


Topic: Relative Error, in ppt  (Read 6751 times)

0 Members and 1 Guest are viewing this topic.

Offline LHM

  • Full Member
  • ****
  • Posts: 144
  • Mole Snacks: +1/-3
Relative Error, in ppt
« on: November 28, 2010, 12:33:53 PM »
The true percentage of Na2CO3 in a sample of soda ash is 40.24%. A 0.4134 g sample was analyzed by titration with HCl. The sample required 28.56 mL of 0.1106 M HCl for complete neutralization. The equivalent weight of Na2CO3 is 53.00. The relative error, in ppt (parts per thousand) would be

A) 13 ppt
B) 6.5 ppt
C) 6.4 ppt
D)12.8 ppt

I started it by doing 0.02856 L*0.1106 M HCl = 0.003313 mol HCl required to titrate the Na2CO3. And then I wrote the neutralization equation: Na2CO3 + 2HCl  :rarrow: 2NaCl + CO2 + H2O.

So there's 2 moles of HCl per mole of Na2CO3. So I divided the 0.003313 mol HCl by 2 to get 0.00166 mol Na2CO3. And then I did 0.4024 * 0.4134 g = 0.1664 g Na2CO3. 0.1664 g/0.00166 mol = 105.328 g/mol Na2CO3. They gave that the equivalent weight of Na2CO3 is 53.00, but that didn't give a reasonable answer, so I redid it and got that the molar mass of Na2CO3 is 105.99. and then I did (105.99-105.328)/105.99 * 100 = 6.24 ppt, which is close to B and C, but neither one of those. I realize that it could've been a rounding error, but if there was one, it would be closer to C) 6.4, whereas the answer is B) 6.5. So is there something I did wrong, or is it just like a really huge rounding error or something?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Relative Error, in ppt
« Reply #1 on: November 29, 2010, 04:17:41 AM »
Relative error of what?

Assuming it was relative error of the percentage I got 6.4 like you (6.377). If you are expected to use 53.00g I would not select other molar mass just to get better result, that's manipulating data.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline LHM

  • Full Member
  • ****
  • Posts: 144
  • Mole Snacks: +1/-3
Re: Relative Error, in ppt
« Reply #2 on: November 29, 2010, 06:13:54 PM »
I don't really know what it meant by relative of error. I just assumed that it was kind of like percent error, but thanks.

Sponsored Links