Homework Help: Chemistry

Posted by Zach on Sunday, October 21, 2012 at 9:35pm.

I titrated 0.25g of soda ash mixed with 25mL of deionized water and it took 43.7mL of hydrochloric acid to complete the titration.
I am trying to figure out how to find the moles of sodium carbonate in the soda ash and then the percent of sodium carbonate in the soda ash. How would I go about doing this calculation?

Chemistry - DrBob222, Sunday, October 21, 2012 at 10:05pm
It's important to know what you used as an indicator. If you used phenolphthalein you titrated to the HCO3^- end point. If you used methyl orange or methyl red you titrated to the H2CO3 (H2O + CO2) end point. I will assume that's the one you used.(I think M.R. is better than M.O. but I believe you want to change about pH of 5)
In addition you don't give the molarity of the HCl.

mols HCl = M x L = ?
mols Na2CO3 = 1/2 mols HCl(from the equation of
Na2CO3 + 2HCl ==> 2NaCl + H2O + CO2
g Na2CO3 = mols Na2CO3 x molar mass Na2CO3.
Then %Na2CO3 = (g Na2CO3/0.25g)*100 = ?
Chemistry - Zach, Sunday, October 21, 2012 at 10:18pm
I used bromophenol blue as the indicator, and the molarity of HCl is 0.2M.
Chemistry - DrBob222, Sunday, October 21, 2012 at 11:06pm
You titrated to the CO3^2- end point so the equation chemistry is what I indicated. I've never used that indicator. I always used methyl red and titrated until the indicator turned, heated to drive off CO2 (the m.r. turned back basic, and I titrated to the end point (much sharper end point the second time around). Since bromophenolblue turns at a more acid end point, you may not be required to heat the solution to drive off CO2.
Chemistry - Zach, Monday, October 22, 2012 at 9:27pm
I have done the calculation for this multiple times and I keep getting over 100% for the sodium carbonate in soda ash. I have no idea what I am doing wrong. I did it the way that you explained.

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