[julixtta]

Hello! I am trying to do the calculations for a lab report where I was shown the titration of washing soda using 0.145 M HCL. I know the gist of how to do them, but I am stuck on what numbers from the given data I should be using.

I know that I need to find the moles of HCl used from the 0.145M, but where do I get this information from? I know that after I multiply L by M to get moles, I have to multiple it by 1/2 because of the 2:1 HCl and Na2CO3 ratio, and then multiply that number by the molecular weight of Na2CO3 to get the mass in the mixture. I can then divide the calculated mass from the data mass and then multiply by 100% to get the percent.

Here's the data that was given (I couldn't do the lab myself due to COVID-19).

Flask #1
Mass of washing soda: 0.1554g
Initial buret reading: 0.20mL
Final buret reading: 15.18mL
Post heat initial buret reading: 15.18mL
Post heat final buret reading:15.31mL

Flask #2:
Mass of washing soda: 0.1514g
Initial buret reading: 15.33mL
Final buret reading: 29.88mL
Post heat initial buret reading: 45.29mL
Post heat final buret reading: 45.45mL

I know how to do titration calculations, but this is the first time where post-heat buret readings were given to us. How do I include this in my calculations? I've tried researching this, and I can't reach my teacher.

Thank you so much for any possible explanations!

[sjb]

Do you have the full procedure? Is it possible that the heat does something to the sample that makes it react differently?

[Borek]

Without knowing the procedure hard to tell anything.

In general

Initial buret reading: 0.20mL
Final buret reading: 15.18mL

these are the numbers you need to use. Do calculations using them.

"post heat" numbers don't make any sense to me. They are not about heating the sample, as they refer only to the final volume, not to the volume spent, plus in the case of the second  flask they refer to some random volume not related to anything.

[julixtta]

Here's the procedure. I was told the molarity of the HCl is 0.145 M.

1. Accurately mass roughly 0.15g of washing soda. Dissolve the washing soda in deionized water. Add 6 drops of bromocresol green indicator solution.
2. Begin titrating with the roughly 0.17M hydrochloric acid. When your solution turns green (or getting close to green) you will remove the excess carbonic acid generated by gently heating the solution on a hotplate. Allow the solution to gently boil for 3 minutes.
3. Allow the flask to air cool initially and then submerge it in an ice-bath to continue the cooling process. Wash down the inside walls of the flask with DI water to return any unreacted carbonate to the flask. The solution will likely be blue again once it cools.
4. Continue titrating the solution until the solution first begins to change from green to yellow- green.

The post-heat readings were after boiling the flask's solution. I was wondering if that had any contribution to the total volume? That's where I got confused. Thank you!!

[julixtta]

To do the calculation for flask #1, it would be then:
15.15 mL - 0.20 mL = 14.98 mL which is 0.01498 L

0.01498L * 0.145 M HCl = 0.0021721 mol for HCl
Due to the ratio, it becomes 0.0021721 x 1/2 =0.001086 mol for Na2CO3
0.001086 mol Na2CO3 x molecular weight of 106 g/mol of Na2CO3 = 0.115 grams Na2CO3
(0.115 g/0.1554 g from given data)*100% = 74%

For flask #2,
29.88mL - 15.33mL  = 14.55 mL which is 0.01455 L
0.01455 L * 0.145 M HCl = 0.00210975 mol HCl
0.00210975 mol HCl * 1/2 ratio = 0.00105 mol Na2CO3
0.00105 mol Na2CO3 * 106 g/mol Na2CO3 = 0.1118 grams Na2CO3
(0.1118 grams Na2CO3/0.1514g from given data) * 100% = 73.9%

Am I in the right spot for the calculations? Thank you for the help

[Borek]

Ah, ok, yes, boiling to get rid of carbon dioxide is a normal procedure. It still doesn't make everything obvious, as the data is inconsistent, but in this case for the calculation you should use the end point titrant volume after the heating. Dissolved carbon dioxide lowers the pH and in its presence titration ends a bit too early.

Other than that logic behind your calculations looks OK.

[julixtta]

So end-point titrant volume and the initial reading? For flask #1, I get around 75% so it's pretty close to the initial calculations I did. For flask #2, I got 153%... I could see why the number is high because the data is inconsistent. I'll try to ask my teacher tomorrow what to do about that.

For flask 2, this is the calculations with the end titrant volume post heating:
0.145 M * 0.03012 L = 0.0043674 mol HCl
0.0043674 mol HCl * 1/2 ratio = 0.0021837 mol Na2CO3
0.0021837 mol Na2CO3 * 106 grams = 0.2314722 grams
0.2314722 grams/0.1514 grams * 100% = 153%

Maybe I shouldn't be using the 0.145M HCl?? I know how to calculate the exact molarity of the HCl, which would change it to
(grams of washing soda / 106.0 g/mol of Na2CO3) * (2 mol HCl/1 mol Na2CO3) * (1000 mL/final mL of HCl used)
which would be:
0.1514g/106 g/mol * 2/1 * 1000/30.12 mL = 0.0948 M. If I substitute that in from the previous calculation, I get 100%.

Regardless, thank you for helping!