Calculations for back titrations

[chemkid_23]

I'm a bit lost with this sac at the moment. We performed a back titration. We had 0.1M of HCl and 0.1M of NaOH. Titres were 6.15, 6.65 and 6.20 with average 6.3.

I worked out the first equation NH4+(aq) + NaOH (aq) --> NH3 (aq) + H2O(l) + Na+(aq)

Second equation would be NaOH(aq) + HCL(aq) --> NaCl(s) + HCL(aq)

Then the n(HCl)= 0.1 x 6.3 x 10-3
                     =6.3 x 10-4 mol

So I presume that n(NaOH)= n(HCl) = 6.3 x 10-4mol          <(this is the n(NaOH) in excess???)

Then the actual n(NaOH) from first equation would be 0.1M x volume used
Then the excess - actual   ?


What do I do next and how? I'm struggling to get this.

Thanks

[luken93]

So for back titrations, you work out the second reaction to find the amount of excess of NaOH that was left over from the first reaction.

So if you had 1 mol of NaOH to begin with, and in the second equation 0.2mol of NaOH reacted, then you would be left with...

1.0 - 0.2 = 0.8 mol reacted in the first reaction.

See if  you can go from there, if not I'll work it out for you

[bawse]

What's the volume of the NaOH?

[chemkid_23]

So basically your saying i do
0.1M - 6.3 x 10-4mol = 0.09937mol
?/

[chemkid_23]

This was the method

1)Accurately weigh about 1.3g of fertiliser using a weighing bottle.

2) Transfer the sample of fertiliser to a 250mL volumetric flask. Rinse the weighing bottle with a bit of distilled water and add this to the contents of the volumetric flask. Add more water until the flask is almost half full, stop and swirl the contents to dissolve the ammonium compounds in the fertiliser. Add more water to make up the volume of the solution to reach calibration line. Mix thoroughly.

3) Using a 20mL pipette, dispense aliquots of the fertiliser solution into each of three 250mL conical flasks.

4) Thoroughly wash the pipette and rinse with a small volume of standard sodium hydroxide solution. Place a 20.00mL aliquot of sodium hydroxide solution in each of the flasks containing fertiliser solution. Record the concentration of the standard solution of sodium hydroxid.

5) To one of the flasks add 50mL of distilled water and boil the mixture for about 10 mins. If needed, add more water to maintain constant volume of solution while boiling. Test the vapour at the neck of the flask with a moist strip of red litmus paper. If ammonia is still present the paper will turn blue and heating should continue until the litmus paper no longer detects ammonia. Cool the flask once completed.

6) Repeat Step5 for the other two conical flasks.

7)Rinse and fill a burette with standard hydrochloric acid; record the concentration of the acid and initial burette reading.

Add 2-3 drops of methyl red indicator to each conical flask containing the fertilser mixture, and titrate. The end point occurs when the initially yellow solution becomes orange. For each flask, record the burette reading once the end point has been reached.