The purpose of this experiment is to find the composition of a sample of sodium carbonate mixture by titration.

Results and calculation

Table 1 (Preparation of the Standard solution)

Table 2(The titration data and results with indicator solutions)

First stage of titration

Na2CO3(aq) + HCl(aq) >> NaHCO3(aq) + NaCl(aq)

Since HCL : Na2CO3 = 1:1

No of mol of Na2CO3 = Mean volume 1 of HCL used  x Concentration of HCL

                  = 7.28 /1000 x 0.5

                  = 3.64 x 10-3 mol

Second stage of titration

NaHCO3(aq) + HCl(aq) >> NaCl(aq) + H2O(l) + CO2(g)

No of mole of NaHCO3 = Mean volume 2 of HCL used x Concentration of HCL

                      = 21.7/1000 x 0.5

                      =0.01085 mol

Calculation

From above , No of mole of Na2CO3 in 250 cm3 solution

         = 3.64 x 10-3 mol

No of mole of NaHCO3 in the 250cm3 solution

= (No of mole of NaHCO3 in second stage)  -  (No of mole of NaHCO3 in first stage)

= 0.01085 – 3.64 x 10-3

= 7.21 x 10-3 mol

Mass of Na2CO3 = 0.0364  x 106  = 3.8584 g

                             Which is around 3.9 g

Mass of NaHCO3 = 0.0721  x 84  = 6.0564 g

                           Which is around  6.1 g

Therefore the percerage of Na2CO3 is 39% and the perecentage of NaHCO3 = 61 %

Discussion

In doing the experiment of titration , several precaution should be taken.

• Wear safety goggles to prevent the splitting of acid onto our body
• take readings at eye level to avoid parallax errors
• The beaker,pipette and the flask should be washed properly with distilled water.
• The flask containing the indicator (phenolphthalein or methyl orange) must be shaken well while acid is added to it.
• Excess of indicator should not be used.
• To read the buret accurately, hold a white card with a black stripe behind the buret, with the black stripe below the meniscus, and the meniscus itself in front of the white region above the black stripe
• Mix the solution in the titration flask thoroughly after each addition of titrant, to ensure complete reaction before adding more.

Some inaccurate result may due to :

• The molarity of sodium carbonate solution was not accurate
• Calculation errors (e.g. converting mL to L)
• Not rinsing and drying the beaker for the acid
• Over titrating
• Acid or base left on the side of the flask or on the tip of the burette
• Errors reading volumes.
• Using pipette (10 mL is measured from 0 mL to 10 mL, not from 10 mL to empty)

There are many kinds of titration in chemistry. In the experiment , it is an acid – base titration. First, phenolphthalein is used , it is colorless below around pH 8.5, but turns red above around pH 9.0 , so in step I , when the phenolphthalein turn from pink to colourless , it is nearly to neutral but not exactly neutral because NaHCO3 is slightly alkali.

After that , methyl orange is used because NaHCO3 is a weak base . It changes from red below pH 3.1 to yellow above pH 4.4. Therefore , in the second part of titration, when the indicator turn from yellow to orange, the solution is in acidic but not exactly neutral . Therefore , the result obtained from above is not accurate. If we want to get a more accurate result , ph meter should be used but not indicator.

The calculation above has a little bit complicated. Since Na2CO3 only involve in the first titration , no of mol in Na2CO3 will exactly equal to the no of mol of HCL used. After that , we need to calculate No of mole of NaHCO3 in the mixture by using the

(No of mole of NaHCO3 in second stage) - (No of mole of NaHCO3 in first stage) because the No of mol of NaHCO3 in second stage contain both NaHCO3 from the mixture and that from the first titration. Thus , we need to minus the no of mole of NaHCO3 in first stage to obtain the no of mol of NaHCO3 in the mixture.

References

Data – from Lam Chi Ho (absent on the day of experiment )