Apparatus
50 cm3 burette, 20 cm3 pipettes, dropping pipette, tile, 250 cm3 measuring beaker, and 100 cm3 sodium hydroxide, 400 cm3 of unknown sulphuric acid, clamp stand filter funnel, conical flask and bromothymol blue.
Method
The solution of sodium hydroxide has a concentration of 0.2 mol/ dm3. Set up the apparatus. Wash equipment with solution three times. Measure 25 cm3 of the standard solution into a conical flask, using a pipette. Add a few drops of bromothymol blue indicator. Pour the acid into a 50 cm3 burette. Record the level. Drip the acid slowly into the conical flask, swirling the flask continuously. Stop adding acid when a single drop turns the indicator to light yellow. Record the new level of acid in the burette. Calculate the volume of acid used. Carry out four titrations, one trial and three accurate. Give your answer to 2 decimal places.
Results
Calculations
Average volume of unknown sulphuric acid used, from three accurate titrations:
26.55 + 26.50 + 26.40 = 26.48 cm3
Moles of HaOH = Concentration * volume in dm3
= 0.2 * 0.025
= 0.005 moles
From the equation
2NaOH + H2SO4 --------- Na2SO4 + 2H2O
1 mole of NaOH = 0.5 moles of Na2SO4
Concentration = moles / volume
0.005 moles of NaOH = 0.0025 moles of Na2SO4
Concentration of Na2SO4 = moles / volume
= 0.0025/ 26.48/1000
= 0.0025/ 0.02648
= 0.094 moldm3
Conclusion
The morality of the unknown sulphuric acid was 0.094 moldm3. The results were close together and the indicator changed quickly at the end point. If you were to repeat this experiment you could make it better by using a different indicator e.g. methyl orange, indicators are weak acids, where the acid and its conjugate bases are different colours. The ph of the indicators changes over 2 units. Or using a different concentration of sodium hydroxide.
Discussion
The end point might be different due to human error, they might have added a couple of extra drops or they might not have added enough drops. Or they might have lost some of the solution. Another factor instrument error which can be built in, due to it will not give a precise point. The pipette could have been a different scale e.g. 10, 15, 25 cm3 etc. which might have had some sodium hydroxide left in its tip. The reading could not be exactly clear due human usage of the instrument time after time. Précis accuracy. The numbers could have been blurred so you can not read it to. The meniscus of the solutions could be out a little. We could have lost some of the solution when we were transferring it from the pipette to the conical flask. It could have remained on the side of the pipette and the conical flask. We could not collect all the solutions no matter how hard we tried. Due to equipment error.
Instrument accuracy/ lowest reading * 100 = ans
0.05/ 26.40 * 100 = 0.19
This shows that the equipment are not absolute accurate. Another factor could have been the air around us. Or standing near the window and the outside temperature could have its effect.
Other instruments could be used which have a better accuracy point. More sophisticated measuring equipment could also be another point to notice.
