Formula of a hydrated salt

2. Using a pipette filler, pipette 20.0cm3 of the solution of iron (II) sulphate into a conical flask and add approximately 20cm3 of the 1 mol dm-3 H2SO4 (aq) solution provided.

3. Titrate the acidified Fe2+ (aq) solution with 0.0100 mol dm-3 potassium permanganate, KMnO4 (aq), and continue the titration to the normal end point.

4. Repeat the titration until you have obtained concordant results.

Results and Calculations:

Mass of hydrated iron (II) sulphate is 59.12 – 56.10 = 3.02 g

To find the mean titre I used the average of only titrations that were concordant with each other (they agreed to at least 0.1 cm3).

So to find the mean I used (17.45 + 17.50) / 2 = 17.475 cm3

This mean titre is more accurate than the burette can measure so it has to be rounded to 17.50 cm3.

To find the moles of MnO4- you have to use the formula: Moles = Concentration x Volume (dm3)

Concentration = 0.01 mol dm-3

Volume = 17.50 cm3 / 1000 = 0.0175 dm3

Therefore 0.01 mol dm-3 x 0.0175 dm3 = 1.75 x 10-4 mol

Now using the equation: 5Fe2+ + MnO4- + 8H+ = 5Fe3+ + Mn2+ + 4H2O, I know that there are 5 times the amount of Fe2+ (FeSO4) being used than MnO4- in the 20 cm3 titration.

So 5Fe2+ = (1.75 x10-4) x 5 = 8.75 x 10-4 mol

To find the moles in the 250 cm3 of solution I have made, I need to multiply this figure by 12.5 (250 / 20 cm3).

So (8.75 x 10-4) x 12.5 = 0.0109375 mol

To find the mass of Fe2+ (FeSO4) in the 250 cm3 of solution you need to use: Mass = Moles x RFM

Moles = 0.0109375 mol

RFM = 151.92

Therefore 0.0109375 mol x 151.92 = 1.661625 g

I can now calculate the mass of H2O in the original sample 3.02 - 1.661625 = 1.358375 g

To find the moles of H2O in the original sample you use the formula as before: Moles = Mass / RFM

Mass = 1.358375 g

RFM = 18

Therefore 1.358375 g / 18 = 0.075465277778 mol

Again, to find the value of (x) in the formula FeSO4.xH2O you need to find the formula ratio. To do this you divide the moles of H2O by the moles of FeSO4.

 So 0.075465277778 / 0.0109375 mol = 6.90 (3sf) = 6.9 (2sf).

Therefore the ratio I have found is 1:7 for FeSO4:H2O. The 6.9 has to be rounded up because it is not possible to have 0.9 of a molecule, you have to make it an integer so you have a whole number of molecules.

Evaluation

Overall both my experiments went really well, obtaining accurate and reliable results enabled me to work out the formula of hydrated iron sulphate crystals. I also carried both experiments safely by following the risk assessments and I tried to be as fair as possible.

Method 1:

This experiment went really well as it was quite accurate and I obtained good reliable results which enabled me to work out the formula of the hydrated iron sulphate crystals. I obtained no anomalous results as I took great care weighting out the mass accurately to two decimal places on a digital top pan balance. When I was heating the hydrated iron sulphate crystals I used a stopwatch to make sure I didn’t overheat it. I also reheated the iron sulphate crystals this ensured that all water was vaporised.

However, there were still errors in the experiment. One was that there was no specific time for cooling was given, and it stated in the method to heat for “about” two minutes. This could have affected the overall mass reading. Another error is that when weighting out the mass on the top balance, there may have been other substances on there which could have affected the mass reading. This was also the same for the crucible as I could see some other substances stuck at the bottom, which couldn’t be removed. This may have again affected the mass reading and the overall calculations, resulting in inaccurate results.

To improve the experiment and the accuracy of results I could firstly try finding brand new clean crucibles or clean the crucibles for longer until all other substances are removed. To ensure all water is vaporised I could reheat the hydrated iron sulphated crystals again and carry out preliminary work and repeats to gain a more accurate and reliable average. I would also use a more pure hydrated iron sulphate crystals as there were many impurities in the crystals I used. I would also used a digital top pan balance which measures to more than just two decimal places and to also make sure the top is cleaned and all other substances are removed to prevent inaccurate results and minimise errors. This would make my results more reliable and accurate.

Method 2:

Overall, method 2 went quite well, as I obtained two concordant results straight away after my preliminary titration where both of the two results had only 0.1cm3 differences which were 18.8cm3 to 18.9cm3. I also had no anomalies in my results, which showed that the experiment was carried out safely, accurately and the method was followed well. To obtain accurate results like these I carried a preliminary titre to help me estimate the next titration and when the end point will be. I also cleaned out all my equipment fully, and dried it afterwards to ensure all other substances are removed. To gain a great degree of accuracy with the mass I used a digital top pan balance and measured to two decimal places. I also used a white tile which was placed beneath the conical flask. This helped me see the slightest of changes from light green to light pink. The results obtained from the titration were used in my calculations and showed that my calculated formula was quite accurate and reliable.

Although I did receive accurate results, they were still some errors in the experiment in both procedure and measurements which I encountered. One error during measuring was it was difficult to see how much sulphuric acid was inside in the plastic cylinders as the measurement markers were had to see as they were clear too. Another error was that the plastic cylinders were used a lot, and some were slightly bent and irregularly shaped which could have lead to slight inaccurate measurements. When titrating using the KMnO4, it was difficult to see the measurements on the burette because it is dark purple. This could have also leaded to errors in measurements and inaccurate results. Also they may have been other substances on the top pan balances which could lead to inaccurate measuring of the mass.

Errors faced in procedure was when trying to estimate when the end point is in the titrations, more KMnO4 then needed was added this lead to a darker shade of pink. This is also down to human error and reaction time when turning the tap just as the colour changes. It also became difficult when trying to shake the conical flask and titrating at the same time. The solution inside the volumetric flask seemed to settle down really quick and required regular shakes. When pipetting the solution from the volumetric flask and pipetting it out into the conical flask there was still drops that were inside the pipette. This could lead to inaccurate results.

To improve my method and increase the reliability I would use a glass pipette to accurately measure out 20cm3 instead of a plastic measuring cylinder. I could also use a different burette whilst titrating the KMnO4 where the measurement markers stand out more clearly this could minimise errors. I would also use a more pure hydrated iron sulphate crystals as there were many impurities in the crystals I used. I also used a digital top pan balance which measures to more than just two decimal places to increase accuracy and to also make sure the top pan is cleaned and all other substances are removed to prevent inaccurate results and minimise errors. I could also carry out more preliminary and repeat titrations, and gain a more accurate average. This way I could estimate the end point much more precisely

To conclude and compare both methods I think that method two was more accurate and reliable than method one. This was because in method one there were no preliminary and maybe not all of the water was vaporised. Where as with method two there was a preliminary titre and repeats where I obtained concordant results straight away. However, there were still errors in method two, which could have affected the accuracy and reliability of the results.