SULPHATE CONTENT OF A FERILISER Date 5th -6th May 2011 Aim To perform a first had investigation using gravimetric analysis to determine the sulphate content of a lawn fertiliser (a mixed fertiliser containing ammonium sulphate and potassium (or ammonium) nitrate).
Background information There are a variety of techniques that are used by chemists to determine the concentration or amount of a particular element, ion or compound in a sample of solution or material. One of these techniques used by scientists is gravimetric analysis. Gravimetric analysis involves weighing materials and determining the percentage composition (by weight) of elements in compounds or components of mixture. The technique techniqu e of gravimetric analysis usually involves the use of precipitation reactions1. Gravimetric analysis often involves some of the steps shown in the following diagram:
In this investigation, the sulphate ion in the fertiliser is precipitated with barium ions from acid solution. The precipitate is then collected, dried and weighed2
Ba2+ (aq) + SO42- (aq)
1 2
BaSO4 (s)
uantitative analysis: Thickett, Geoffrey. Chemistry 2: HSC course (pg 266-267) Q uantitative Equations for precipitation reactions: http://dl.clackamas.edu/ch105-04/equation.htm
Mat ia ls: y
-
Solid (soluble) f ertiliser
-
Concentrated HCl solution
-
Barium Chloride solution - 7% (w/v)
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Electronic balance
-
250mL beaker
-
Distilled water
-
Burette
-
Hotplate
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Glass rod
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Q uantitative filter paper
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Funnel
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Retort stand and clamp
M thod:
1. All saf ety precautions were ensured. 2. 60 grams of the powdered lawn f ertiliser was accurately weighed into a 250mL beaker. The mass of the f ertiliser was recorded. 3. 25mL of warm distilled water from a measuring cylinder was added into the beaker to dissolve the crystals. 4. 10 drops of concentrated hydrochloric acid were added into the dissolve solution. 5. A hotplate was used to heat the mixture until it just boiled. 6. From a burette the 7% barium chloride solution was slowly added into the hot sulphate solution until no further white precipitate formed. The solution was stirred with a stirring rod after each addition. 7. The mixture was gently re- boiled for a further 5 minutes to coagulate the precipitate. The beaker was removed from the hotplate and onto the heat proof mat where it was allowed to cool for approximately 5-10 minutes. 8. Meanwhile, a circle of quantitative filter paper was weighed on the electronic balance. The mass of the filter paper was recorded.
9.
The filter paper was folded and placed into a glass funnel, the apparatus was set up for filtration (as shown below)
10. The warm supernatant was poured and then the suspension of barium sulphate through the filter paper. It was ensured that the entire solid was transferred from the beaker into the filter using small amounts of warm washing water (distilled water).
11. The apparatus was left overnight to ensure that water had completely been removed from the residue laying on the filter paper. 12.
hen they are dry, weigh the filter paper with the barium sulpha te collected. Calculate the mass of barium sulphate collected.
iagrams from:http://www.micromountain.com/sci _diagrams/sci _app/sci _app _assets/filter _lab _eng. jpg
Results: The table below shows the mass of the f ertiliser used, quantitative filter paper as well as the calculated mass of barium sulphate that precipitated.. Mass of f ertiliser (g)
60 g
Mass of quantitati e filter paper (g)
Mass of quantitati e paper + bariu sulphate (g)
Calculated mass of barium sulphate(g)
1.86 g
2.45 (g)
2.45 - 1.86 = 0.59 g
1. Calculate the number of moles of barium sulphate collected. Thus determine the number of moles of sulphate present in the fertiliser sample.
Step 1: Determine the moles of barium sulphate n (BaSO) =
massg
=
059 233 97
= 2.5216 x 10³mol
Step 2: Write a balanced equation of the precipitation reaction to determine the moles of sulphate present in the fertiliser sample
Ba2+ (aq) + SO 42- (aq)
BaSO4 (s)
From equation n (BaSO): n (SO42- ) = 1:1 n (SO 42- )= 2.5216 x 10³mol
2 .
Calculate the weight of sulphate in the sample.
Mass (g) = Molar mass (g/mol) x Moles (mol) = 2.5216 x 10³ x 96.07mol = 2.2422 g
3. Calculate the percentage by weight of sulphate in the sample.
% sulphate by weight=
mass of sulphateg
x100 = 2.2422g/0.59g x100=41.06%
4 . Briefly describe the outcome of your investigation.
In this investigation, the sulphate ion in the f ertiliser is precipitated with barium ions from acid solution (Barium Chloride solution). The balanced ionic equation for this precipitation reaction is shown below:
Ba2+
aq)
2-
+ O
BaSO
aq)
)
Step 1: Determine the moles of barium sulphate
n (BaSO) =
massg
=
059 233 97
= 2.5216 x 10³mol
Step 2: Write a balanced equation of the precipitation reaction to determine the moles of sulphate present in the fertiliser sample
Ba2+ (aq) + SO 42- (aq) From equation n (BaS ): n (S
n (S
24
4
2-
BaSO4 (s)
) = 1:1
)= 2.5216 x 10³mol
Step 3: Determi ne the wei ght of sulphate i n the sample.
Mass (g) = Molar mass (g/mol) x Moles (mol) = 2.5216 x 10³ x 96.07mol = 2.2422g
Step 4: C alcul ate the percent age by wei ght of sulphate i n the sample.
% sulphate by weight=
mass of sulphateg
x100 = 2.2422g/0.59g x100=41.06%
The steps above were followed by all of the groups in the class. The percentage by weight of sulphate in the sample calculated by each group is shown in the table below:
Group no.
1
2
3
4 (my group)
% sulphate by weight
65.87%* (outlier, not included in class average)
44%
49%
41.06
Class average of % sulphate by weight
44.69%
3. The solution is acidified prior to the addition of barium ions. Explain why the solution is acidified.
The purpose of acidif ying the solution prior to the addition of barium ions is to eliminate the possibility of interf ering ions including carbonate ions and phosphate ions that may precipitate when barium ions is added. Carbonate ions that may be in the solution (as an impurity) will form a precipitate with barium ions to form barium carbonate which is insoluble.
Ba² (aq) +CO ²(aq) BaCO(s) However in the presence of a dilute acid (usually HCl or HNO), the carbonate ions will decompose to give carbon dioxide gas. Phosphate ions that may be in the solution (as an impurity) would also form a precipitate with barium ions to form barium phosphate which is insoluble.
2 PO43-(aq) + 3Ba² (aq) Ba (PO) (s) By eliminating the carbonate ions and phosphate ions in the solution, it becomes assured that all the precipitate formed when the barium ions are added are only barium sulphate (with no interf erence by the carbonate ions and phosphate ions) 4. Explain why the precipitate is heated for some time prior to filtration.
Barium sulphate ions are very small and have the possibility to pass through the pores of the filter. However, heating the precipitate formed prior to filtration causes the barium sulphate particles to coagulate (clot) to ensure that they will not pass through the pores of the filter paper, thus giving us more accurate results. 5. One student considered varying the mass of the fertiliser used in the experiment. Explain why the accuracy would be improved by precipitating 0.90G of barium sulphate rather than 0. 40g of barium sulphate.
The accuracy would be improved by precipitating 0.90G of barium sulphate rather than 0.40g of barium sulphate as the percentage error would be reduced.Percentage error is the measure of how far off an estimate is from its true value. Percentage of error is dependent on the size of the sample. As sample size increases, standard error decreases because estimates become more precise. 6. Explain why a quantitative filter paper leads to greater accuracy than using a qualitative filter paper.
Q uantitative filter paper is chiefly used for weight analysis test in quantitative chemical analysis,
and relevant analysis experiments as they have finer pores. These very fine pores prevent the small barium sulphate particles from being lost (i.e.: they ar e trapped on top of the filter paper) in our experiment. Qualitative filter paper however, is mainly used for qualitative chemical analysis as they have larger pores and do not restrict very small particles (i.e.: used for rough work where it is not important to completely retrieve all t he solids, thus not very accurate in dealing with very small barium sulphate particles) 5
5
Quantitative and qualitative filter paper: http://answers.yahoo.com/question/index?qid=20100828073836AAhveEH
7 . Explain how reliability could be improved in this
experiment.
In order to improve the reliability of this investigation, the experiment must be repeated a minimum of five times whilst ensuring that all the variables are controlled. The investigation must also be repeated with extreme accuracy and precision as we are dealing with very fine particles (barium sulphate) and a very sensitive balance (i.e.: any small error may impact dramatically on our results). The results of the repeated trials must also be averaged (making sure not to include the outrider result/s) 8. On what assumption is the validity of the experiment dependant?
The validity of the experiment is dependent on the following assumptions: - The hydrochloric acid eliminated all the carbonate and phosphate ions that were contaminating the solution (no foreign ions -other than sulphate ions-were in the solution) - The precipitate that formed was entirely barium sulphate. - All the precipitate collected on the filter paper (i.e.: no barium sulphate was lost)
9. Explain why the precipitate on the filter paper should be heated to constant dryness?
The precipitate on the filter paper should be heated to constant dryness to ensure that all the remaining water molecules are removed. All water molecules must be removed to ensure that the results are unaff ected and valid. Only be constant heating do we know that all the water molecules have been removed. 10. The accuracy of the experiment can be improved by filtering through a pre-weighed fine-pore sintered glass filter rather than a filter paper. Explain why this procedure is more accurate.
Filter paper is made out of cellulose. The cellulose forms strong bonds with water molecules (due to the high polarity of water and cellulose). It becomes very difficult to break these strong bonds even if the filter paper is heated, this will adversely eff ect the validity of the results(i.e.: the mass of the water will be mistaken for barium sulphate) In order to improve accuracy, a sintered glass filter can be used instead of filter paper. The sinister glass can be heated to complete dryness as the glass will not absorb any water molecules. The very fine pores of the sintered glass filter also ensure that all the barium sulphate will collect at the top of the glass disk. Chemist also use sintered glass filters, and not filter paper, whenever the solid matter must be dried in an oven, as filter paper might burn in an oven. 11. Identify the main safety concerns of this experiment.
The main saf ety concerns of the experiment include: - Heated gl assw are, materi als and solutions: If glassware that is heated over the Bunsen flame/hotplate may become extremely hot and if it gets into contact with skin, the skin may produce redness, pain, and swelling. If heated solution may splash into a persons eye it may damage the eye. - Barium Chloride (BaCl 2 ): Barium chloride is poisonous. It is harmful if it is inhaled and if it comes into contact with the skin (irritant), and may be fatal if swallowed, thus any contact must be avoided.
- Hydrochloric Acid ( HCl): HCl is a corrosive liquid and exposure to skin may cause irritation. Prolonged contact may result in burns. It is irritating and may cause conjunctivitis, ulceration and corneal burns if it comes into contact with the eyes. In order to prevent any of these saf ety concerns, the following must be followed and implemented: y
Wear safety glasses
y
Wear safety gloves.
y
Wear closed shoes.
y
Wear long sleeved clothing to cover skin
y
Ensure
that the glassware is completely cooled before handling
Conclusion:
An investigation was conducted to determine the amount of sulphate present in the lawn f ertiliser using gravimetric analysis. The amount of sulphate in percentage by weight in the f ertiliser was calculated to be 41.06% and on class average it was calculated to be 44.69%.
