Analysis
Table 2: Mass of reactants
Table 3: Theoretical and experimental mass and percent error of products 1. Balanced chemical equation
2KI( aq ) + Pb(NO3)2(aq) ⎯⎯→ PbI2 ( s ) + 2KNO3(aq )
It can be presumed from the chemical formula of potassium iodide and lead (II) nitrate that double replacement reaction would occur, which produces lead (II) iodide and potassium nitrate. According to the solubility table −
I is insoluble with and
Pb2+
NO3 is soluble with all cations. Therefore it can be presumed that the yellow precipitate, solid product, is and the liquid product is .
PbI2KNO3(aq)
2. Limiting reagent
The chemical equation shows that the limiting reagent of the reaction is potassium iodide and the excess reagent is lead (II) nitrate.
Molar mass = mmchemical formula Mole = n chemical formula
Mass = mchemical formula
Mass
mKI = (21.60± 0.05 g) – (20.80± 0.05 g) = 0.80± (Refer to Table 1)
Calculation of uncertainty: 0.05 g + 0.05 g = 0.10g
mKI = 0.80 ± 0.10g (Table 2)
mPb(NO 3 ) 2 = 1.10 ± 0.10g (Table 2)
Molar mass
mmKI= 39.10g/mol + 126.90 g/mol = 166.00 g/mol
mmPb(NO 3 ) 2 = 331.21 g/mol
Mole
n =
molar mass
Mole ratio
KI : Pb(NO3)2 = 2 : 1 mole × molar ratio
KI: 0.0048 × 1 = 0.0024
2
Potassium iodide is the limiting reagent and lead nitrate is the excess reagent.
- Theoretical value and percent yield
T = theoretical mass
E = experimental mass
Lead (II) iodide
nKI = 0.004819mol mPbI = 460.99 g/mol
2
KI: PbI2 = 2 : 1
mPbI = 1.41 ± 0.10g = E (Table 3)
2
The percent yield exceeds 100%, which suggests that more was produced than
PbI2
expected. This value is explainable when it is presumed that substance other than
PbI2Pb(NO3)2 KNO3
, or was left in the filter paper when it was removed from the
funnel.
-
Percent error and percent uncertainty of PbI2
The result of the experiment is accurate by 73%. The experimental value deviated from the theoretical value by 27%.
Since percent uncertainty is smaller than percent error, the systematic error of the experiment is great. A cause of systematic error could be the lost of stuck on lab
PbI2
apparatus.
Conclusion
The limiting reagent of the chemical reaction that determines the amount of products produced is potassium iodide (KI). Based on this fact, the theoretical values of products were calculated. Percent error, which shows the accuracy of results, can be
determined only when both the theoretical and experimental value is measured. The
experimental value (mass) of precipitate, lead (II) iodide (), can be easily gained
PbI2
through filtering, while the mass of potassium nitrate () requires more steps due to
KNO3
-
1st method: A substance that is soluble to one substance but insoluble to another
can be added to separate and . An example of such substance is
KNO3Pb(NO3)2
nitric acid (), which is insoluble to but soluble to. When
HNO3Pb(NO3)2KNO3
nitric acid is added to solid and left over (which can be gained
KNO3Pb(NO3)2
by boiling the filtrate) only will remain solid; therefore, it can be
Pb(NO3)2
filtered to measure the mass. The mass of can be fou
KNO3
-
2nd method: First, add enough amount of KI to the filtrate so that left over
Pb(NO3 )2 is all used up. Now the limiting reagent is not
Pb(NO3)2 KI , and
therefore the amount of produced is dependent on the amount of .
PbI2 Pb(NO3)2
Remove produced using filter paper and heat it in laboratory oven to obtain
PbI2
the mass. From the mass of , calculate the amount of in the
PbI2Pb(NO3 )2
filtrate using the molar ratio.
Both methods may not produce accurate results since they involve filtering. Filtering may cause errors because all liquid cannot perfectly pass through the filter paper. However, both methods theoretically allow one to obtain the mass of KNO3 or excess reagent left.
Evaluation
1. Limitations
- All liquid products cannot completely pass through the filter paper. Some liquid is
always left on the paper with the solid.
- Some precipitate sticks to beakers and stirring rods.
2. Effects on result
- Gives less accurate mass of the liquid product.
- Gives less accurate mass of the precipitate. 3. Improvement
- Find a different way to filter solid form liquid.
- Try using different tools to clean out the precipitate stuck on lab apparatus.