Precipitation of lead (II) chromate

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Precipitation of lead (II) chromate Folk Narongrit 10 December 2014 (Lab performed 19 November 2014) Data Collection and Processing Qualitative Observations

Figure 1. A sample of 1.00M

solution of potassium chromate

Figure 2. Another sample of 1.00M solution of potassium

chromate

Figure 3. A sample of 1.00M

solution of lead(II) nitrate

Figure 4. Precipitate after the

chemical reaction between lead(II) nitrate and potassium

chromate is completed

Figure 5. Separating the

precipitate from the solution from the process of filtration.

Figure 6. The wet precipitate after

it has been filtered.

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Table 1. Qualitative observations during the experiment

Observations Solution of 1.00M lead(II) nitrate

Lead(II) nitrate appears to be colorles solution.

Solution of 1.00M potassium chromate

Potassium chromate appears to be a yellow (Figure 1)-brown (Figure 2) solution with some suspended yellow solids with with a detergent like odor.

After mixing diluted lead(II) nitrate and diluted potassium chromate

A yellow precipitate is formed. The precipitate is clumped together towards the center of the beaker (Figure 4). The precipitate appears to have a rough texture. The aqueous product is transparent. When filtering (figure 5) , the precipitate becomes a smooth suspension.

Precipitate after filtering

After filtering (Figure 6), the precipitate appears to be a wet, smooth yellow paste. The appearance is similar to wet paint.

Precipitate after drying The dried precipitate appears to be a yellow clumped powder with cracks on the filter paper. Upon contact, the clumped powder turns into fine powder. The precipitate stains most surfaces when it comes in contact.

Theoretical Data Calculations (Pre-Lab) The theoretical data required for dilution, reacting the substances, and percentage yields are made in this section. Reaction Equation: K2CrO4 (aq) +Pb(NO3)2(aq) → PbCrO4(s) + 2KNO3 (aq) The experiment’s aim is to obtain the product mass from the concentrations as listed in table 2. Table 2 Requirements and target yield for this experiment (Lab Requirements)

Quantity Amount* Concentration

of reactants Lead (II) nitrate 0.25 M

Potassium chromate 0.75 M Theoretical

mass of product Lead (II) chromate 1.00 g

*These numbers are target amounts and don’t necessarily require uncertainties

The molar mass of lead(II) chromate (product) is determined: (Elemental Molar masses are taken from the International Baccalaureate Organization) M(PbCrO4) = 207.20 gmol  ±0.01) 52.00 gmol ±0.01)  4(16.00 gmol ±0.01)( −1 + ( −1 +   −1

=323.20 gmol-1 ±0.06 (±0.018%) The theoretical number of moles of the product is determined below in order to calcululate the volume of reactants needed: Theor. Moles PbCrO4= 0.00309 mol ±0.018% Molar Mass of  product

Theoretical Mass of  product (Table 2) = 1.00 g323.20 gmol ±0.018%−1 =

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The volume of the reactants required to obtain a theoretical yield of 1.00 g from the conditions in table 2 are calculated. Table 3 The volume of reactants required to obtain a theoretical yield of 1.00 g from the conditions set in table 2

Reactant Volume /dm3 Volume /cm3

Lead (II) nitrate 0.124 ±0.018% 12.4 (±0.018% ) Potassium chromate 0.00413 ±0.018% 4.13 (±0.018% )

Sample Calculations from table 3: equired V olumeR = Theoretical Moles of  Reactant ( )*

Required Concentration of  reactant (Table 2)

Lead (II) nitrate: 0.0124 dm3 = 12.4 cm3 ±0.018%0.25 M0.00309 mol ±0.018% =

*Since there is a 1:1 mole ratio between each reactant and its product, the theoretical moles of the reactant is equal to the theoretical mole of the product.

Because the current apparatus (1.00M) has reactants with greater concentration that what is required (Table 2), the reactants will be diluted in order to obtain the required concentration. The calculations are made below: Table 4 Theoretical requirements for the dilution of reactants

Reactant Initial Concentration

/M /±0.01*

Target Concentration**

(Table 2) /M

Final solution Volume** /cm3

Initial Volume /cm3

(Calculated) Lead (II) nitrate 1.00 0.25 100. 25 (±0.01%)

Potassium chromate 1.00 0.75 20. 15 (±0.01%) *The absolute uncertainty for the initial concentration is assumed to be 0.01M as it has not been provided. **The final solution volume and target concentration is an arbitrary number. An excess amount of solution is produced (compared to the required volume in table 3) in order to minimize precision errors in the experiment. Sample Calculations from table 4: Calculation of the initial volume of 1M reactant to dilute: Lead (II) nitrate: ⇒ V VC initial initial = C f inal f inal 5 cm .01%V initial = Cinitial

C Vfinal final = 1.00M  ±0.01(0.25M)(100. cm )3 = 2 3 ± 0

Experimental Quantitative Data and Processing (Practical/Lab) This section outlines the raw data measurements and calculations made from performing the experiment. The reactants are diluted by adding 1.00M of each solution then followed by deionized water in the amounts Table 5 Raw Measurements of the dilution of the reactants from 1.00M solutions of each reactant.

Reactant Initial Concentration

/M /±0.01

Volume of water added

/cm3 /±0.5

Volume of solute (reactant) added

/cm3 (See uncertainty calculations below)

Total volume of diluted solution

/cm3/±1

Pb(NO3)2 1.00 75.0 (0.7%) 25.0 (±2%) 100.0 (± 1%) K2CrO4 1.00 5.0 (10%) 15.0 (±3%) 20.0 (± 5%) Uncertainty Calculation of the volume of reactant solute added: Lead (II) nitrate:

Uncertainty in Calculated Initial V olume [Table 4]) Uncertainty in practical volume measurement) ( + ( 25 ) 0.5) .5 (2%)( × 100

0.01 + ( =   ± 0

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Uncertainty calculation of the total volume of diluted solution Lead (II) nitrate:

Uncertainty of volume of water added) Uncertainty of solute added)( + ( 0.5) 0.5)  (1%)= ( + ( =   ± 1

The concentration of the final solution is calculated and compared to the required concentration from table 2 Table 6 Calculation of the final concentration of diluted solutions

Reactant Concentration of diluted solutions /M

Pb(NO3)2 0.25 ±4% K2CrO4 0.75 ±9% Calculations from table 6: Lead (II) Nitrate: iluted ConcentrationD = Diluted V olume

(Initial Concentration)(Initial V olume of  reactant)  .25  %= (100.0 ±1%)

(1.00 ±1%)(25.0 ±2%) = 0 ± 4

The required volume (from Table 3) is drawn out from the diluted solutions using a pipette as shown in table 7. Table 7 Raw measurements of the volume drawn out from diluted solutions

Reactant Drawn Volume /cm3 /±0.1

Pb(NO3)2 12.4 (±0.8%) K2CrO4 4.1 (±2%) After the reaction, the products (Figure 4) are filtered through filter paper (Figure 5), and the masses measured in Table 8, and the mass of the precipitate in Table 9. Table 8 Raw measurements the mass of the product and apparatus required to hold the precipitate

Mass /g / ±0.01

Filter Paper 1.62 (±0.6%) Filter Paper + dried PbCrO4 Precipitate 2.47 (±0.4%) Table 9 Calculation of the actual mass of the dried precipitate (PbCrO4)

Mass /g / ±0.02

PbCrO4 Precipitate 0.85 (±2%) Calculations from Table 9: Mass of PbCrO4 = Mass of F ilter Paper  Dried Precipitate)  (Mass of filter paper)( +   −  

2.47  .01) 1.62  .01) .85  .02 (2%)= ( ± 0 − ( ± 0 = 0 ± 0 Calculation of the Percentage Yield of PbCrO4 Theoretical Yield = 1.00 g (Table 2) Actual Yield = 0.85 g ± 2% (Table 9) Percentage Yield = 85% ± 2%00 00 (Actual Y ield)

(Theoretical Y ield) × 1 = 1.00 g0.85g (±2%) × 1 =

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Conclusion and Evaluation This experiment has successfully obtained lead(II) chromate by reacting lead(II)

nitrate and potassium chromate through double displacement. This reaction (Pb(NO3)2 (aq) + K2CrO4 (aq) → 2KNO3 (aq) + PbCrO4 (s) ) was completed, as evident from the presence of lead (II) chromate precipitate, which does not dissolve in water. The precipitate is then put onto filter paper. The actual mass of the precipitate is determined by subtracting the mass of the filter paper and precipitate from the mass of the precipitate alone. The measured and calculated actual mass of the precipitate, 0.85 g (±2%), has an 85% yield and a 15% error from the theoretical yield of 1.00 g. An 85% yield, or 15% error falls within a good error range of less than 35% indicating that any errors have been minimized to a good extent. There are a plethora of errors which may have caused the percentage yield to be this high.

Overall, the data had low variability and high reliability; all of which contained less than 10% uncertainty. Contributing factors that strengthened the reliability of the experiment were the little number of steps and the nature of the calculations. The few number of steps has prevented the uncertainties to add up too significantly. The use of relatively large volumes and masses compared to the instrumental precision has lowered the effect of measured uncertainties.

A crucial flaw in the experiment is that it was only performed once (one trial), jeopardizing the reliability and accuracy of the results as there are no other trials to support or reject the the data; the one trial performed may well be an outlier. An solution for this problem is to repeat this experiment multiple times and average the data to ensure accurate and reliable results.

A random error encountered in this experiment was the wet precipitate left overnight to dry in an uncontrolled environment. The precipitate may have reacted with stray particles in the air, or dust particles may have stuck onto the wet precipitate. A related error is that since the precipitate was not washed it was contaminated with potassium nitrate solution (another product from the reaction). When water has evaporated, the potassium nitrate would deposit into powder and would be mixed with the precipitate. These factors would have lead to an increase in mass, increasing the actual yield. A way of minimizing this problem is to dry the precipitate in an airtight chamber to prevent any stray dust particles from getting stuck on the precipitate. Another way of minimizing the problem is to wash the precipitate with distilled water and then lightly heating the precipitate to ensure that is is completely dry.

The percent error (yield) of 15% is larger than the total propagated error (2%) suggesting that there could be some systematic errors contributing. This systematic error may be the questionable quality of the chemicals. As seen in Figures 1 and 2, the colors of 2 samples of K2CrO4 are very different. This could be due to contaminants in the chemicals . The contaminants may have reacted with the lead (II) nitrate, creating extra products. All experiments involving these samples would have gone through unwanted reactions and extra products and thus increasing the actual yield and mass by a certain percentage (Depending on the amount of contaminants). This error could be minimized by using new chemicals directly after obtaining it.

Any error affecting the accuracy of this experiment could be reduced by performing it multiple times, especially when there is a question on the accuracy of the experiment. Though much more trials would have benefited the experiment’s results, a more practical way of reducing overall errors would be to improve the design of the experiment to increase the degree of accuracy. Methods such as improving the quality of

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the chemical preparation, washing and drying the precipitate in a controlled manner, and using greater amounts of reactants would have decreased the effects of the errors.