The research objective is to evaluate the concentration of arsenate (mg/kg) and the soil's mass percent. The concentration of arsenate in the samples may be used to determine the safety of drinking water in the sampled area. Possible health effects may also be hypothesized based on the concentration of arsenate in the sample. The study considers that 1.5 mg/L is a lethal dose of arsenic to humans. The procedure involves adding1 M AgNO3 to each of the soil samples before recording the mass of A A formed. The ions in AgNO3 react with the soil sample to form a precipitate A A.
Procedure
Add 1 M AgNO3 solution to sample in small increments of 0.5 mL
Add AgNO3 until the amount of A A formed remains constant.
Record the amount of A A
Repeat the experiment 2 or 3 times for accuracy.
Data Analysis and Calculations
Sample 1
Table 1
A A Formed (g)
| Amount AgNO3 Added (mL) | A A Formed (g) |
|
2.5 |
0.370024 |
|
2.5 |
0.370024 |
|
2.5 |
0.370024 |
|
2.5 |
0.370024 |
Delegate your assignment to our experts and they will do the rest.
Find the molar mass of A A = 1 Ag + 1 AS + 4 O
=3*(107.8682 g/mol) + 74.9216 g/mol + 3 *(15.9994 g/mol) = 462.5238g/mol
Find the number of moles of A A formed
Number of moles =
Number of moles = 0.000800 moles
The stoichiometry ratio A A : As is 1 mole: 1 mole such that 1 mole of A A contains 1 mole of As.
Mass of As ions = 0.059938083 g
Table 2
Amount As in Sample (g)
| Amount AgNO3 Added (mL) | A A Formed (g)) | Number of moles of As ion | Amount As in sample (g) |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
The sample volume = 100 mL
Amount As in Sample (mg/Kg) = 599.38083 mg/Kg
Table 3
Mass percent of As in Sample (g)
| Amount AgNO3 Added (mL) | A A Formed (g) | Number of moles of As ion | Amount As in sample (g) | Amount As in sample (mg) |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
599.38083 mg/Kg |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
599.38083 mg/Kg |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
599.38083 mg/Kg |
|
2 |
0.370024 |
0.0008 |
0.059938083 |
599.38083 mg/Kg |
Mass (%) =
Amount As in Sample (mg/Kg) = 599.38083 mg/Kg
Mass (%) = * 100 = 59.938083 %
Table 4
Mass percent of As in Sample (g)
| Amount AgNO3 Added (mL) | A A Formed (g) | Amount As in sample (g) | Amount As in sample (mg) | Mass (%) of As in sample (g) |
|
2 |
0.370024 |
0.059938083 |
599.38083 mg/Kg | 59.938083 % |
|
2 |
0.370024 |
0.059938083 |
599.38083 mg/Kg | 59.938083 % |
|
2 |
0.370024 |
0.059938083 |
599.38083 mg/Kg | 59.938083 % |
|
2 |
0.370024 |
0.059938083 |
599.38083 mg/Kg | 59.938083 % |
Sample 2
Table 5
A A Formed (g)
Amount AgN
Added (mL)
|
A
A
Formed (g)
|
|
2.0 |
0.231265 |
|
2.0 |
0.231265 |
|
2.0 |
0.231265 |
|
2.0 |
0.231265 |
Molar mass of A A = 1 Ag + 1 AS + 4 O
=3*(107.8682 g/mol) + 74.9216 g/mol + 3 *(15.9994 g/mol) = 462.5238g/mol
Find the number of moles of A A formed
Number of moles =
Number of moles = 0.000500 moles
The stoichiometry ratio A A : As is 1 mole: 1 mole such that 1 mole of A A contains 1 mole of As.
Mass of As ions = 0.03746 g
Table 6
Amount As in Sample (g)
| Amount AgNO3 Added (mL) | A A Formed (g) | Number of moles of As ion | Amount As in sample (g) |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
The sample volume = 100 mL
Amount As in Sample (mg/Kg) = 374.6 mg/Kg
Table 7
Amount As in Sample (mg).
| Amount AgNO3 Added (mL) | A A Formed (g) | Number of moles of As ion | Amount As in sample (g) | Amount As in sample (mg) |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
374.608 mg/Kg |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
374.608 mg/Kg |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
374.608 mg/Kg |
|
2 |
0.231265 |
0.0005 |
0.0374608 |
374.608 mg/Kg |
Mass (%) =
Amount As in Sample (mg/Kg) = 374.6 mg/Kg
Mass (%) = * 100 = 37.4608%
Table 8
Mass percent of As in Sample (g)
| Amount AgNO3 Added (mL) | A A Formed (g) | Amount As in sample (g) | Amount As in sample (mg) | Mass (%) of As in sample (g) |
|
2 |
0.231265 |
0.0374608 |
374.608 mg/Kg |
37.4608% |
|
2 |
0.231265 |
0.0374608 |
374.608 mg/Kg |
37.4608% |
|
2 |
0.231265 |
0.0374608 |
374.608 mg/Kg |
37.4608% |
|
2 |
0.231265 |
0.0374608 |
374.608 mg/Kg |
37.4608% |
Oxidation state of the element arsenic (As) in arsenate (As )
So to calculate the oxidation state, we have to equate
Sum of individual charges = total charge on the ion.
Let the oxidation number of As in arsenate (As) be x
The oxidation number of Oxygen ion = -2
x+4(−2) =−3
x−8=−3
x=5
The oxidation number of arsenic = +5.
The reaction between silver nitrate and two samples is a displacement reaction. A displacement reaction occurs when an atom or group of atoms is displaced by another. In the reaction, As ion is displaced by 3 ions from silver nitrate to form a precipitate A A . The reaction is a single displacement reaction because arsenic ion occurs as free ions in the soil (Stains & Talanquer, 2008). The nitrate ion released by silver ions does not combine with an ion that previously held arsenic ion.
A (aq) + 3 (aq) A A (s)
The reaction also falls under the precipitation reaction. A precipitation reaction occurs when aqueous cations and anions combine to form an insoluble ionic solid, known as a precipitate (Stains & Talanquer, 2008). The soluble ions of As (aq) and (aq) react to form an insoluble compound, A A .
Arsenic is a colorless semi-metal that has no taste. The semi-metal is potentially poisonous even in small amounts and may leak into a water supply or contaminate the soil. Potential sources of arsenic include industrial or agricultural chemicals, natural geological sources, or sea creatures (EPA, 2020). Arsenic occurs in both trivalent arsenic with oxidation number +3 and pentavalent arsenic with oxidation number +5 (García-Lara et al., 2014). Trivalent arsenic is relatively more toxic as compared to pentavalent arsenic. Although both are harmful to humans, the trivalent form is more difficult to remove from water, while the pentavalent form is easily removed using a strong oxidant such as chlorine (García-Lara et al., 2014). Potential health effects include stomachache, diarrhea, nausea, vomiting, partial paralysis, and cancers of the lung, skin, liver, and prostate.
The percentage of arsenic is more in soil sample 1, 59.938083 % compared to sample 2, 37.4608%. The high level of arsenate suggests that the samples were collected from an area with natural geological contamination, deposits from industrial activities, runoff from agricultural fields or seawater (EPA, 2020). The amount of arsenic in samples is more than 1.5 mg/L, meaning that the samples are lethal to humans. If only 50% of the arsenate leaks into the drinking water, the proportions would still be higher than the recommended 1.5mg/L. 50% of sample 1 is 299.690 mg/L, and sample 2 is 187.304 mg/L. Arsenic with oxidation number +5 can be removed using a strong oxidant such as chlorine.
Conclusion
There are significantly high levels of arsenate in samples 1 and 2. Even if 50% of the samples leak into drinking water, the arsenate levels may cause instant death if consumed by humans. For those who survive death, arsenate affects the heart, gut, and nervous system. Other possible effects of arsenic poisoning include pigment spots on the skin, damaged liver, red blood cells, bone marrow, and the brain. The impact of long-term arsenate exposure includes thickening of pigment skin spots and cancer of the lung, bladder, kidney, or skin. Arsenate also inhibits reproduction, causes poor growth or death of other organisms in the environment.
References
EPA. (2020, July 8). Drinking-Water Requirements for States and Public Water Systems: Chemical contaminant rules. US Environmental Protection Agency. https://www.epa.gov/dwreginfo/chemical-contaminant-rules
García-Lara, A. M., Montero-Ocampo, C., Equihua-Guillen, F., Camporredondo-Saucedo, J. E., Servin-Castaneda, R., & Muñiz-Valdes, C. R. (2014). Arsenic removal from natural groundwater by electrocoagulation using response surface methodology. Journal of Chemistry, 2014.
Stains, M., & Talanquer, V. (2008). Classification of chemical reactions: Stages of expertise. Journal of Research in Science Teaching: The Official Journal of the National Association for Research in Science Teaching, 771-793.
