Project+-+Gravimetric+Estimation+of+Chloride+Ions

**Introduction**
//Gravimetric analysis// is used to determine the amount of a substance by finding its mass, and then using the mass to find the quantity of the substance. One of the most common techniques of applying gravimetric analysis is obtaining a precipitate from a solution and removing any impurities from it, in order to find its net mass. //Stoichiometry// is the study of the relationships between the quantities of reactants and products involved in chemical reactions, and is used in many calculations involving molar and mass ratios. //Gravimetric stoichiometry// is a combination of gravimetric analysis and stoichiometry, and in short it is the procedure for calculating the masses of reactants or products in a chemical reaction. Gravimetric stoichiometry is used in this experiment to calculate the theoretical yield of a chemical reaction, and the percentage yield (what percentage of the amount predicted for a substance was actually produced).

The objective of this experiment is to measure the number of chloride ions in a solution of sodium chloride, using silver nitrate as the other reagent. When sodium chloride is added to silver nitrate, a double displacement reaction occurs which results in silver chloride and sodium nitrate. The reaction occurs as follows:

AgNO3(aq) + NaCl(aq) --> AgCl(s) + NaNO3(aq) Aqueous Silver Nitrate + Aqueous Sodium Chloride --> Solid Silver Chloride + Aqueous Sodium Nitate.

Since this equation is already balanced, and all of the coefficients are one, that means that there is a 1:1:1:1 mole ratio between the reactants and products. When the reaction occurs, a white solid (silver chloride) precipitates out of the solution. The precipitate can be extracted and washed free of impurities. The extract is weighed to give a theoretical mass, and using gravimetric stoichiometry, the quantity of chloride ions can be estimated. This quantity is compared with the prediction based on the mass of the reactants to give the percentage yield.

To derive the amount of molecules in a substance, the following formula was used: Mass (g) / Molecular Mass (g/mol) = Mol quantity of the molecule Multiply that quotient by Avogadro's number to get the amount of molecules present in the substance.

Avogadro's number is a constant which is equal to the number of molecules in one mole of molecules, 6.022 x 10^23 to be precise. Also, 1u multiplied by Avogadro's number is 1g. This is where the term molar mass come from. For example, a carbon-12 atom has an atomic mass of 12u. This means that in a mole of only carbon-12 atoms, the mass would be 12g. Consequently, 1u = 1g/mol.

**Materials**

 * Safety Goggles
 * Distilled water
 *  0.117g of NaCl
 * AgNO3
 * Dropper
 * 50mL Erlenmeyer Flask
 * 100mL Erlenmeyer Flask
 * Funnel
 * 1 piece of Whatman filter paper
 * Porcelain crucible and lid
 * Crucible tongs
 * Ring clamp
 * Retort stand
 * Clay triangle
 * Bunsen burner
 * Accurate scale

**Procedure**

 * 1) The mass of the empty crucible was measured.
 * 2) In a 50mL Erlenmeyer flask, the NaCl was dissolved in 20mL of distilled water.
 * 3) Using a dropper, small amounts of AgNO3 was added until all of the NaCl had reacted; the products were NaNO3(aq) and AgCl(s).
 * 4) The filter paper was placed over the funnel which was placed over the 100mL Erlenmeyer flask.
 * 5) The products were poured over the funnel, so that the NaNO3 would be in the 100mL Erlenmeyer flask while the AgCl would be on the filter paper.
 * 6) Distilled water was added to the 20mL Erlenmeyer flask and poured over the filter paper so that all of the AgCl would be left on it.
 * 7) The ring clamp, retort stand, clay triangle and bunsen burner were set up.
 * 8) The filter paper was gently placed in the crucible and the crucible was placed on the clay triangle.
 * 9) The bunsen burner tube was attached to the gas nozzle and was carefully turned on.
 * 10) Once the whatman filter paper was allowed to burn and all that was left was the silver chloride, the bunsen burner was turned off.
 * 11) The crucible, and its contents, were allowed to cool.
 * 12) The mass of the remaining silver chloride was calculated by subtracting the mass of the empty crucible as well as the one with the substance.
 * 13) Using the molecular mass and the grams of silver chloride remaining, the moles of silver chloride was found.
 * 14) Multiplying that amount by Avogadro's number resulted in the amount of molecules of silver chloride present in the precipitate.
 * 15) Since the ratio of silver to chlorine in silver chloride is 1:1, the amount of chloride ions present was that quantity found in the previous step.

**Safety Precautions**

 * Safety goggles were worn at all times
 * The work space was free of clutter
 * No chemicals or apparatus were tasted or smelled; if necessary, the wafting method was used
 * All long hair was tied back and all loose clothing was tied or removed
 * Made sure that the bunsen burner was far enough away from the operator when it was being used
 * The bunsen burned was turned off when not in use
 * The retort stand was firmly attached to the base
 * All hot materials were handled with care and were never touched directly
 * All chemicals were disposed off properly

**Observations**
Prior to the experiment, the NaCl and AgNO3 were both transparent aqueous solutions. Once the two were combined, a white precipitate of AgCl (from the double displacement reaction) began to form; small amounts of AgNO3 were added until it seemed like all the NaCl had reacted with it. AgCl was a white powdery, insoluble substance in the NaNO3 solution:



Since not all of the filter paper had burned, some black substances were left in the crucible. These black substances were burnt forms of Carbon (from the filter paper) which had not totally dissipated; this skewed the actual mass of the precipitate obtained:

**Data Table**

 * ~ Objects ||~ Mass (g) ||
 * Mass of empty crucible || 10 ||
 * Mass of crucible + lid || 15.871 ||
 * Mass of crucible + NaCl || 10.117 ||
 * Mass of crucible + NaCl + lid || 15.988 ||
 * Mass of NaCl || 0.117 ||
 * Mass of crucible + AgCl || 10.3191 ||
 * Mass of AgCl || 0.3191 ||

**Calculations**
__**Mass of NaCl**__ (Mass of crucible + NaCl) - (Mass of crucible) = Mass of NaCl 10.117 g - 10 g = 0.117 g Therefore, the mass of NaCl used was **0.117g**.

__**Mass of AgCl**__ (Mass of crucible + AgCl) - (Mass of crucible) = Mass of AgCl 10.3191 g - 10 g = 0.3191 g Therefore, the mass of AgCl obtained was **0.3191 g**.

__Theoretical Calculations__
__**Mass**__ n = (mass)/(Molar mass) n (NaCl) = (0.117g)/(58.44247g/mol) Moles of NaCl = ~0.002 mol

n (AgCl) = 0.002 mol (Since the balanced equation indicates that both coefficients are the same) mass (AgCl) = (moles of AgCl) * (Molar mass of AgCl) mass (AgCl) = (0.002 g) * (143.3207 g/mol) mass (AgCl) = 0.2866414 g

__**Amount of Chloride ions**__ n = (mass)/(Molar mass) n (NaCl) = (0.117 g)/(58.44247 g/mol) Moles of NaCl = ~0.002 mol

n = (# of molecules) / (Avogadro's number) n (AgCl) = (# of molecules of AgCl) / (Avogadro's number) Number of molecules of AgCl = (moles of AgCl) * (Avogadro's number) Number of molecules of AgCl = (0.002) * (6.022*10^23) Number of molecules of AgCl = 1.2044 * 10^21

Mole to Mole ratio of Ag : Cl = 1 : 1 Therefore, there will be roughly **1.2044 * 10^21** Chloride ions in the precipitate.

__Actual Calcuations__
__**Mass**__ According to the observations, the mass of the precipitate (AgCl) was 0.3191 g.

n = (mass)/(Molar mass) n (AgCl) = (0.3191 g)/(143.3207g/mol) Moles of AgCl = 0.002226475 mol
 * __Amount of Chloride ions__**

n = (# of molecules) / (Avogadro's number) n (AgCl) = (# of molecules of AgCl) / (Avogadro's number) Number of molecules of AgCl = (moles of AgCl) * (Avogadro's number) Number of molecules of AgCl = (0.002226475) * (6.022*10^23) Number of molecules of AgCl = 1.3407843432 * 10^21

Mole to Mole ratio of Ag : Cl = 1 : 1 Therefore, from the experiment's results, there were **1.3407843432 * 10^21** Chloride ions in the precipitate.

__**Percentage Yield and Error**__
__**Percentage Yield**__ Percentage Yield = [ (Number of Chloride ions - Actual) / (Number of Chloride ions - Theoretical) ] * 100% Percentage Yield = [ (1.3407843432 * 10^21) / (1.2044 * 10^21) ] * 100% Percentage Yield = ~111.32%

Therefore, the percentage yield is ~111.32%; the reasons for this are listed in the sources of error / modifications to the experiment.

__**Percentage of Error**__ Percentage of Error = | [ (Number of Theoretical Chloride ions - Number of Actual Chloride ions) / (Number of Theoretical Chloride Ions) ] * 100% | Percentage of Error = | [ (1.2044 * 10^21 - 1.3407843432 * 10^21 ) / (1.2044 * 10^21) ] * 100% | Percentage of Error = | [ -0.1132 ] * 100% | Percentage of Error = | -11.32% | Percentage of Error = 11.32%

Therefore, the percent of error from the experiment is 11.32%.

**Discussion**
This experiment could have been performed in a number of ways. Since it was decided that gravimetric analysis would be used, the largest problem faced was separating the final products of sodium nitrate from silver chloride. One way was to burn it, as silver chloride has a higher boiling point than sodium nitrate. However this would be impractical, as the boiling point of sodium nitrate is still greater than 600 ° C. Another option was to use filter paper, which would drain the aqueous sodium nitrate through and leave the solid silver chloride. This was done, but then there was the problem of how to get it off the filter paper. The filter paper had miniscule holes to let the solution drain through, but some of the particles of silver nitrate could have gotten lodged in the holes. In this case, simply scraping the precipitate off of the paper might leave some residue behind, throwing off the results of the gravimetric analysis. So the filter paper was burned until only the silver chloride was left. Gravimetric analysis was not the only the way to perform this experiment, although it was by far the most convenient. Mass is very easy to measure, and through a few simple calculations the quantity in moles or molecules can be found. Gravimetric analysis is only one of many branches of analytical chemistry, though. Analytical chemistry is the study of the chemical composition of natural and artificial materials. Although many issues are studied in analytical chemistry, everything boils down to the questions of what chemicals are present in reactions, the characteristics of those chemicals and the quantities of those chemicals in the reaction. It can also be divided into two kinds:
 * Quantitative: Based characteristics of those chemicals and the questions of the presence of what chemicals in on finding out the amount of a chemical present
 * Qualitative: Based on establishing the presence of a chemical

Between the 17th and 20th centuries, most of chemistry was analytical, as there was much focus on what the elements and all of their properties were. The branch of analytical most studied today is instrumental analysis, and the first of this kind was flame emissive spectrometry, developed by Robert Bunsen in 1860. Aside from gravimetric analysis, another traditional analytical technique is volumetric titration. In this, there is a reagent of known volume and concentration (called a titrant), which is used to react with a solution (called the titrand) of unknown volume. The titrant is added to the titrand in small quantities until the equivalence point is reached (the point at which the quantity in moles of the titrant is equal to the quantity in moles of the titrand). This is mostly used in chemical reactions where acids and bases are involved, and it is easy to tell when the equivalence point has been reached by using an indicator (such as phenolphthalein or litmus). In the instrumental analysis branch of analytical chemistry, there is also thermogravimetric analysis. This involves studying weight changes in relation to temperature, and is most often used to determine characteristics of polymers and other materials.

**Conclusion**
The amount of chlorine ions present in the solution of sodium chloride and silver nitrate is   1.3407843432 * 10^21 molecules. This figure was calculated by first combining aqueous liquids of sodium chloride and silver nitrate to form sodium nitrate and silver chloride through double displacement. AgNO3(aq) + NaCl(aq) --> AgCl(s) + NaNO3(aq) Since the molecule ratio between AgNO3(aq) + NaCl(aq) is 1 : 1, NaCl was the limiting reagent as there was a set amount of salt to begin with. The double displacement formed a solid silver chloride as a precipitate due to silver chloride having a low solubility at room temperature. The silver chloride was then isolated from sodium nitrate and was weighted to be 0.3191grams. The number of moles was found though the equation of n = (Mass/Molecular mass) and was found to be 0.002226475 mol. The number of molecules was calculated from the equation of n = (# of molecules) / (Avogadro's number). The number of molecules was found to be 1.3407843432 * 10^21 and since the mole ratio is 1 to 1, the amount of chlorine ions was identical to the aforementioned figure. In terms of percentage yield, the figure came to be approximately 111.32%. The number was found by subtracting the actual mass from the theoretical mass, dividing by the theoreical mass and multiply it by 100. Usually the number would be below 100% since it’s impossible to obtain more than the theoretical mass or the maximum mass. The reason why the percentage went over 100% was that carbon particles still remained after the burning process and had added weight to the entire compound. This partly explains the percentage error of 11.32% found through this equation. | [ (Mass of Theoretical Yield - Mass of Actual Yield ) / (Mass of Theoretical Yield) ] * 100% |

**Suggested Modifications to the Experiment**

 * Use of quantitative filter paper as opposed to qualitative filter paper
 * Use of a single, more accurate scale
 * Determining the exact amount of Silver Nitrate required to fully react with the salt

**Sources of Error**
In the experiment, several steps that could not have been feasibly avoided may have caused incorrect results with regards to mass, leading to incorrect calculations. On the other hand, some errors could have been avoided. The following sources of error are divided into avoidable and unavoidable: 

Unavoidable Sources of Error

 * 1) Whatman filter paper was used to separate the precipitate (silver chloride) from the aqueous sodium nitrate. The specific filter paper used was qualitative, when quantitative filter paper was required. The qualitative paper used did not disintegrate as much as quantitative paper would, when burned. This had added carbon from the filter paper to contribute to the excess mass of the precipitate.
 * 2) Drops of aqueous silver nitrate were added to the solution until all of the sodium chloride had completely reacted. The only measure of determining if all the salt had reacted was adding more and more silver nitrate until easily visible precipitates of silver chloride had ceased to form. This is extremely inaccurate and it's possible that not all of the sodium chloride had reacted with the silver nitrate.
 * 3) Two difference scales were used to measure the mass of the crucible, lid, reactants, precipitates, etc. One measured to the nearest thousandth of a gram and the other to the nearest hundredth. This shows an immediate incongruence with regards to obtaining the mass. In addition, human error during calibration of the absolue zero mass on each scale seemed to result in slightly different values for each measurement.


    

**Avoidable Sources of Error:**

 * 1) As the procedure required transportation of substances from container to container, it is possible that amounts of the substance may have been spilled or may have escaped from the containers, which would result in different masses. 
 * 2) Measurement of the amount of the amount of reactants used may have been slightly skewed as graduated cylinders were used to estimate a reasonable value for the amount of distilled water and salt.