Charcoal Analysis and Percentage of Water in a Hydrate, Chemistry tutorial

Introduction:

Fossil fuels symbolize a big portion of the world's energy supply. They as well serve as an economical source of raw materials for the manufacture of products like plastics, synthetic fabrics and medicines. A very significant property of a fuel is its heat of combustion which is the amount of heat (that is, calories, joules or BTU's) discharged per amount of fuel (that is, barrels, cubic feet or tons). Energy rating of the fossil fuel provides us information regarding its chemical composition. However charcoal is not a fossil fuel, however amorphous carbon from the incomplete combustion of animal or vegetable matter, its chemical composition can be examined in the similar fashion as fossil fuels. The average person is more familiar by charcoal as the energy source for cooking on outdoor grills.

Procedure:

A) Moisture Content:

Support a clean, empty crucible and lid on a clay triangle by employing a ring stand. Heat the crucible by a hot burner flame for some minutes to burn off any impurities. Handle the crucible and lid by crucible tongs for the remainder of the experiment. Move the crucible and lid to the carrying tray, allow cooling and weighing to the closest 0.001 g. Add around 1 g of charcoal to the crucible and weigh again. The lid might be weighed separately as its use is not needed in the first part of the procedure. Place the crucible in a 100 ml beaker marked for simple identification and put in a drying oven for 1 hour. Let crucible to cool and reweigh. The weight loss symbolizes the moisture content of the charcoal.

B) Volatile Combustible Matter, VCM:

Now support the crucible and dried charcoal in the clay triangle and cover by the lid leaving a slight opening at one side.

At first, heat the apparatus quietly, steadily increasing to a hot flame for some minutes. Use tongs to close the lid fully and let the sample to cool to room temperature. Before weighing the crucible, observe the upper part of the crucible and the lid for deposits and discolorations. If deposits emerge on the lid, heat it directly in the flame. If deposits appear on the crucible, cautiously heat the upper part of the crucible devoid of the lid and avoid heating the charcoal sample. Weigh the crucible, lid and remaining sample, whenever cool, to the closest 0.001g.

The weight loss is the VCM evolved throughout the heating process.

C) Ash and Fixed Carbon:

Support the crucible in the clay triangle and partly cover it with the lid. Heat the sample intensely and if the sample ignites, make use of the tongs to cover the crucible until flame is extinguished. Carry on intense heating till no black residue remains on the lid or in the crucible. This will need that you periodically make use of the tongs to rotate the crucible's position in the flame. The residue in the crucible must appear to be a light gray-brown color whenever the combustion has been completed. Cool the crucible, lid, and contents and weigh. The residue remaining in the crucible is the ash content. The weight loss after the VCM elimination is fixed carbon content. Repeat the whole process for a second trial.

Requirements for Report:

Tabulate the data collected in the experiment and compute the percentages of ash, fixed carbon, moisture and VCM from data. Pay concentration to significant digits in the data and reported numbers. Comment on the inconsistency or consistency of values from the two trials.

Percentage of water in Hydrate:

Most of the pure substances join with water in a fixed mole ratio to yield compounds known as hydrates. For illustration - zinc sulphate combines with water to form crystalline ZnSO4.7H2O which is a stable compound at normal atmospheric conditions. All the pure samples of this hydrate exhibit the similar percentage of water by analysis. Therefore, this hydrated compound follows the law of constant composition. On heating a sample of such a hydrate, it might lose all its water of hydration and revert to the anhydrous salt. Substances that have adsorbed water on the surface don't exhibit constant composition and thus are not hydrates. An illustration of this would be common table salt that becomes very sticky on humid whenever the relative humidity is high. In such cases, the percentage of water is not constant for all the samples of a specific compound and the water is not chemically bonded as portion of the crystal structure.

Theory of percentage of water in hydrate:

Hydrates are crystalline salts which are bonded to water molecules in a definite proportion. The weakly bound water is acknowledged as either the water of hydration or water of crystallization. The fixed numbers of water molecules which are weakly bonded to the salt are symbolized as follows:

Salt • number of waters

Zinc sulphate heptahydrate

(ZnSO4 • 7 H2O)

The dot symbolizes the weak salt • water bond in the chemical formula. The bond is so weak that simply heating the hydrated salt to discharge the water molecules as vapor can in general break it. Whenever water is added to the now anhydrous salt, the reverse will occur with the waters reattaching themselves to the salt. This is recognized as a reversible action.

There are three closely associated substances which act similar to hydrates however distinct individual characteristics have. These are hydroscopic, deliquescent and efflorescent substances. 

a) Hydroscopic substances:

It readily absorbs moisture from the air and is employed as drying agents (like desiccants).

b) Hygroscopy:

It is the capability of a substance to attract and hold water molecules from the surrounding atmosphere.

c) Deliquescent substances:

It continues to absorb water from the air till they form a solution.

d) Deliquescence:

It is the method in which the soluble substance picks up water-vapor from the air to form a solution.

e) Efflorescent substances:

These are the hydrates which lose water whenever simply exposed to the atmosphere.

f) Efflorescence:

(The term signifies 'to flower out' in French) is the loss of water (or a solvent) of the crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.

As formerly illustrated, each and every hydrated salt has water molecules bonded to them in definite proportions. The percent water in the hydrated salt can be computed, theoretically, employing the chemical formula of the hydrate.

% H2O = [{(number of H2O molecules)(molecular mass of H2O)}/(molecular mass of hydrate)] x 100

Procedure:

Clean and dry the porcelain crucible and cover. Put the empty, covered crucible on a clay triangle supported via a ring on a ring stand. Heat the crucible and cover in the hottest flame of the Bunsen burner for 5 minutes. A dull red glow should be observed on the crucible and cover. This will require that you manipulate the burner to heat uniformly. This will make sure that all the combustible and volatiles materials are eliminated prior to the analysis method and that a constant weight for the crucible and cover might be recorded. The crucible and cover should be permitted to cool to laboratory temperature for around 15 minutes. By using crucible-tongs transfer the crucible and cover to a carrying tray and weigh them to the closest 0.001 g. Now add 1 to 1.5 g of zinc sulphate heptahydrate to the crucible and weigh the covered crucible to the nearby 0.001g.

Place the covered crucible on the clay triangle by the cover slightly opened. Heat the crucible gently for some minutes to avoid loss of material from spattering throughout initial heating. Keep in mind the contents for around 15 minutes steadily increasing the temperature of the flame. Let the crucible to cool on the triangle after removing the flame till it has reached the room temperature. 

Transfer it to carrying the tray and weigh the covered crucible to the nearby 0.001 g. Reheat the crucible and contents for around 5 minutes and, after cooling, weigh it again. Repeat this heating, cooling and weighing process till two consecutive weighing is within 0.005 g. This process of heating, cooling and weighing to get consistent results is acknowledged as heating to a constant weight.   

That water boils at around 100oC. Continue to heat the crucible and compute the real percentage of water in the zinc sulphate by using the gram formula weight of the hydrate and the weight of water pointed out in the formula multiplied by 100. Compute the experimental percentage of water in your hydrate based on water lost as an outcome of heating divided by the hydrate sample weight multiplied via 100. To estimate your results and find out how well you did the analysis, you can compute relative error by using the given formula.    

[(Actual value - Experimental value)/(Actual value)] x 100 = Percent of relative error.

The absolute relative error of 3% or less is excellent. If the percent of error is more than 3%, think about the factors which you noticed throughout the process that might have influenced your results. Change your actions accordingly for the unknown hydrate sample. Get an unknown hydrate sample from your instructor. Repeat the method which you used on the acknowledged hydrate. Do a minimum of two trials and report the average percentage of water in your unknown hydrate. Do not remember to record your sample number and/or letters.         

Requirements for Report:

Data tables must be constructed for all the hydrate trials in such a way that the information is readily found in the report. One sample computation should be illustrated for a completed trial. Record all the observations and adjustments made to enhance your method.

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