Lab Report 8 stoichiometry PDF

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Learning the basics stoichiometry is essential in chemistry to make sure that the right relative
amount of a certain chemical can be found in the medicine that they dispense. From determining
the amount of mg/ml to finding the amount of reactant a reaction needs to proceed can be deeme...

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Technological Institute of the Philippines College of Information Technology Information Technology Department

CHM 001A - IT22S2 Chemistry

LABORATORY ACTIVTIY NO.8 STOICHIOMETRIC CALCULATIONS: IDENTIFY AN UNKNOWN COMPOUND USING GRAVIMETRIC ANALYSIS

Group No.: 1 Group Members: Enriquez, Mhar Vincent (Leader) Aban, Vincent Estoesta, April Mae Galinato, Frances Louise Magparangalan, Joyce Manali, Sittie Asmah Tiu III, Benjamin Zaguirre, Froilan Jesus

Submitted to: Prof. Anne Marie Valdez

Date Performed: April 17, 2021 Date Submitted: April 22, 2021

I. OBJECTIVES: 1.1 To learn about Avogadro’s number and how the molecular weight, mass and number

of molecules correlate.

II. INTENDED LEARNING OUTCOME: The students shall be able to: 2.1 Explain the relationship between mass, molecular weight, and numbers of atoms or molecules and perform calculations deriving these quantities from one another 2.2 Perform mass-to-mass stoichiometric calculations via conversions to mole -Identify the limiting and excess reagents in a chemical reaction 2.3 Calculate the theoretical, actual and percent reaction yield 2.4 Define Avogadro’s number and describe the mole quantification of matter

III. DISCUSSION: Learning the basics stoichiometry is essential in chemistry to make sure that the right relative amount of a certain chemical can be found in the medicine that they dispense. From determining the amount of mg/ml to finding the amount of reactant a reaction needs to proceed can be deemed part of stoichiometry. Identify an unknown compound. In order to identify a compound where the label has been partly destroyed, you must apply the technique of gravimetric analysis. To do so, you must first learn to understand the relationship between mass, moles and molecular weights and how to perform stoichiometric calculations from mass to mass via conversions to mole. Stoichiometric calculations with moles You will perform a realistic gravimetric analysis with detailed instructions on what to do and why to do it in every step of the experiment. From balancing the equation to recognizing the stoichiometry of the reactants and finding out which equation to employ in the calculations, the theory behind the experiment is explained step-by-step in the order of the experiment. What compound is it? At the end of the simulation, you will have finalized all of the stoichiometric calculations and the answer to the question should be clear… Can you see what compound it is?

IV. MATERIALS: • Gloves • Spatula • Analytic Balance • Weighing Dish • Unlabeled Chemical XCI2 • 1m AgNO3 • H2O • Heating Plate • Filter Paper • Buchner Funnel • Rubber cone

• • • • • • •

Conical Flask 500ml Vacuum Tube Vacuum Valve Water Glass Oven 100ml Measuring Cylinder Magnetic Stirrer

V. PROCEDURE: Refer to Labster: Stoichiometry Simulation Manual https://theory.labster.com/welcome_sto/

VI. DATA AND RESULT: The Alkaline Earth Metals The elements of the main group 2 of the periodic table are called the alkaline earth metals. The constituents of the group are Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra). Common for the elements of this group is that they are metals in their pure form, with low densities and low melting and boiling points. They all react with halides (F, Cl, Br, I) to form metal halides. Since the number of valence electrons of the alkaline earth metals is 2, the general formula of the formed metal halides is MX2, where M is the metal and X is the halide. All of the alkaline earth metals occur naturally, although Radon only occurs as a decay product from Uranium or Thorium, and is highly radioactive. Magnesium and Calcium are both among the top-8 most abundant elements on the earth’s surface.

Balanced chemical equation A chemical equation shows what happens in the Reactant → Product chemical reaction. The basic chemical reaction can Substance A + Substance B → Substance AB be written as follows:

In a balanced equation, the total number of atoms of each kind (e.g. A) in the reactants and products is the same. The relationship between the different components of the reaction is referred to as the reaction stoichiometry.

Working example: Na + O2 → Na2O

Unbalanced equation

Balanced equation

The reaction above, between sodium and oxygen, is not balanced. We need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrows. 4Na + O2 → 2Na2O The equation is now balanced. Be sure to double check the number of atoms on the left side of the arrow matches the one on the right.

How to balance a chemical equation When balancing a chemical equation, you change the coefficients in front of each compound. It is most common to use only integers, but you can also use fractions if you like. Like in a mathematical equation you can always just multiply with the same number throughout the equation and obtain integer coefficients that way. Be aware, that you are not allowed to change the numbers in subscript. These are part of the chemical formula, and cannot be changed. Example: On the images, you see the reaction between Hydrogen molecules, H2, and Oxygen molecules, O2, to form water, H2O. When you count the number of atoms on either side of the arrow in the equations on the first image, you should reach the conclusion that the equation is not balanced: On the left-hand side we find two hydrogen atoms and two oxygen atoms, whereas on the right-hand side we find two hydrogen atoms and only one oxygen atom. In order to balance the equation above, we need to add more water molecules to the right-hand side. See the equation on the second image. Now the number of oxygen atoms are the same on both sides, but now there are four hydrogen atoms on the right-hand side and only two on the left-hand side. We need to add another hydrogen molecule to the left-hand side. Now the equation is balanced, see the third image.

Reaction Stoichiometry The relationship between the different components of a reaction is referred to as the reaction stoichiometry. When the chemical equation is balanced it tells you, on a numerical basis, how many of each molecule reacts: For example, two molecules of hydrogen reacts with one molecule of oxygen to form two molecules of water.

Gravimetric analysis Gravimetric analysis is a technique to determine the amount of an analyte based on mass. A common example of a gravimetric analysis is to determine the amount of chloride in a solution - for example, to determine the amount of salt in seawater, or to determine the mass of the counter-ion of an unknown chloride compound. But the analyte could also be another ion. The analyte is reacted with a counter-ion with which it forms an insoluble compound, which can then be isolated and weighed. From the mass of the precipitate the amount of the analyte can be calculated. If the analyte is the chloride ion, then the counter-ion could be silver, Ag+, because AgCl is insoluble in water. In order for the gravimetric analysis to be valid, we want all of the chloride ions to precipitate. This we obtain by letting chloride be the limiting reagent and the silver ions being in excess. In other words: There should be more silver ions than chloride ions. It is equally important that silver ion is introduced via a compound that is highly soluble in water so that the only precipitate is the silver chloride. The typical choice would be silver nitrate, AgNO3: NaCl (aq) + AgNO3 (aq) --> Na+ (aq) + NO3- (aq) + AgCl (s)

➢ Analyte An analyte is the substance or compound being analyzed for in a chemical analysis. ➢ Counter-ion A counter-ion is an ion with an opposite charge, which constitutes the other half of an ionic compound. For example, in table salt, NaCl, the counter-ion to Na+ is Cl-.

Moles and Avogadro's number A mole is defined as the number of atoms present in 12 grams of 12C. Since the atomic mass of 12C is 12 amu, and 1 amu is 1.66x10-24 g, then 12 grams of 12 C must contain 6.022x1023 atoms:

This number is also referred to as Avogadro’s number in recognition of the great Italian chemist Amedeo Avogadro. It follows from this that since the mass of one atom of 12C is 12 amu, then the mass of one mole of 12C is 12 grams. The latter is called the molar mass and its unit is grams per mole (g/mol or gᐧmol-1). This is true for every atom in the periodic table: The mass stated in the periodic table is the mass of one atom in amu, but it is also the mass of one mole of those atoms in grams. Examples: The molar mass of Iron (Fe) is 55.845. It means that one atom of iron weighs 55.845 amu and that one mole of iron weighs 55.845 g. The molecular weight of water is 18.015. It means that one molecule of water weighs 18.015 amu and that one mole of water weighs 18.015 g.

How to count molecules Molecules are immensely small and two molecules of water are really not much. Suppose you would like to describe a reaction forming one drop of water. A single drop of water contains 1,670,957,400,000,000,000,000 or 1.67x1021 water molecules. We know from the balanced chemical equation that it takes the same number of hydrogen molecules but only half that number of oxygen molecules for the reaction to occur. So, we need 1.67x1021 hydrogen molecules and 8.35x1020 oxygen molecules. When counting molecules, we are dealing with particles so tiny that they cannot be seen, even with a microscope, and with numbers so big that it is impossible to count. Therefore, atoms and molecules are counted by mass, and in very large numbers called moles. Just like eggs are often counted in dozens, so atoms and molecules are counted in moles. Don’t let the name confuse you, a mole is just a number.

Atomic mass As the mass number reflects the sum of protons and neutrons in the nucleus of an atom, the Atomic mass, reflects the weight of these particles in the nucleus. As the weight of a proton and a neutron is almost equal to 1 amu in every atom, the atomic mass and mass number of an atom are almost the same.

Calculating molecular weights The molecular weight Mw of a molecule can be found by adding the atomic weights of all the atoms that molecule is made of.

Let’s take Sulfuric Acid as an example. The formula of Sulfuric Acid is H2SO4. To calculate the molecular weight we take:

The relationship between M, m and n The number of moles, n, of a substance can be found by using the following equation

n(mol) = m(g) / M(g/mol) where m is the mass and M is the molecular weight of the given substance. The molecular mass, M or Mw, is the mass in grams per one mole (g/mol or gᐧmol-1) of a substance. It can be calculated from the atomic weights of its constituent atoms, but when you look at the unit (g/mol) you can see that it can also be calculated if you know the mass of a substance (g) and how many moles that corresponds to (mol): M(g/mol) = m(g)/ n(mol) m(g) = n(mol) * M(g/mol) Limiting and excess reagents In the chemical equation where we form water from hydrogen and oxygen, we can see that hydrogen and oxygen reacts in a 2:1 ratio. This means that if you have two moles of hydrogen molecules and one mole of oxygen molecules, you have stoichiometric amounts of the reagents, and you form two moles of water. When you only have one mole of hydrogen molecules, then we say that hydrogen is the limiting reagent, and that we have an excess of oxygen. You can only form as much product as the limiting reagent, so in this case you can only form one mole of water molecules.

Theoretical, experimental and percentage yield a) The theoretical yield of a reaction is the amount of product you would get if you use up all of the limiting reagent, and if there is no loss, e.g. by degradation of reactant or product or by formation of byproducts. b) The experimental yield is the actual amount of product you obtain when performing a given experiment. c) The percentage yield is a calculation of how large a percentage of the theoretical yield you obtained in a given experiment. It is calculated by:

Conversion between moles and molarity When a compound is dissolved in a solvent the concentration of the compound is usually given in moles per liter (mol/l or mol·l-1) which also called molarity, M. To calculate how large a volume, V, of a solution with a known concentration, c, to use in order to obtain a certain number of moles, n, the following equation is used:

The equation can be rearranged to isolate for n or c if it should be desired.

Experimental thoroughness and accuracy • •

When performing chemical experiments in the laboratory a great deal of thoroughness and accuracy is necessary if you want to obtain reliable and reproducible results It is important when you are performing a reaction that you measure accurate amounts of reactants to ensure the highest possible conversion, and thoroughness when you isolate your product, to minimize the loss and achieve the highest possible yield.

Below are some examples of things to be aware of: a) Contamination with other chemicals ➢ You should always make sure that the glassware and equipment you apply is clean. ➢ The chemicals and solvents in the lab can easily be contaminated as well. To avoid that it is important that you always use clean spatulas for dipping into solids, and clean pipettes for liquid chemicals. b) Causes for deviating yields The yield is lower than the expected yield. It could be that: ➢ You lost some product in transferring from one container to another. ➢ One of your reagents were diluted or contained an impurity, so you didn’t add as many moles as you thought you did. ➢ The reaction was not complete when you isolated the product. ➢ Some byproducts were formed. ➢ Some degradation of your reactants or products has occurred. ➢ You made an error somewhere in your calculation of moles and masses. The yield is higher than 100 %. It could be that: ➢ Your product still contains some solvent (not completely dry). ➢ You forgot to tare the container before putting your product in there. ➢ You made an error somewhere in your calculation of moles and masses. Suction filtration ▪ ▪ ▪ ▪ ▪

Place a rubber cone in the mouth of the suction flask. Place the Büchner funnel in the rubber cone. Place a filter paper inside the Büchner funnel. Turn on the vacuum. Pour a little of the solvent onto the filter to make it stick to the funnel.

▪ ▪

Pour the mixture to be filtered into the funnel. After all the liquid has been sucked through, wash the filter cake with at little solvent.

VII. QUESTIONS: 1. Atoms and molecules are way too small to be seen, even with a microscope, so how can we count how many there are? - You count them by mass

2. In which of the pictures, do you find the most particles? By particles I mean, sand grains, water molecules and iron atoms - There are roughly the same amount of particles on each picture

3. Why is it important to use demineralized water? - Because demineralized water doesn’t contain minerals

4. In order for the gravimetric analysis to be valid, we want all of the chloride ions to precipitate. How can we ensure this? - By ensuring silver ions are in excess

5. Look at the reaction schemes in your journal. How does the number of moles of AgNO3 correlate to the number of moles of the alkaline earth metal chloride? - There should be twice as many moles of AgNo3 as XCl2

6. How many moles of AgNO3 should we use to be sure that we have excess, no matter which of the three compounds it is? - 0.100 mol

7. Look at your lab journal. Can you see from the reaction stoichiometry how the number of moles of AgCl formed correlates to the number of moles AgNO3 consumed? - The number of moles are the same

8. Which of the compounds do you think it is? You were very accurate with the experimental procedure, so I would expect the yield to be more than 90%. - MgCl2

9. Which of the following answers in NOT a valid reason for the percentage yield of the reaction being below 100%? - The product is not completely dry

VIII.CONCLUSION:

In this Labster simulation, we learned about the Stoichiometric Calculations. We got to identify an unknown compound by using gravimetric analysis. But before that, we first learned the relationship between mass, moles and molecular weights and we performed a stoichiometric calculation by converting it to mole. We also did a gravimetric analysis with detailed instructions on what do with the experiment. We balanced the equation to recognize the reactions and then we find out which equation to use it with the calculations. We calculated the theoretical, actual and percent reaction yield....