Stoichiometry
The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C. The number of entities (NA) is 6.0221367 x 1023 and is defined as the Avogadro's constant.
Molar mass is the mass of 1 mole of any element or molecule expressed in grams. Therefore, 1 mole of 12C weighs 12g and contains 6.0221367 x 1023 atoms. Similarly, 1 mole of Li weighs 6.941 g and also contains 6.0221367 x 1023 atoms.
Notice that the molar mass is equal to the atomic mass for elements.
Try this: How many atoms are in 0.551 g of potassium.
We know that the atomic mass of potassium is 39.10 g (check the period table). So 1 mole of potassium weighs 39.1g. How many moles are there in 0.551g? This is equal to 0.551/39.1 = 0.014 moles. Now, using the Avogadro's constant, 1 mole contains 6.0221367 x 1023 atoms, so 0.014 moles will contain 0.014*6.0221367 x 1023 atoms, which equals 8.49 x 1021 atoms. (you might get 8.48 x 1021).
The molar mass of a molecule/compound can be obtained by adding the molar masses of each element in the molecule. This then is called the molecular weight or molecular mass. For example, the mass of one mole of CO2 is equal to the molar mass of Carbon + 2(molar mass of Oxygen); 12+2(16); 44g.
Notice that the number of moles corresponds with the number of atoms present in the compound. So one mole of CO2 contains 2 moles of oxygen and one mole of CO only contains one mole of oxygen. We can take this even further, for example,
How many hydrogen atoms are present in 72.5 g of C3H8O; You should be able to calculate the mass of 1 mole of C3H8O as 60g. (3*12) + (1*8) + 16; How many moles are in 72.5g; 72.5/60 equals 1.21 moles; The number of moles of hydrogen therefore is 8*1.21 which equals 9.67 moles. Now, 1 mole contains 6.0221367 x 1023 atoms, so 9.67 moles contain (9.67*6.0221367 x 1023) atoms. Try this on your calculator and I expect you to get 5.82 x 1024 atoms.
Percent composition: is the percentage composition of each element in a compound. i.e. [(n x molar mass of the element)/molar mass of the compound] X 100%. Where n represents the number of moles of that element in that compound. if there is one mole of the compound, then there are k moles of an element with the formula Ak. For example, in CO2, with a total mass of 44g, the composition of C is [(1*12)/44] X 100%; which should result in 27.2%. The composition of O is [(2*16)/44] X 100% which should result in 72.7%.
A chemical equation uses chemical symbols to show what happens during a chemical reaction. In the formula 2 Mg + O2 -> 2 MgO means 2 moles Mg + 1 mole O2 makes 2 moles MgO, OR 48.6 grams Mg + 32.0 grams O2 makes 80.6 g MgO. A chemical equation is made up of reactants on the left, and the products on the right. An arrow indicates the direction of the reaction.
1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation.
2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts.
3. Start by balancing those elements that appear in only one reactant and one product.
4. Balance those elements that appear in two or more reactants or products.
5. Check to make sure that you have the same number of each type of atom on both sides of the equation.
The limiting reagent is the reactant that is completely consumed by the reaction. Assume you want to create ammonia (NH3), each atom of Nitrogen needs three atoms of Hydrogen. If you have 6 atoms of N, and 24 atoms of H in the mixture, you will only be able to create 6 molecules of ammonia, and there will be 6 atoms of hydrogen remaining unused, therefore Nitrogen in this case will be the limiting reagent. If it happened that we had 15 atoms of hydrogen, then hydrogen would be the limiting reagent.
When you have the balanced chemical reaction, which clearly shows the number of moles needed of each element/molecule, it is possible to identify the limiting reagent by calculating the actual available moles of each reactant as compared to how much of that reactant is needed for the reaction. For illustration purpose;
In the example provided previously for the formation of (NH3), we know that for each mole of (NH3), we need 1 mole (14g) of N, and (3 moles) 3 g of hydrogen. This mole ratio of 1:3 must always remain for us to make (NH3). If we had 126g (9 moles) of Nitrogen and 20g (20 moles) of Hydrogen, then Hydrogen is the limiting reagent. The reaction of 9 moles of nitrogen requires at least 27 (9*3) moles (27g) of hydrogen to react fully.
Theoretical Yield is the amount of product that would result if all the limiting reagent reacted. Actual Yield is the amount of product actually obtained from a reaction. The percent Yield (% Yield) is (Actual Yield / Theoretical yield) X 100%.
To analyze unknown chemical samples we can use both qualitative and quantitative analysis. Qualitative analysis involves identification of actual substances present. Whereas Quantitative analysis identifies the quantity of the substance present.
Colorimetry, which is the analysis by color identifies substances by light emitted, absorbed, or transmitted by the chemical.
Gravimetric analysis uses stoichiometric calculations from a measured mass of a reagent.
Titration analysis uses stoichiometric calculations from a measured solution volume of a reagent.
Gravimetric analysis is the same as the (NH3) example provided under 'limiting reagents'
Gas stoichiometry is the quantitative relationship (ratio) between reactants and products in a chemical reaction with reactions that produce gases. Gas stoichiometry applies when the gases produced are assumed to be ideal, and the temperature, pressure, and volume of the gases are all known. The ideal gas law is used for these calculations. Often, but not always, the standard temperature and pressure (STP) are taken as 0 °C and 1 bar and used as the conditions for gas stoichiometric calculations. Gas stoichiometry calculations solve for the unknown volume or mass of a gaseous product or reactant.
Titration is a common experimental design used to determine the amount concentration of substance in a solution.
Titration curves. (Source: Wikipedia-CC BY-SA 3.0)
Download a pdf copy of Unit 4: Quantitative Relationships in Chemical Changes for offline use.
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