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Alexis Dzikowich

on 13 March 2011

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Transcript of Stoichiometry

STOICHIOMETRY Example 2: moles to moles How many moles of hydrogen are needed to react with 53.8 moles of nitrogen? We will start with the given(53.8 moles of nitrogen)
N (g)+ 3H (g) 2NH 2 3 2 You will need to use this balanced equation
The coefficients (3 and 2) determine the number of moles of that element/compound are used in the reaction 53.8 moles N 1 mole N X 3 moles H 2 1 mole N 2 2 2 = 161.4 moles H 2 Third Example: Grams to Grams using molar mass for elements Lets start from the beginning What is Stoichiometry? First you will need to balance a chemical equation REACTANTS: are any substance that undergoes a chemical change in a given reaction PRODUCTS: are substances obtained from another substance through a chemical reaction Reactants Products When balancing equations you need to balance the coefficients and the subscripts Subscripts: are the numbers that are written smaller than the element symbol, this number is determined by the charge of the elements Coefficients: this number is as large as the element symbol and is the number that can be changed when balancing equations For example: F # This number is the Coefficient and can be changed when balancing # This number is the subscript and CANNOT be altered when balancing When you are balancing an equation, there must be the same amount of a certain element on the reactant side as there is on the product side of the equation. WHY? Because of the law of mass conservation Which states that the mass of the reactants = the mass of the products Subscripts Charges on the periodic table The charges on the table are up at the top, and help you to determin the subscript number These are the charges for the families of the periodic table The subscript balances the charges, in a compound the charge must =0. Example: Copper + Oxygen Copper Oxide Copper =Cu Oxygen=O 2 0xygen is diatomic so when oxygen is by itself there is always a subscript of 2 behind it Cu + O 2 CuO We are not done yet because on the reactant side there is 2 oxygen molecules and on the product side there is only 1 So we must put a coefficient of 2 in front of CuO so the O's are balanced Cu + O 2CuO But now there are 2 coppers on the products side and only 1 on the reactant side so we need to put a coefficient of 2 in front of the Cu 2 So the finished balanced equation is: 2Cu + O2 2CuO 2Cu + O 2CuO 2 Lets get back to stoichiometry! To To To There are 3 types of conversions used in stoichiometry Moles Moles Moles Grams Grams Grams What is a Mole? A mole is the molecular weight of a substance expressed in grams; Grams per mole molar mass 1 Mole Or molar mass 1 Mole When finding the molar mass, you must look at the mass of the element, found on the periodic table, If you are trying to find the molar mass for a compound you must add the molar masses of each element together Example: What is the molar mass of CO ? 2 Molar mass Molar mass The molar mass for Carbon (C) is 12.0107 and the molar mass for Oxygen (O) is 16 times 2, you will just need to add the masses together When you are finding the molar mass you must look to see if there is a subscript, because you will need to multiply the molar mass of that element by that # Oxygen has a subscript of 2 so you must multiply the molar mass of oxygen by 2, The are not used to find the molar mass but the are important Coefficients Subscripts 1. Add the molar masses of CO together
=12.0107 + 16(2)

=12.0107 + 32

= 44.0107 grams/ mole 2 So the molar mass for Carbon Dioxide or CO is 44.0107 grams per mole 2 Now lets use molar mass in a chemistry problem! Example: How many grams are in 2.5 moles of H O? 2 First we will start with the given number in the problem, which is 2.5 moles of H O ( water) 2 2.5 moles H O 1 2 You will then multiply this by the molar mass of water molar mass for H O 2 Molar mass 1. Add the molar mass of (O) and (H )
=1.00794(2) + 15.99
=2.01588 + 15.99
=18.00588 grams / mole We will take this molar mass and multiply it by our given number- 2.5 moles of H O 2 How many grams are in 2.5 moles of H O? 2 2.5 moles water 1 X 1 mole water 18.00588 grams/ moles The unit you are trying to find must always be on top in the last step of your problem The units that need to be cancelled out have to be diagonal from eachother In this example you are trying to find grams, so grams have to be on top In this example you want moles to cancel out so the molar units have to be across eachother 2.5 moles water 1 1 mole water 18.00588 grams/ mole X = 45.0147 grams of water in 2.5 moles of water What does this mean? This means that for every 1 mole of water there is 18.00588 grams of water, so if you want to know how many grams are in 2.5 moles of water you multiply the by the number of moles given grams per mole So in 2.5 moles of water there is 45.0147 grams-of water Lets do another example How many moles of aluminum sulfate are in 100.0 grams? Aluminum= Al Sulfate= SO 4 Sulfate is made of one Sulfur atom and 4 Oxygen atoms, so when finding the molar mass you must find the mass of each and add them to Aluminum's molar mass 1. Add the molar masses AlSO (Aluminum Sufate)
=16(4) oxygen + 32.065 sulfur + 26.982 aluminum
=123.047 grams/ mole 4 How many moles of Aluminum Sulfate are in 100.0 grams? 100.0 grams AlSO 4 1 X 123.047 grams 1 mole AlSO 4 1 Mole is on top because that is the unit you are trying to find = 100 grams divided by 123.047 grams = 0.81269 moles of Aluminum sulfate in 100 grams of AlSO 4 Now for the big stuff Example 1: How many grams of sodium cholorate must be decomposed to form 2.5 moles of oxygen? Equation (unbalanced): NaClO O + NaCl 3 2 Balance the equation The balanced equation is : 2NaClO 3O + 2NaCl 3 2 Start with the given number, which is 2.5 moles 2.5 moles of O 2 1 X 3 moles of O 2 moles NaClO 2 3 In this step you must go from the element you start with to the element you need to find, the coefficients infront of the element symbols in the balanced equation are used here 2 NaClO 3O + 2NaCl 2 3 The coefficients represent the # of moles of that substance that react in the equation Next you have to find the molar mass of NaClO , so you can find how many grams were used 3 NaClO 3 1. Add the molar masses
= 22.99 grams Na + 15.99(3) grams O + 35.45 grams Cl
=22.99 +47.97 + 35.35
=106.41 grams / mole Now insert this molar mass into the equation 2.5 moles O 1 2 X 3 moles O 2 moles NaClO 2 3 X 106.41 grams NaClO 1 mole NaClO 3 3 Remember you need to put the units you want to cross out diagonal from eachother Put the molar mass on top because you need to find grams , and molar mass is in grams = 177.35 grams NaClO 3 Sodium Chlorate
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