Volumetric analysis+-+How+Much?

THE MOLE AND MOLAR MASS
media type="youtube" key="xPdqEX_WMjo" height="315" width="420" When carrying out a reaction it is generally important to know how much is reacting. A balanced equation tells the ratio in which species will react. 2H2 + O2 ®  2H2O tells us that 2 molecules of hydrogen react with 1 molecule of oxygen to produce 2 molecules of water. Hydrogen and oxygen will always react together in a numerical ratio of 2 : 1.
 * The Mole **

If we don't want unused reactants then we must ensure we have counted out the correct number of molecules. Clearly atoms, molecules and ions are far too small for us to count e.g. 1 teaspoon of copper sulfate contains 3 x 1022 ions. Instead we measure materials by mass, just as a banker weighs out large numbers of coins knowing exactly how many coins correspond to1 kg.

Rather than count in millions or trillions, chemists use a much larger number called a **MOLE (abbreviated to mol, symbol n).** **(also called Avogadro's number). ** 1 mole is the number of atoms in 12 g of pure carbon-12 (about a handful of the solid).
 *  1 mole = 6.02 x 10 23 **
 * NB: ** When a question refers to **an amount** of a substance it generally means the number of moles.

If the actual number of particles is known (atoms, ions or molecules) then the amount, n, is easily calculated since
 * Converting number of particles to amount, n **
 *  n = __number of particles__ **
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 6.02 x 10 <span style="font-family: Arial,sans-serif; font-size: 11pt; vertical-align: super;">23 **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">For example 12 x 1024 sodium ions is equivalent to __12__ __<span style="font-family: Arial,sans-serif; font-size: 11pt;"> x 10 __<span style="font-family: Arial,sans-serif; font-size: 11pt; vertical-align: super;">24 <span style="font-family: Arial,sans-serif; font-size: 15px;">mol = 20 mol of sodium ions.

<span style="font-family: Arial,sans-serif; font-size: 11pt;"> 6.02 x 10 <span style="font-family: Arial,sans-serif; font-size: 11pt; vertical-align: super;">23

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The formula of a compound tells the ratio of moles of atoms or ions in a sample. e.g. since 1 molecule of H2SO4 contains 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms, then 1 mol of H2SO4 <span style="font-family: Arial,sans-serif; font-size: 11pt;">molecules contains 2 mol of hydrogen atoms, 1 mol of sulfur atoms and 4 mol of oxygen atoms. <span style="font-family: Arial,sans-serif; font-size: 11pt;">When considering an ionic solid we may consider either the individual atoms from which the <span style="font-family: Arial,sans-serif; font-size: 11pt;">compound is made, or alternately the ions <span style="font-family: Arial,sans-serif; font-size: 11pt;">2 mol Na2CO3.10H2O contains 2 x 2 = 4 mol of Na+ ions, <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2 x 1 = 2 mol of CO32 - <span style="font-family: Arial,sans-serif; font-size: 11pt;"> ions, <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2 mol of carbon atoms, 2 x 10=20 mol water molecules, <span style="font-family: Arial,sans-serif; font-size: 11pt;"> or 2 x (3+10) = 26 mol of oxygen atoms!
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Moles of particles from a given formula. **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">A balanced equation gives the ratio of the number of moles of reactants to products. <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2Mg + O2 ® <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2MgO <span style="font-family: Arial,sans-serif; font-size: 11pt;">2 mol of Mg react with 1 mol of O2 to produce 2 mol of MgO. <span style="font-family: Arial,sans-serif; font-size: 11pt;">Similarly 0.50 mol of Mg reacts with 0.25 mol of O2 to produce 0.50 mol of MgO. <span style="font-family: Arial,sans-serif; font-size: 11pt;">Clearly the molar ratio of O2 : MgO is 1:2 while the molar ratio of Mg : MgO is 1:1.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Mole Ratio in a Balanced Equation **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">When doing a chemical reaction it is not possible to count out large numbers. Rather we weigh out the sample and use a quantity called the **molar mass.** Molar mass**, //M//**, is the mass of 1 mol of any substance, pure element or compound. (It has units of grams per mole, g mol-1.) <span style="font-family: Arial,sans-serif; font-size: 11pt;"> //M// = where m is the mass in grams, and n is the number of moles.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Molar Mass **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The equation can be rearranged and written in the forms
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;"> n = ****<span style="font-family: Arial,sans-serif; font-size: 11pt;"> or m = n x //M// **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The molar mass of an element is known and can be looked up in a data table. It has the same numerical value as the atomic mass number of an element e.g. the molar mass of oxygen = 16 g mol-1 and the atomic mass of O is 16.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">(**Note:** For oxygen gas O2, molar mass = 32 g mol-1). <span style="font-family: Arial,sans-serif; font-size: 11pt;">If the formula of a compound is known then the molar mass of the compound is readily calculated. <span style="font-family: Arial,sans-serif; font-size: 11pt;">hence the molar mass of CuSO4 = 63.5 +32 + (4 x 16) = 159.5 g mol-1 <span style="font-family: Arial,sans-serif; font-size: 11pt;">and the molar mass Cu(NO3)2 = 63.5 + 2 x ( 14 + 3 x 16) g mol-1 = 187.5 g mol-1 //<span style="font-family: Arial,sans-serif; font-size: 11pt;">Exercise 1: Using data tables calculate the molar mass, M, of the following: // //<span style="font-family: Arial,sans-serif; font-size: 11pt;">(a) H2SO4 (b) CH3COOH // //<span style="font-family: Arial,sans-serif; font-size: 11pt;">(c) Ca(OH)2 (d) Na2CO3.10H2O //
 * //<span style="font-family: Arial,sans-serif; font-size: 11pt;">M //****<span style="font-family: Arial,sans-serif; font-size: 11pt;"> (g mol-1) **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> Cu = 63.5 O = 16 N = 14 S = 32

<span style="font-family: Arial,sans-serif; font-size: 11pt;">If you know the mass of a pure substance and the correct formula, it is possible to calculate how many moles in the sample. <span style="font-family: Arial,sans-serif; font-size: 11pt;">10 g of CuSO4 is = 0.063 mol (n = ) //<span style="font-family: Arial,sans-serif; font-size: 11pt;">Exercise 2: Using the M values calculated above find the number of moles in each of the following: // //<span style="font-family: Arial,sans-serif; font-size: 11pt;"> (a) 100 g CH3COOH (b) 5 g Ca(OH)2 (c) 20 g Na2CO3.10H2O //
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Calculating the number of moles in a sample given the mass and vice versa. **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">It is similarly possible to calculate what mass corresponds to a given number of moles. <span style="font-family: Arial,sans-serif; font-size: 11pt;">e.g. 2 mol of Cu(NO3)2 weighs 2 x 187.5 = 375 g (m=n x //M//) //<span style="font-family: Arial,sans-serif; font-size: 11pt;">Exercise 3: What is the mass of each of the following amounts: // //<span style="font-family: Arial,sans-serif; font-size: 11pt;"> (a) 2.2 mol H2SO4 //

//<span style="font-family: Arial,sans-serif; font-size: 11pt;"> (b) 5 mol KMnO4 //

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Sometimes information is provided in terms of the relative atomic mass, //A//r, of an element rather than its molar mass. This has exactly the same numerical value but has no units e.g. //Ar// of oxygen is 16 and the //M// of oxygen is 16 g mol-1.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Relative Atomic and Molecular Masses, //A//r and //M//r **


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Note: **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> The relative atomic mass is, in fact, the mass of an atom relative to the mass of an atom of the isotope carbon -12 which has a mass of 12. This means an atom of oxygen-16 is [[image:https://skcchemistry.wikispaces.com/site/embedthumbnail/placeholder?w=200&h=50 width="200" height="50"]] times heavier than an atom of carbon-12. The relative molecular mass, //M//r, is the mass of a molecule relative to carbon-12. e.g. //M//r of CuSO4 is 159.5, the same numerical value as the molar mass but having no units, (//M// = 159.5 g mol-1).