Avogadro’s constant and the mole

Mass on its own is not very useful because when reactions take place they depend on # of atoms not their mass. Atoms are way to small and numerous to count so you need a way to relate # of atoms to masses that can be measured In science it is important to measure amounts accurately and conveniently

The Mole People find it easier and useful to work in groups when counting Pair=2 Dozen=12, Gross=144 To count the number of atoms a dozen or gross isn’t large enough since elements contain a huge # of atoms Instead chemist use the mole (symbol mol)

The Mole The amount of substance that contains 6.02214199X1023 particles(atoms, molecules, formula units) of that substance This is called Avogadro Constant Has the symbol NA Rarely need 9 sig figs when working with the number so 3 is enough NA=6.02X1023mol-1

Examples One mole of carbon contains 6.02X1023 carbon atoms One mole of Magnesium Chloride contains 6.02X1023 formula units of MgCl Etc

Application When someone ask you to buy them 2 dozen eggs, they are also thinking of 24 eggs 2dozen X 12eggs=24eggs 1dozen You can convert between moles and particles in the same way 2molX6.02X1023atoms=1.20X1024atoms mol

Converting moles to # of particles Moles are used to help us count atoms and molecules Relationship can be written using formula below N=n X NA # of particles (atoms, molecules, formula units) Multiply by # of moles 6.02X1023 Where: N=# of particles n= amount(mols) NA= Avogadro's constant(mol-1)

Converting # of particles to moles Usually express in moles To convert # of particles in a substance to number of moles, just rearrange the formula learned on pervious slide N= n X NA rearranges to n= N/NA Since there are far too many particles to work with we usually express the amount of substances in moles

Example How many moles are present in a sample of potassium floride, KF, made up of 5.83X1024 molecules? Solution: To solve we use Avogadro constant and the number of molecules given and substitute into equation: n= 5.83X1024 molecules KF 6.02X1023 molecules KF/mol KF n=9.68mol KF So there are 9.68 moles of KF in the sample

Converting from moles to mass Suppose you were asked to weigh out three moles of a substance In order to do so you must figure out how much 3 moles is in grams to use the scale The formula below relates moles and mass by molar mass Mass=#of moles X molar mass m= n X M where m=mass n=# moles M=molar mass

Example A flask contains 0.750 mol of carbon dioxide gas. What mass of CO2 gas is in this sample? First we must find the molar mass of CO2 to use in the equation along with the # of moles that was given 1Carbon + 2Oxygen 12.01g/mol+2(16.00g/mol)=44.01g/mol Using the equation m=n X M m= 0.750molX44.01g/mol m=33g of CO2 in 0.750mol of CO2

Converting from mass to moles Opposite of moles to mass Use formula below Mol= mass or n= m molar mass M

Example How many moles of acetic acid, CH3COOH, are there in a 23.6g sample? First need to find molar mass: There are 2C, 2O, and 4H M=2(12.01g/mol)+(2(16g/mol)+4(1.01g/mol) M=60.06g/mol Next substitute values into equation n=m/M n= 23.6g n= 0.393mol of CH3COOH in 60.06g/mol 23.6g of CH3COOH

Time to practice!