Formulas and equation
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1. This document discusses writing chemical formulae and balancing chemical equations. It explains how relative atomic mass is used to compare the masses of different atoms by using carbon-12 as the standard. 2. Isotopes are discussed, which are atoms of the same element that have different numbers of neutrons and masses. The average atomic mass of an element takes into account the different isotopes and their abundances. 3. It also explains how the mole concept relates grams of a substance to numbers of atoms or molecules using Avogadro's number. Empirical formulae are derived from determining the ratios of elements that combine in a compound.
Formulas and equation
- 1. WritingFormulae and Balancing Equations The massesof atoms Relative AtomicMass The mass of an atom is tiny. A single hydrogenatomisonlyabout 0.000 000 000 000 000 000 000 002 grams! Dealingwithsuchsmall numbersisdifficult.Therefore, scientistsfoundasimplerwayof comparingthe mass of differentatoms. Theychose the carbon atom and comparedall the otheratomswithit. Since a carbonatom consists of 6 protonsand 6 neutrons,theygave ita massof 12 units(theyignoredthe electrons). The mass of an atomrelative tothat of carbon-12 is calledthe Relative AtomicMass. If we compare the mass of a hydrogenatomwiththat of the carbon-12, we finditsmassis a 1/12th of the carbon-12 atom. Therefore,a hydrogenatom isassignedthe mass unit 1. Whena magnesiumatomwascompared,itsmass wasfoundto be twice that of carbon-12, therefore magnesiumwasassignedthe massunit 24. Massand Isotopes Many elementspossessatomswith differingmassesdue to them having differentnumbers of neutrons. The atoms shownabove belongtothe isotopesof chlorine.Theyhave differentmassesbecauseone has two more neutrons than the other. Also itwas found that chlorine-35 is more abundant than chlorine-37. For everyfourchlorine atoms,one will be achlorine-37the otherthree will be chlorine-35.Therefore, the average massof a chlorine atomis35.5. The presence of isotopesandtheirabundancesmustbe takenintoaccountwhencalculatingRelative AtomicMass (RAM). The RAM of an elementisthe average mass of itsisotopesrelative to an atom of carbon-12. Avogadro'sNumber The MagicNumber! If you calculate the RAMof a substance,andthenweighoutthatnumberof grams of the substance you can calculate the numberof atoms or moleculesthat itcontains.
- 2. Carbonhas a RAMof 12, if you were toweighoutexactly12grams of carbon it wouldcontain 602 000 000 000 000 000 000 000 carbon atoms. Thisis calledamole of atoms.The numberiscalled Avogadro'snumber, usuallywrittenas 6.02 x 1023 Examples: 1. 24 grams of magnesiumwouldcontain6.02 x 1023 magnesiumatoms. 2. 56 grams of ironwouldcontain6.02 x 1023 ironatoms. 3. 18 grams of waterwouldcontain6.02 x 1023 watermolecules. One mole ofa substance is6.02 x 1023 particlesof that substance. It is obtainedby weighingout the RAM or the formula mass in grams. What informationcan we get from a chemical formula? The formula ofwater isH2O. Thisisbecause 1 atom of oxygencombineswith 2atomsof hydrogen. Alternatively,we cansaythat 1 mole of oxygenatomscombineswith 2 molesof hydrogenatomsto form1 mole of watermolecules. Molescan be changedtograms; therefore we cansay 16 grams of oxygencombine with 2 grams of hydrogento form 18 grams of water. The formula of carbon dioxide is CO2. Thisis because 1 atom of carbon combines with 2 atoms of oxygen. Therefore,12grams of carbon combine with32 gramsof oxygentoform44 grams of carbon dioxide. Alternatively, 1mole of carbon atoms combine with 2molesof oxygenatoms to produce 1 mole of carbon dioxide. To findthe formulaofa compound 1. Start withthe numberof grams that combine 2. Change the grams to moles 3. Thisgivesyouthe ratio whichtheycombine 4. So nowyouknowthe formula A formulaobtainedinthiswayiscalledthe Empirical Formula. Example: 1. 32 grams of sulphurreactwith32 grams of oxygen 2. 1 mole of sulphurreactswith2 molesof oxygen
- 3. 3. Ratioof 1:2 4. FormulaSO2 - sulphurdioxide Findingmass by experiment The Reactionbetween MagnesiumandOxygen The apparatus belowis usedtocalculate the massof magnesiumoxide,butfirstlyyoumustknowthe massesof the elementsthatcombine. Method: 1. Weighthe massof the crucible andlid 2. Adda coil of magnesiumribbonandreweigh. 3. Heat crucible strongly,liftinglidoccasionallytoallow oxygenin. 4. Whenburningiscomplete allowthe apparatustocool. 5. Withthe lidonreweighthe crucible anditscontents. Results: mass of magnesiumoxide- massof magnesium=mass of oxygen 4.0g - 2.4g = 1.6g 2.4g of magnesium= 0.1 moles 1.6g of oxygen= 0.1 moles Therefore the ratioof magnesiumtooxygenusedis1:1 Conclusion: The formulaof magnesiumoxide isMgO. How to write an equation Four stepsto writingequations: 1. Write the equationinwords 2. Write the equationinsymbols.Checkthatyou are usingthe correct formulae.
- 4. 3. Checkthat the equationisbalanced.Balancingmeansthatyouhave the same numberof atom on one side asyou do onthe other.The reasonforbalancingisbecause atomsare notlostor createdduringa reaction.Rememberwhenyoubalance youmultiplythe wholeformulae whetheritsanelementormolecule- youdonot change its formulae. 4. Addstate symbols. Example1: Magnesiumburns in oxygento produce magnesiumoxide. Because oxygenhastwoatomson the left,we multiply by2the MgO sothat we now have 2 oxygen atomson the right. Howeverwe nowhave twomagnesiumatomsonthe right,so we needtomultiplyMgby 2 on the left to balance. Example2: Hydrogen gas reacts with oxygengas to form water when a spark isplaced in the mixture. There are twooxygenatomsonthe leftbutonlyone onthe right.Hence we needtomultiplythe H2Oon the right by2. Thisgivesustwo oxygenatomsonbothsidesbutwe are now leftwithonlytwohydrogens onthe left and fourhydrogenatomsonthe right.Therefore,we multiplythe H2 by2.