Download Stoichiometry, Molecular and Empirical Formulas - Class Notes | CHEM 1315 and more Study notes Chemistry in PDF only on Docsity! Unit 2-Stoichiometry-Chapters 2, 3, & 4 02/12/2009 Empirical vs. Molecular Formulas Problem: What is the molecular formula for a compound having a molar mass of 138g/mol and a mass % analysis of 52,14% C, 13.13% H, and 34.73% O? Can use any size sample; 100 gram sample is easy; o C-52.14% of 100 grams=52.14 o H-13.13% of 100 grams=13.13 o O-34.73% of 100 grams=34.73 Convert to molar mass o C-52.14% of 100 grams=52.14—(1mol/12g)=4.345 mol o H-13.13% of 100 grams=13.13—(1 mol/1g)=13.13 mol o O-34.73% of 100 grams=34.73—(1 mol/16g)=2.17 mol Divide by smallest number o C-52.14% of 100 grams=52.14—(1mol/12g)=4.345 mol/2.17=2 o H-13.13% of 100 grams=13.13—(1 mol/1g)=13.13 mol/2.17=6 o O-34.73% of 100 grams=34.73—(1 mol/16g)=2.17 mol/2.17=1 Use molecular mass to get g/mol, C=12, H=1, O=16 C2H6O—46 g/mol 138=molar mass given in problem; 138/46=3; Multiply C2H6O by 3=C6H18O3 Conservation of atoms: Must have same number of atoms after a chemical reaction as before the reaction Some atoms have a consistent weight; mass is also conserved From experiment C-2: Co(No3)2+Na3PO4 Co=+2 No3=-1 Na=+1 PO4=-3 Elements and compounds can now come in contact with any of the others; double replacement chemical reaction happens when Co bonds with PO4 and Na bonds with NO3: Co3(PO4)2+NaNO3; formula is not balanced, has different number of atoms, Na for example, than we started with Adjust coefficients, trial and error __Co(NO3)2 +__Na3PO4 -->__Co3(PO4)2 + __NaNO3 _3_Co(NO3)2 +_2_Na3PO4 -->_1_Co3(PO4)2 + _6_NaNO3 Rules for balancing equations: same number and kind of atoms on both sides same overall net electrical charge cannot change the subscripts of compounds: only change the coefficients coefficients should be the lowest ratio integers mathematically they are similar to algebraic equations Steps to balancing a chemical reaction formula: 1) Write a balanced chemical equation. (2) Convert given quantities into moles. (3) Convert moles of given into moles of unknown. (4) Convert moles of unknown into desired units. 3.0 mol NH3 (2mol N2/4 mol NH3) =1.5 mol N2
