11 chem, week 10 - Review Time!

Hey my loyal followers.  This week is a bit different as I only prepared for my test, so I thought that I'd run through all of the key concepts in the third unit.

Module 1: Significant Figures

An easy-to-understand diagram.  Tadah!

• No measurement can ever be complete, as there will always be another decimal place to a measurement that cannot be measured by known tools
• Precision vs. accuracy!
• Accuracy: How close you can come to the correct result
• Precision: How consistently you can get that result with the same method
• Sigfigs!
• A number's sigfig is the number of numbers in the first number.  However, there are some rules:
• Any integer that ends in n zeroes has a sigfig of the total number of numbers minus n
• Any zeroes in a number that are surrounded by other non-zero numbers do not count
• Any numbers after the decimal spot of an non-zero integer are included in the sigfig
• If the number is a decimal answer (i.e. 0.00506 or 0.1667) the sigfig is the number of numbers starting with the first non-zero number (i.e. sigfig of 0.00506 starts at 5, and sigfig of 0.1667 starts at 1)
• Numbers are always rounded to lowest common sigfig

Module 2: The Mole and Molar Mass

A cuter mole

• The Mole!
• Also called Avogadro's Number, it is ~6.022*10^23, and describes (theoretically) the number of atoms in one gram of any pure substance
• Used a lot in stoichiometry to deal with numbers of atoms in amounts of substances, because no one wants to do math with massive numbers
• WHEN DOING CALCULATIONS, DO NOT ROUND UNTIL THE ANSWER IS FOUND
• Calculations
• N(# of molecules/particles) = n(# of moles)*Na(Avogadro’s constant)*a(# of atoms)
• Symbol of molar mass (mass, in grams, of one mole of something) is M (units are g/mol)
• m(mass in grams)= n(# of moles)*M(molar mass of atom)

Module 3: Empirical/Molecular Formulas

Look!  A super-duper easy way to understand this concept!

• To find % composition of a compound (with weight):
• Take overall molar mass of each element (M*coefficient*subscript)
• Divide by mass (g) of compound
• Multiply by 100
• Tadaaaa
• Empirical/molecular formulas:
• Molecular: full compound with NO reduced subscripts
• Empirical: reduces subscripts where possible

Module 4: Stoichiometry

Poor guy

While I can't do stoichiometry the justice it deserves here, every problem uses these four steps:

1. Write a balanced chemical equation
2. Convert given values to moles (if required)
3. Use mole ratio to determine unknown mole ratio
4. Convert to required units, if necessary

Module 5: Limiting Reactants and Percentage Yield

Clever cat

Oh... this poor guy thinks he's so good...

• Ideas about limiting reactants
• If there are not enough reactants, there will be no product
• If there are excess of one reactant but the perfect amount of another, there will be leftovers of only the first reactant
• Similar to stoichiometry questions, limiting reactant questions follow a 4-step solving process:
• Write a balanced chemical equation
• Determine amount of moles of each reactant
• Use mole ratio to determine unknown mole ratio of each reactant - smallest # corresponds to limiting reactant
• Use limiting reactant to determine unknown value (convert to required units, if necessary)
• % yield is what was product was measured, theoretical yield is what should have been produced (using stoichiometry)
• % yield = (experimental yield / theoretical yield) * 100%

And there you go!  I hope you enjoyed this.  I will be back to my regular template next week.  Thank you for reading!

 - L