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If you're given the mass of each isotope of an element, and the average atomic mass, you can calculate the percent (%) abundance of each isotope. Let "x" be one of the abundances, and the other abundance is "1-x" ... then solve the equation using basic algebra.

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How to draw the Bohr-Rutherford Diagram for Potassium. 2 electrons can go in the first shell, 8 in the second, 8 in the third, and so on...

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If you're given the Percent Composition of a compound, you can find the Empirical Formula for it. I have the shortest method ever to do it, although it's not a "full solution" like your teacher probably asks for.
1. Divide each % by the atomic mass of the element
2. Divide each of THOSE answers by whatever's smallest
3. Adjust these numbers into their lowest whole-number ratio.
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Sigma bonds are the FIRST bonds to be made between two atoms. They are made from hybridized orbitals.
Pi bonds are the SECOND and THIRD bonds to be made. They are made from leftover "p" orbitals.
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The pKa of an acid is exactly the same as the pH HALFWAY to the equivalence point!
Then, Ka = 10^-pKa and you're done. Super easy.
Thanks Henderson-Hasselbalch!
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How many significant figures are in a number? Don't memorize complex rules ... here's THREE that will get you the right answer each time.

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Use Bronsted Lowry Acid/Base Theory to identify conjugate acid base pairs.
More free chemistry help at www.chemistnate.com

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How to draw the Bohr-Rutherford Diagram for Phosphorous. 2 electrons can go in the first shell, 8 in the second, 8 in the third, and so on...

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How to find the average atomic mass of an element. You need to know the mass of each isotope and the percent (%) abundance of each as well. Multiply each mass by its corresponding percentage, and add these products together.

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What are the Pauli Exclusion Principle, Aufbau Principle, and Hunds Rule? They are rules we use to fill electron orbital filling diagrams. Fill from the bottom up, Spread them out before you double up, and always have one up one down in each orbital.

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What are the periodic trends? Electronegativity, Atomic Radius, Ionization Energy, and Electron Affinity. How to remember them? F has the highest electronegativity, electron affinity, and one of the largest ionization energies. It is also one of the smallest atoms.

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Stronger intermolecular forces = LOWER vapor pressure
Weaker intermolecular forces = HIGHER vapor pressure
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How to write total and net ionic equations.
1. Write a balanced chemical equation
2. Break up all the (aq) compounds into its ions (this is TOTAL)
3. Get rid of spectator ions (things that appear on both sides) (this is NET)
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How to assign oxidation numbers to the atoms in a molecule.
1. Elements have oxidation number = 0
2. Hydrogen's always +1 (except in "hydrides")
3. Oxygen's always -2 (except in "peroxides")
4. Other atoms get the charge they prefer, as long as the sum of oxidation numbers for all atoms = the total charge on the atom.

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More free chemistry help videos: http://www.nathanoldridge.com/chemistry-videos.html
Here I show how to calculate the pH of a solution made with a Strong Base
Summary: Use the concentration of base and the number of OH- ions per molecule to calculate the concentration of OH-. Then, you can calculate the pOH as I did, or calculate [H+] since Kw = [H+][OH-] (more on this formula in a later video).

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How to tell if a bond is Ionic, Covalent or Polar Covalent. You have to calculate the difference in electronegativities between the atoms ... the difference tell you which you have!

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How to use an ICE Table to find the solubility of a salt, if you're given the Ksp.
I'll do it on the calculator with you, to show you how to take care of x to the power of 5.
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More free chemistry help videos: http://www.nathanoldridge.com/chemistry-videos.html
This is the easiest way to figure out how each atom's orbitals are hybridized.

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How to balance a redox reaction in basic solution.
Same process as balancing in acidic solution, with one extra step:
1. Make sure electrons gained = electrons lost
2. Add H2O to whichever side doesn't have enough O
3. Add H+ to whichever side doesn't have enough H
4. Add OH- to both sides. These combine with H+ to form H2O.

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Use ICE Tables to figure out equilibrium concentrations.
In this example, I use the quadratic formula to solve.
There are simplifications you can make, like the Rule of 100, but I don't talk about that here.
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Esters are molecules connected by a =O on one carbon, and a -O- on the SAME carbon.
The carbon chain is broken into two bits, so each gets its own part of the name.
You can identify esters with OATE at the end of the name.
They are kinda polar, and them smell fruity.
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Ksp is really just an equilibrium constant (Keq), but it's for a solid dissolving in water. This is special, since all of the reactants are solid, and so they AREN'T included in the equilibrium expression.

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Hydrolysis is breaking apart molecules by reacting with water. It can happen in lots of places, but if SOMETHING + water = Two other somethings, they it's hydrolysis.
Examples: Breaking apart Esters, Bronsted/Lowry Acids, and ATP.
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How to Calculate Wavelength if you're Given the Energy of a photon. Wavelength is Planck's Constant times Speed of Light divided by Energy.

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Which atoms can violate the octet rule? The answer is basically all of them.
H, Li, Be, B and Al can make do with less than 8.
Everything beyond P can have MORE than 8 because of hybridization of "d" orbitals.
Basically only C, N, O and F follow this rule.
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How to Calculate the Molar Mass of elements and molecules
Elements: Easy, just look at the periodic table.
Molecules: Add up the masses of the atoms that form the compound.
For example, an oxygen atom weighs 16.00 g/mol according to the periodic table. So an O2 molecule would weigh 2 x 16.00 g/mol = 32.00 g/mol

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The Lewis Structure (Lewis Dot Diagram) for HCN.
1. Count electrons
2. Put least electronegative atom in centre
3. Put one electron pair in each bond
4. Fill outer atoms with electrons
5. Move electrons so all atoms (esp. the centre one) has a full octet
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How to calculate the frequency of a photon, given its energy. Frequency = Energy divided by Planck's Constant

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There are 5 isomers of hexane ... here I show how you can come up with all of them.
n-hexane; 2-methylpentane; 3-methylpentane; 2,2-dimethylbutane; 2,3-dimethylbutane
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How to draw the Bohr-Rutherford Diagram for Oxygen. 2 electrons can go in the first shell, 8 in the second, 8 in the third, and so on...

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To determine if a reaction is spontaneous, use this formula to find Delta G. Gibbs Free Energy is NEGATIVE for spontaneous reactions.
You can also determine above which TEMPERATURE the reaction will be spontaneous. Check it out!
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This video shows how you can calculate the Ka of an acid, if you're given the pH of the solution (and its concentration, of course). It's pretty straightforward if you understand where the Ka expression comes from (Equilibrium).
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How to Draw Lewis Structures (aka Lewis Dot Diagrams)
In this video I explain how to draw simple Lewis Structures, while drawing the Lewis structure for a simple molecule (HCN).

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How to Convert an Amount (in moles) into the number of molecules a sample contains. Just multiply by Avogadro's Number (6.022 x 10^23)!

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An updated (i.e. better-looking) video that explains how to create equilibrium expressions (Keq)
- Products over Reactants
- Coefficients become exponents
- Don't include solids or liquids
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How to turn a regular old equilibrium expression into the Henderson-Hasselbalch equation.
Uses laws of logarithms!
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How to Calculate the number of moles, given the mass of a substance.
n = m / M
where n = number of moles
m = mass in grams
M = molar mass (see http://www.youtube.com/watch?v=F9NkYSKJifs)

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How to Convert a Number of Molecules into an Amount in Moles. Just divide the number of molecules by Avogadro's Number (6.022 x 10^23)!

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More free chemistry help videos: http://www.nathanoldridge.com/chemistry-videos.html
Here I show how to calculate the pH of a solution made with a strong acid (like HCl, H2SO4, HNO3, HBr, HClO4, HI, etc)
Remember to account for how MANY H+ ions come from each molecule ... H2SO4 gives 2 H+ when it dissociates.

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What is the expression for Half-Life of a Second Order Reaction?
Here, I derive it from the integrated rate law.
The answer is t = 1/ (k [A]0)
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Sorry this one's so long. But Slater's rules aren't the easiest things in the world.
Summary:
For an "s" or "p" electron:
Add up 0.35 for each electron in same group;
plus 0.85 for each electron in the shell below
plus 1.00 for each electron two or more shells lower
Then subtract this sum from the atomic number to get Z(eff)
For a "d" or "f" electron:
Add up 0.35 for each electron in the same group
plus 1.00 for every electron in any other lower group
Then subtract this sum from the atomic number to get Z(eff)
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Here, I find the solubility of calcium phosphate in phosphoric acid. There's already phosphate in solution, so the solubility of the salt is LOWER.
I even use a simplifying assumption to spice things up.
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Add some acid. Add some conjugate base. What's the pH?
This is the old-school way. You can also use the Henderson-Hasselbalch Equation.
In this example I use 0.2 M HF and 0.1 M NaF
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The Lewis Structure (Lewis Dot Diagram) for ClO3-
1. Count electrons
2. Put least electronegative atom in centre
3. Put one electron pair in each bond
4. Fill outer atoms with electrons
5. Move electrons so all atoms (esp. the centre one) has a full octet and so formal charges are minimized (in this case, chlorine can have an EXPANDED OCTET!)
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How to Draw the Lewis Structure of N2 - with explanation!
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How to Get Frequency from Wavelength. Frequency = Speed of Light divided by wavelength.

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If you have rate constants at different temperatures, you can use Microsoft Excel (or OpenOffice) to find the Activation Energy.
The slope of a (ln k) vs (1/T) graph is -Ea/R. So just take the slope, flip the sign, and multiply by 8.314 ... the answer will be in J/mol
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The pH of a weak base.
Set up equilibrium and K expression. Use ICE Table. Solve for x, which is [OH-]. Take the negative log to get pOH and then subtract from 14 to get pH.
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