solve chemistry math problems when in doubt, go to moles
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Solve Chemistry Math Problems
“When in doubt, go to moles.”
Use Dimensional Analysis
• Analyze the labels
• Change them with converting fractions
• Do the “number math” last (after all the labels cross off so they are the same on both sides of your equation.)
1: Identify your goal unit:
What label goes with the number you want to find?
• Is it grams, moles, liters, joules, or some other unit?
• Is it a combined unit, like moles/liter, or cal/(gram*Celsius)?
2. Write the label
• Write an = sign, and leave a space for the answer number on the far right side of your paper.
• Then write the label you want with your answer.
3. Choose a number to convert
• Choose one of the numbers you’re given, based on its similarity to the goal unit. On the far left side of your paper, write that number as a two line, flat fraction.
4. Convert units
• multiply by two line, flat fractions
• Cross out any unit label you find on both the top line and the bottom line.
• Continue adding converting fractions and crossing out labels.
• The top must equal the bottom.
• Memorize the common converting fractions.
• When all the labels left before the equals sign match the labels after the equals sign, you’re almost finished.
6. Multiply then divide
• Multiply all the numbers across the top line of your two line fractions.
• Then divide that answer by each of the numbers across the bottom.
• Write the result before the goal label on the right.
Common converting fractionsAtomic mass, formula mass:X grams = 1 mole
STP liters of any gas:22.4 Liters = 1 mole
Avogadro’s Number:6.02X1023 particles = 1 mole
Gas EquationsDalton’s Law of Partial Pressures:
P1 + P2 + P3 … = Ptotal
Combined Gas Law:
P1V1T2 = P2V2T1 (T must be Kelvin)
Ideal Gas Law:PV = nRT
Solutions
Molarity = M
M = moles of solute / Liter of solution
Heat and Energy
Specific Heat : changing temperature
# calories/gramoC
Heat of fusion: melting or freezing
#Joules/mole
Heat of vaporization: boiling or condensing
# Joules/mole
Units
• Moles • Grams• Liters• Energy: calories or Joules• Temperature: Kelvin, Celsius, Fahrenheit• Pressure: Atmospheres, Torr, mm Hg,
KiloPascals• Particles (atoms, ions, electrons, molecules,
formula units…)
Prefixes
• Use these as converting fractions, too.
• Mega = M = 106
• Kilo = K = 103
• centi = c = 10-2
• milli = m = 10-3
• micro = “mu” (Greek letter) = 10-6
Mole Map
Grams of A
Liters of A
Moles of A
Moles of B
Grams of B
Liters of B
Particles of A
Particles of B
Mole/Grams
X Ratio
Grams/Mole
Grams/Mole Mole/Grams
X Ratio
1/6.02
1023
X6.02
1023
1/22.4X
22.4
1/22.4 X
22.4
X Ratio
X Ratio
X Ratio
X Ratio
X6.02
10231/6.02
1023
Learn the system
• Dimensional analysis is a procedure you can use to solve most problems.
• It’s not just for chemistry.
• You will be given many equations on the SOL test.
• You still need to memorize some equations.
Good Luck!
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