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!