This is a long answer.
There are four steps to solving any problem:
- What do I know?
- What must I find?
- Decide on a plan.
- Carry out the plan.
Here's a typical stoichiometry problem:
What mass of iodine must react completely with 10.0 g of aluminium to form Al₂I₆?
Step 1. What do I know?
- Mass of Al
- Molar mass of Al (I can get it from the Periodic Table.)
- Molar mass of I₂ (Also from the Periodic Table.)
- The balanced equation (I may not know it right now, but I will need it, so I can figure it out.)
Step 2. What must I find?
- The mass of I₂
Step 3. What's my plan?
I know that I will need a balanced equation, so I'll do that first,
Then I have to get somehow from
grams of Al → → → grams of I₂
I know that I can convert grams to moles and vice versa (I might not be able to do it this minute, but I can figure it out). This gives me:
grams of Al → moles of Al and moles of I₂ → grams of I₂.
How do I connect the two pieces?
Aha! I know. I can use the molar ratio from the balanced equation.
My plan will be to convert
grams of Al → moles of Al → moles of I₂ → grams of I₂
Note: To this point I haven't used a single number! But now is the time to carry out my plan and insert the numbers.
Step 4. Carry out the plan
The balanced equation is
Since I have a plan, I can do a chain calculation:
Note: Plugging numbers into formulas is the LAST thing you do. You must first develop a plan or strategy to solve the problem.