Question #d36a3

This is a long answer.

There are four steps to solving any problem:

1. What do I know?
2. What must I find?
3. Decide on a plan.
4. 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?

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

#2"Al" + 3"I"_2 → "Al"_2"I"_6#

Since I have a plan, I can do a chain calculation:

#"Mass of I"_2 = "10.0 g Al" × "1 mol Al"/"26.98 g Al" × ("3 mol I"_2)/"2 mol Al" × ("253.8 g I"_2)/("1 mol I"_2) = "141 g I"_2#

Note: Plugging numbers into formulas is the LAST thing you do. You must first develop a plan or strategy to solve the problem.