2-Step Stoichiometry

Multi-Step Stoichiometry

Some problems cannot be solved with a single conversion factor.

If the problem includes.....

• ...particles (atoms, molecules, ions, formula units, etc.), then use Avogadro's number (6.02 x 10²³ particles/1 mole)

• ...mass (or grams), then use molar mass/1 mole

• ...more than one compound, then use a conversion factor from the balanced equation

Multiple Select

Which conversion factors does the following problem need?

Find the mass of 1.6 x 10²⁹ helium atoms.

• molar mass

  (4.003 g/1 mol)

• conversion factor from equation

  (2 moles He/3 moles H₂)

• Avogadro's number

  (1 mol/6.02 x 10²³ atoms)

Multiple Select

Which conversion factors does the following problem need?

N₂ (g) + 3 H₂ (g) → 2 NH₃ (g)

How many grams of NH₃ can be produced from 6.0 moles of H₂ ?

• molar mass

  (17.031 g NH₃/1 mole NH₃)

• conversion factor from equation

  (2 moles NH₃/3 moles H₂)

• Avogadro's number

  (6.02 x 10²³ molecules NH₃/1 mol NH₃)

Multiple Select

Which conversion factors does the following problem need?

CH₄ (g) + 2 O₂ (g) → 2 H₂O (g) + CO₂ (g)

Burning 3.5 moles of CH₄ produces how many H₂O molecules?

• molar mass

  (16.043 g CH₄/1 mole CH₄)

• conversion factor from equation

  (2 moles H₂O/1 mole CH₄)

• Avogadro's number

  (1 mole H₂O/6.02 x 10²³ molecules H₂O)

How do we do the math?

Step 1

As always, start with the known over 1.

Step 2

• Next, choose the conversion factor that cancels out the known.

• If the known is grams or particles, start with molar mass or Avogadro's number.

• If the known is moles, start with the equation conversion factor.

Step 3

Add a conversion factor that cancels out everything EXCEPT the unknown.

Step 4

Just like normal, multiply the top and divide the bottom.

This math will be 14 x 1 x 31.998 / 1 / 2 / 1

Calculate the answer and type it onto the next slide.

Fill in the Blank

What is the answer to example problem?

(Round to 3 digits, please.)

Fill in the Blank

Try a couple more questions!

Mg (s) + 2 H₂O (l) → Mg(OH)₂ (aq) + H₂ (g)

What mass of Mg is needed to react with 0.5 moles of H₂O ?

(Please round to 2 digits.)

Fill in the Blank

2 N₂O (g) → 2 N₂ (g) + O₂ (g)

52 grams of N₂O are decomposed. How many moles of O₂ are produced?

(Please round to 2 digits.)

Unit

Finally, your unit is whatever did NOT cancel out.