General Chemistry
Zumdahl Chapter 9 Notes
Chemical Quatities
Week of 2/9/15, expected test date 2/19/15
Standard: HS-PS1-7 Use mathematical
representations to support the claim that
atoms, and therefore mass, are
conserved during a chemical reaction.
What is the goal?
Your test will essentially be this one
question…
How many molecules of water are formed
if 453 grams of oxygen gas reacts with an
excess of hydrogen gas?

Step 1: Write out the equation

Hydrogen (H2) gas reacts with oxygen
(O2) gas to form water (H2O) liquid?
H2(g) + O2(g)  H2O(l)
Step 2: Balance equation

H2(g) + O2(g)  H2O(1)
2H2(g) + O2(g)  2H2O(l)
Step 3: Find moles of given

Given: 453 grams of oxygen gas O2(g)
◦ Note: atomic mass of oxygen is 16.00
453 grams of O2 are given
The atomic mass of O2 is 32 grams.
453/32 = 14.16 moles of O2
Step 4: Determine amount of moles
of wanted by mole ratio

2H2(g) + 1O2(g)  2H2O(1)
Two moles of water are formed for every 1 mole
of oxygen
Hence there is a 2:1 ratio of hydrogen to water
Hence if you are given 14.16 moles of O2 you will
form 2 times as much H2O.
You will form 28.32 moles of H2O
Step 5: Determine amount of grams are
made from the molar mass and desired
moles
 Molar mass of H2O(l)?
Not needed! You would
reversing step 4. SKIP!
Step 6: Determine amount of molecules of
wanted formed using Avogadros number.
= 6.02X1023 molescules/mol
Things to remember
- 1 mole of anything is 6.02X1023 units
- Hence 1 mole of bananas is 6.02X1023
bananas
- Or 1 moles of water molecules is
6.02X1023
molecules
So there are 28.32 X 6.02 X 1023 molecules of
water formed in this reaction.
28.32X6.02 = 170.49X1023 molecules of water
Step 7: Write it all out in
stoichiometry method and circle
answer
Correction!
1) Write in scientific notation
2) Consider your number and write your answer in correct
significant figures
1) All scientific calculations are based on accuracy. Hence you
need to go with the low amount of significant figures.
My Calculation: 28.32X6.02 = 170.49X1023 molecules of water
1) Write in scientific notation
1) 1.7049X1025 molecules of water
2) Consider your number and write your answer in correct
significant figures
1) My Calculation: 28.32 (4 sig figs) X6.02 (3 sig figs) =
1.7049X1025 molecules of water
2) Final answer should have 3 significant figures
1) 1.70X1025 molecules of water