Stoichiometry and You
A fun guide to handling those pesky mole questions
Solutions for the questions are in red
Here are some rules to make your life much easier when you have to do mole calculations
(and believe me, you will).
1. Balance the equation. If the equation isn’t balanced, do that step first. This is an
extremely important step, as it will show you the stoichiometric coefficients you
will need later.
2. Determine what the question is asking you to find out. I know what you’re
thinking – maybe this should have been done first, but balancing the equation is
more important. Highlight the important information in the question and pay
attention to any distractors that may throw you off the trail.
3. Doing the calculations - depending on what is being asked:
a. Example: You have the following equation –
i. HCl + NaOH  NaCl + H2O (You will note this is balanced)
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You are given 45 grams of solid NaOH to work with. How many
grams of NaCl will be produced and how many moles of H2O will result?
Step 1 – Determine the number of moles of NaOH. You will need to determine
the mole mass of NaOH by looking at the mole masses of Na O and H on the periodic
table and adding them together. In this case, the mole mass of NaOH is 40 grams/mol.
To determine the number of moles, divide the given mass (45g) by the mole mass
(40g/mol). This will give you 1.125 moles.
By looking at the stoichiometric ratios above, you can say if we have 1.125 moles
of NaOH, we will have 1.125 moles of the other reactants and products because of the
1:1:1:1 ratio.
Therefore we have 1.125 moles of NaCl produced and the mole mass of NaCl is
58g/mol. You then take the 1.125mol and multiply it by 58g/mol – giving 65.3 g of NaCl.
(We’ve answered the first part of the question).
Because of the 1:1:1:1 ratio and the value of 1.125 moles we determined above,
we can say we have 1.125 moles of water (We’ve answered the second part of the
question).
4. Other formulae you will use in this unit:
a. M1V1 = M2V2 - this formula will be used when doing Molarity problems
involving aqueous solutions.
i. Example: How many mL of 16M HCL will be required to make
2000 mL of a 3M HCl solution.
1. What we know: M1 = 16M V1 = ? M2 = 3M V2 2000mL
2. Plug those values into the above equation and we end up
with: (3M)(2000mL) / 16M = 375mL
3. Therefore we need 375mL of 16M HCl
ii. You can use this same equation to determine a question like
this: You have 50mL of a 1M NaOH solution. How many mLs of
3M HCl will be required to neutralize the NaOH? The equation
shown is in Q.3 is a neutralization equation. The ratio is
1:1:1:1.
iii. You will need to determine the number of moles of the 1M
NaOH solution first. Remember that MOLARITY (M) is simply
moles/litre so to determine the number of moles in 50mL of the
1M solution you can make it a ratio: x moles/50mL = 1
mole/1000mL so divide 50/1000 to get 0.05 moles of NaOH.
iv. Therefore you will need 0.05 moles of 3M HCl – use the same
ratio idea: 0.05moles/? mL = 3 moles/1000 mL = (0.05)(1000)/3
= 16.7 mL of 3M HCl will neutralize the 50mL of 1M NaOH
b. The other formula that you will use is this one for gases:
i. P1V1 = P2V2 where P is pressure and V is volume.
1. Example: You have 3000mL of Hydrogen gas at a pressure
of 150 kPa (kilo pascals). What pressure is required is
the new volume of 4500 mL is to be realized?
2. P1 = 150kPa V1 = 3000mL P2 = ? V2=4500mL
a. (150)(3000) = (?)(4500)  (150)(3000) / (4500) =
100kPa. The new pressure is 100kPa
ii. IF YOU ARE GIVEN MOLE AMOUNTS OF GAS, PLEASE
REMEMBER THAT 1 MOLE OF ANY GAS OCCUPIES 22.4
L OF VOLUME AT STP (25°C and 101.3 kPa)
1. Example: You have 4 moles of Chlorine gas at a pressure
of 103.4 kPa. What is the new volume of Chlorine gas if
the pressure is increased to 250kPa?
a. Remember the 22.4 L stuff above. 4 moles of
Chlorine gas occupies 22.4 x 4 = 89.6 L
b. You can now take this value of 89.6 L and plug it
into the P1V1 = P2V2 equation.
(103.4)(89.6) = (250)(?)
? = (103.4)(89.6) / (250)
= 37.1 L
Questions to poke in the eye with a sharp stick – so to speak.
1. When you dissolve solid NaOH in water, a great deal of heat is produced. This
heat value is 426.7 kJ (kilo Joules) per mol. How much heat is produced if you
dissolve 150g of NaOH pellets in deionized water?
You need to know the number of moles of NaOH to answer this question.
150g / 40g per mole for NaOH = 3.75 moles
Then times the heat value of 426.7 kJ per mole by the number of moles (3.75) to get
1600 kJ of heat.  answer
2. The following reaction takes place: H2SO4 + 2 NaOH  Na2SO4 + 2 H2O
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a. You are the plant chemist for the Acme Company. How many kilograms
of Sodium Sulphate will be produced if you start with 16,500 kg of
Sodium Hydroxide?
Convert the 16500 kg to 16500000 g and then divide that figure by 40
g/mole of NaOH to get 412500 moles of NaOH. Because we use 2 moles
of NaOH for every 1 mole of Na2SO4 produced, we will have 206250
moles of Sodium Sulphate. Multiply the number of moles (206250) by the
mole mass of Sodium Sulphate (142g/mole) to get 29287500 grams.
Dividing this by 1000 will give 29287.5 kg  answer.
b. How many Litres of water will be produced? HINT: Look at the mole
ratios above. We know we will have 412500 moles of water produced.
Multiply this value by 18g/mole to get 7425000 grams of water. This is
equal to 7425000 mL or 7425 L  answer
3. You have the following reaction: Zn + 2 HCl  ZnCl2 + H2 gas + 115.3 kJ/mol heat
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a. How many moles of Zinc Chloride will be produced if you react 120g of Zn
with 200mL of 6M HCl?  this is a distractor
Find the number of moles of Zn in the 120g – 120g / 65g per mole = 1.85 moles.
This value of 1.85 moles holds true for Zinc Chloride too because of the mole ratio.
b. How many kJ of heat will be produced (the value of 115.3 kJ is per mole of
Zn) Take the 1.85 moles and multiply it by 115.3 kJ to get 213.3 kJ
c. How many litres of Hydrogen gas is produced if the reaction takes place at
STP? 1.85 moles of Hydrogen gas is produced so we multiply that by 22.4 L
per mole to get 41.4 L of Hydrogen gas.
4. You have been given the task of making 4000mL of a standard 2M HCl stock solution
from 16M HCl. How many mL of the 16M HCl will you use? Use M1V1 = M2V2 for
this one. M1 = 2M V1 = 4000mL M2 = 16M and V2 =? (2M)(4000mL)/(16M)(?) =
500mL of 16M HCl  answer
5. For this question let’s consider the following: For every 10 meter in depth you
descend in the ocean, the pressure increases 1 atm (atmosphere) or 101.3 kPa. So
20 m deep will have a pressure of 2 atm or 202.6 kPa.
You are in a specially built submersible going down into the Mariana
Trench. You reached a confirmed depth of 10898 m. What was the
pressure, in kPa, on the submersible at this depth?
10898m/10m per atm = 1089.8 atm. 1089.8atm x 101.3 kPa = 110407 kPa
FYI 1atm is equal to 15 pounds per square inch, so 1089.8 atm is approx..
equal to 8 tons of pressure per square inch. ( A Great White Shark has a
bite strength of 0.33 tons per square inch.)
If you took a mole of Oxygen gas down with you, what would the volume
be at the depth of 10898 m if the gas was in a bottle inside the ship? It would be
22.4 L
NOTE: James Cameron, the director of Titanic and an avid explorer of the
deep ocean, did this dive solo in 2012. It took him 2 hours and 36 minutes to
reach this depth and he spent around 6 hours exploring the trench and collecting
samples on behalf of the National Geographic society and the Scripps
Oceanographic Institute.