1. Recap – Molar Mass and Moles
Chem 1001 Lecture 4
Recap – Molar Mass and Moles
Relative Atomic Mass – weighted average mass of all isotopes of a particular element relative to 12C.
Avogadro number – x 1023 this is the number of atoms of 12C in exactly g of 12C.
The mole – name we use when we have an Avogadro number of objects.
Amount in moles, n = mass / molar mass

2. Chem 1001 Lecture 12
Stoichiometry
Q: What masses of Na and Cl2 are needed to produce 100 g NaCl?
2Na Cl2  2NaCl
Mass/g ? ?
Molar mass
Moles  
Dr Toby Hudson

3. Chem 1001 Lecture 12
Stoichiometry
Q: What masses of Na and Cl2 are needed to produce 100 g NaCl?
2Na Cl2  2NaCl
Mass/g ? ?
Molar mass
Moles  
Use balanced equation
Dr Toby Hudson

4. Chem 1001 Lecture 12
Stoichiometry
Q: What masses of Na and Cl2 are needed to produce 100 g NaCl?
2Na Cl2  2NaCl
Mass/g
Molar mass
Moles  
m = n M
Dr Toby Hudson

5. Chem 1001 Lecture 13
Limiting Reagent
So far we have talked about reactions where we have exactly the right amounts of each reactant.
Most of the time, however, we will not have enough of one reactant (reagent) to react with all of the other.
The reagent which we are short of is called the limiting reagent – it determines how much product we get.
Dr Toby Hudson

6. Chem 1001 Lecture 13
Limiting Reagent
Q: 5.00 g of H2 and 10.0 g of F2 are mixed. When the reaction ceases, what is the mass of each type of molecules present?
H F2  2HF
Mass/g
Molar mass
Moles
\ F2 is the limiting reagent – amount of F2 will determine amount of product.
Dr Toby Hudson

7. Chem 1001 Lecture 13
Limiting Reagent
Q: 5.00 g of H2 and 10.0 g of F2 are mixed. When the reaction ceases, what is the mass of each type of molecules present?
H F2  2HF
Mass/g
Molar mass
Moles (initially)
Moles (used)
Moles (after react.)
Mass (after react.)  
Dr Toby Hudson

8. Chem 1001 Lecture 13
% Yield
So far we have assumed that all of the limiting reagent is used up – 100 % reaction – the reaction ‘has gone to completion’.
This may not always be the case.
Dr Toby Hudson

9. Chem 1001 Lecture 13
% Yield
Q: In the reaction of 5.0 g of sodium with excess chlorine, the mass of sodium chloride obtained was 12.4 g. Calculate the % yield.
2Na + Cl2  2NaCl
Mass/g
Molar mass
Mole
Dr Toby Hudson

10. Chem 1001 Lecture 13
% Yield
Q: In the reaction of 5.0 g of sodium with excess chlorine, the mass of sodium chloride obtained was 12.4 g. Calculate the % yield.
2Na + Cl2  2NaCl
Mass/g
Molar mass
Mole
% yield = 12.4 / 12.7  = 97.6 %
Dr Toby Hudson

11. Learning Outcomes: By the end of this lecture, you should:
be able to perform stoichiometric calculations including:
determining the mass of product given the mass of starting materials
determine the mass of starting materials needed to give a required mass of product
identifying the limiting reagent and using this in the calculations
determine the % yield in a reaction given the mass of starting materials
be able to complete the worksheet (if you haven’t already done so…)

12. Questions to complete for next lecture:
Chem 1001 Lecture 4
Questions to complete for next lecture:
Write a balanced equation for the complete combustion of carbon to form carbon dioxide.
How many moles of carbon are there in 100 g of carbon?
If the combustion is carries out in air, which is the limiting reagent?
How many moles of carbon dioxide would this produce on complete combustion of 100 g of carbon?
What mass of carbon dioxide is produced from the complete combustion of 100 g of carbon?
If 348 g of carbon dioxide is isolated from complete combustion of 100 g of carbon, what % yield does this represent?