Stoichiometry in Chemistry

1. General Chemistry—
Stoichiometry
Instructor: Helen (Chihan) Huang

2. Reflective Question
• Quantify things
– Take 1 g of XYZ medicine?
– Quantification in Chemical
reaction

3. Outline
• Amount of Substances
• Reaction Stoichiometry

4. Amount of Substances
• Atomic Mass Units
– 1 a.m.u. = mass of one atom of 12C
– Masses of all elements are relative to this
• Avogadro Constant: The number of atoms/
molecules in one mole of substances
– NA=6.022*1023 mol-1
– 1 mole= 6.022*1023 of things
• Practice Problems:
– How many atoms are in 0.25 moles of helium?
• Avogadro number: 6.022*1023 in 12 g of 12C

5. Mass vs. Mole
• 1 mole (6.022*1023 ) of 12C = 12 g
• Practice Problem:
• Calculate molar mass of CO
• How many moles of carbon monoxide are in 25.5 g?

6. Stoichiometry
• Meaning of coefficients
– Relationship—amount of reagents & products
1H2 +
1
2
O2 1 H2O
– Atoms are conserved
• Practice:
2C + 1O2 2 CO
– Moles of different reactants are required to
produce 2 moles of CO?
Lego

7. Balance reaction equation
1. Balance weird guy first
2. Hydrogen or oxygen
3. Uncombined elements last
NH3 + O2 NO + H2O

8. Limiting and Excess Reactants/
Reagents
• Limiting Reagent:
– The one exhausted first
• Excess Reagent:
– The reactants remained
• Practice:
Q: Which one is Limiting
Reactant?
2NH3 +
5
2
O2 2 NO + 3H2O
4 mol 2.5 mol
Limiting reagent=?
Lego

9. Review Question
• What mass of ammonia can be produced from
56.0 g of nitrogen and 10.0 g of hydrogen?
MW: N2=28.0, H2=2.0 (g/mol)
1. Balance reaction equation
2. Calculate amount of given reagents
3. Identify limiting reagent