1. Chemistry 141
Lecture 7
Wednesday, September 20, 2017

2. Your turn For each pair below:
Predict the formula of the compound they form
Write the name of the compound
Ca and NO3-
Cr(III) and SO42-
NH4+ and S
H and PO43-
2 N’s and 5 O’s
Ca(NO3)2 calcium nitrate
Cr2(SO4)3 chromium(III) sulfate
(NH4)2S ammonium sulfide
H3PO phosphoric acid
N2O dinitrogen pentoxide

3. Objectives for today Write and balance chemical equations
Identify types of reactions (combination, decomposition, and combustion)
Determine formula and molecular weights and percent composition from compound formulas
Understand and use the mole concept
Determine empirical formulas from mass data

4. The Law of Conservation of Mass
Matter cannot be created or destroyed
(Lavoisier, “The father of modern chemistry”, 1789)
“…an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends.”

5. Your turn
How many atoms of Mg, O, and H are represented by the notation 3 Mg(OH)2?
1 Mg, 2 O, and 2 H
2 Mg, 2 O, and 2 H
6 Mg, 6 O, and 6 H
3 Mg, 6 O, and 6 H
Answer: d

6. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
States of matter
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
The states of the reactants and products are written in parentheses to the right of each compound.
(g) = gas
(l) = liquid
(s) = solid;
(aq) = in aqueous solution

7. Balance the following equation:
AlCl3 (s) + Ca3N2 (s)  AlN (s) + CaCl2 (s)
What is the coefficient of CaCl2 in the balanced equation?

8. Three Types of Reactions
Combination reactions
Decomposition reactions
Combustion reactions

9. Combination Reactions
In combination reactions two or more substances react to form one product.
2 Mg(s) + O2(g) MgO(s)

10. Decomposition Reactions
In a decomposition reaction one substance breaks down into two or more substances.
2 NaN3(s) Na(s) + 3 N2(g)

11. Combustion Reactions CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
Combustion reactions most often involve oxygen in air as a reactant and can produce a flame.
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

12. The formula weight of Na3PO4 is _______ grams per mole.70
164
265
116
Answer: b

13. The percentage by mass of phosphorus in Na3PO4 is
44.0.
11.7.
26.7.
18.9.
Answer: d

14. What number would you use to convert grams of CH4 to moles of CH4?
Avogadro’s number, 6.02 × 1023 particles/mol
Inverse of molar mass of CH4, 1 mol CH4/16.0 g CH4
Molar mass of CH4, 16.0 g CH4/1 mol CH4
Formula weight of CH4, 16.0 amu
Answer: c