Chemistry

Answers to Chapters 6 & 7 Study Questions

 

1. a) 3(12.0) + 8(1.01) + 3(16.0) = 92.1 g/mole b) 92.1 g c) 6.02 x 1023 molecules

d) e)

 

2. a) 4 atoms (one N + 3 H)

b)

c)

 

3. Molar mass of NaNO2 = 23.0 + 14.0 + 2(16.0) = 69.0 g/mole

% Na = 23.0/69.0 = 33.3% Na; % N = 14.0/69.0 = 20.3% N; %O = 2(16.0)/69.0 = 46.4% O

33.3% Na, 20.3% N and 46.4% O.

4. a) In 100 g of this compound, there are 40.7 g C, 5.1 g H, and 54.2 g O

3.39/3.39 = 1 x 2 = 2

5.1/3.39 = 1.5 x 2 = 3

3.39/3.39 = 1 x 2 = 2

empirical formula = C2H3O2

 

b) Molar mass of C2H3O2 = 2(12.0) + 3(1.0) + 2(16.0) = 59.0 g/mole

118/59.0 = 2 molecular formula = C4H6O4

 

5. a) In 25.0 g of compound, there are 7.20 g Mg, 3.55 g C and 25.0-(7.20+3.55) = 14.25 g O.

0.296/0.296 = 1

0.296/0.296 = 1

0.891/0.296 = 3

formula = MgCO3

b) % Mg = 7.20/25.0 = 28.8% Mg; %C = 3.55/25.0 = 14.2 % C; % O = 14.25/25.0 = 57.0% O. 28.8% Mg, 14.2 % C and 57.0% O.

(You should get the same result if you used molar mass and atomic masses.)

c)

d)

 

6. Chemical reactions are frequently accompanied by:

a) bubbles which show that a gas is one of the products of the reaction.

b) heat changes; heat is evolved in exothermic reactions; heat is used up in endothermic reactions. Exothermic reactions also often result in the production of light and sound.

c) color changes which often signify a change in chemical composition.

d) the formation of a precipitate which represents the formation of an insoluble ionic compound from soluble ionic compounds.

 

7. a) 4 Fe(s) + 3 O2(g) 2 Fe2O3(s)

b) B2H6(l) + 3 O2(g) B2O3(s) + 3 H2O(l)

c) 4 PH3(g) + 8 O2(g) 6 H2O(l) + P4O10(s)

 

8. a) PH3(g) and O2(g) b) H2O(l) and P4O10(s) c) 6

 

9. a) 6 Li(s) + N2(g) 2 Li3N(s)

b) C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l)

 

10. mass of Zn = 33.64 g - 32.00 g = 1.64 g Zn

mass of O = 34.04 g - 33.64 g = 0.40 g O.

# moles Zn: 0.0251/0.025 = 1

# moles O: 0.025/0.025 = 1

 

formula = ZnO