Observatory Workbook p57-60 Moles, Stoichiometry

Observatory Workbook p34 (#6 to 9) Electrolytes and pH

Observatory Workbook p36 to 38 (several topics)

To the authors and publisher of Observatory: I am not violating copyright laws! If you look at the solutions, you’ll see that they are entirely my own. The method of solving stoichiometry problems is standard and radically different from what’s in the teacher's manual.

p57               N₂ + 3 H₂ à 2 NH₃

5.       a)       6 moles NH₃ (1 N₂/2 NH₃) = 3 moles N₂

                    6 moles NH₃ (3 H₂/2 NH₃) = 9 moles H₂

b)              2 moles NH₃ (1 N₂/2 NH₃) = 1 moles N₂ = 28 g

2 moles NH₃ (3 H₂/2 NH₃) = 3 moles H₂

3 moles H₂ (2g /mole) = 6.0 g

c)               10 g N₂ ( mole/28 g) = 0.357 moles N₂

0.357 moles N₂(2 NH₃/1 N₂/) = 0.714 moles NH₃

0.714 moles NH₃(17g/mole) = 12.1 g

d)              150 g NH₃ ( mole/17 g) = 8.82 moles NH₃

8.82 moles NH₃ (3 H₂/2 NH₃) = 13.235 moles H₂

6.                 a)       122 g H₂O(moles/18g) = 6.78 moles H₂O

                              6.78 moles H₂O (1 O₂ /2 H₂O) =  3.39 moles O₂

                    b)       6.78 moles H₂O (2 H₂ /2 H₂O) =  6.78 moles H₂

7.                 50 g Fe₂O₃ ( moles/159.5 g)= 0.313 moles Fe₂O₃

0.313 moles Fe₂O₃ (4 Fe/ 2 Fe₂O₃ ) = 0.6269 moles Fe

0.6269 moles Fe (55.8 g/mole) = 34.98 g

 8.                a)         0.012 moles

                    b)         1:1 ratio, so 0.012 moles of HCl

9.             a)       0.017 moles

                 b)       0.017 moles Mg(OH)₂ ( 2 HCl/1 Mg(OH)₂) = 0.034 moles HCl

10.                    1 g of Mg(OH)₂ neutralizes more HCl than 1 g of NaHCO₃(see #8 and 9)

11.                     a)       1000 g C₇H₆O₃ (mole/138 g) = 7.246 moles C₇H₆O₃

7.246 moles C₇H₆O₃ ( C₄H₆O₃/ 2 C₇H₆O₃ ) = 3.623 moles C₄H₆O₃

3.623 moles C₄H₆O₃ ( 102 g/mole) = 369.6 g C₄H₆O₃

b)       7.246 moles C₇H₆O₃ (2 C₉H₈O₄ / 2 C₇H₆O₃) = 7.246 moles moles C₉H₈O₄

7.246 moles moles C₉H₈O₄ ( 180 g/mole) = 1304 g C₉H₈O₄

p34 (#6 to 9)

6.               10^-2/10^-5 = 1000 times more acidic

7.               10000 = 10⁴, so go up 4 units to pH = 10. As you go up on the scale it gets more basic.

8.               a) basic

b) bases

c) a strong electrolyte

d)   it is a strong electrolyte from looking at the bright light, and from the colour, it is an acid.

p36 to 38

6.

a)                                      diphosphorus pentoxide

b)                                      lithium hydride

c)                                      CaF₂ (should be calcium fluoride)

d)                                      Magnesium sulphate

e)                                      CCl₄

f)                                       Sodium phosphate(do not use tri!) Na₃PO₄

g)                                      Calcium chloride

h)                                      AgNO₃

6. Use n = CV

5000

0.425

0.04

255

0.0002 ppm

7. They are showing 10^-10, so the pH is 10.

a) true

b) false; you need two or more non-metals

c) false; there are rules based on type of bonding

d) false; dissociation has to do with whether or not ions are formed

e) false; it's based on H⁺ concentration

9. The weak electrolyte is either A or B. Use a stronger bulb(50W or 100W), and the weak electrolyte will not be able to create a bright glow.

10. 500 g CaSO₄(mole/136 g) = 3.67 moles

C = n/V = 3.67 moles/1.2 L = 3.1 moles/L

11. A molecule of N₂ is more stable because through sharing with another N atom it fills the second shell.

In so doing thee bonds form within te molecule. The N atom has a valence of 5 and the ability to pull in more electrons

12. Convert molecules to moles by dividing by 6.02 X10²³. You'll get about 5 moles.

Then C = n/V =5 moles/0.600L = 8.3 moles/L

13. 0.0069 moles/L

14. Get moles from molecules(see #12)= 2 moles.

n = CV

2 = 4V

V = 0.5 L

15. AgCl àAg⁺ + Cl^-

Ca(OH)₂ àCa⁺² + 2 OH^-

HCH₂CO₂à H⁺ + CH₂CO₂ ^–

HCl à H⁺ + Cl^-

NaClà Na⁺ + Cl^-

NaOHà Na⁺ + OH^-