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Name Student #

Chemistry 301 - MIDTERM EXAM #1

Wednesday, October 5, 2005, 9:00 am

 Explain your answers whenever possible, Calculators are permitted.

Worth 20 points (20% of total marks for course)

1) (8 points; 2 points each) True or False? And briefly explain why. If the statement is false,

correct it. (NO credit is given for T/F alone – you NEED an explanation!)

a) If two lakes have the same pH, they have the same alkalinity.

 b) Water stabilizes our climate due to its high heat capacity.

c)  Polyphosphates are good chelators, but not as good as their breakdown product,

orthophosphate.

d)  Oxygen is often supersaturated in surface waters.

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2) (3 points) While traveling in some other region, you experience an odd difference in the

way soap feels when you shower – it doesn’t feel as “soapy” as it does here in Vancouver.

Why? What is this due to? Explain how this effect is produced.

3)  (3 points) What is the residence time of water in Lake "Small" if it's sole input is the onlyriver flowing out of Lake "Big", when "Small" is half the size of "Big", and the residence

time of water in "Big" is 20 years? (Assume that both are at steady state)

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4)  (6 points) At pH 9.33, what is the ratio of [HCO3-] to [CO3

-2]? If carbonate alkalinity is

1.00 x 10-3

 equivalents/litre, what is [HCO3-]? If 1 millimole of acid is added to 100 litres

of this water, what do you expect the pH to be? If the alkalinity were essentially zero,what would the pH be after the addition of the same amount of acid?

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Potentially useful data:

Reaction Kind of 

Constant

Value

CO2(aq)Í HCO3- + H+ pKa1 6.35

HCO3- Í CO3-2 + H+ pKa2 10.33

CO2(g)Í

 CO2 (aq) pKH 1.47H2S(g)Í H2S(aq) pKH 0.99

H2S(aq)Í HS- + H+ pKa1 7.02

HS- Í S-2 + H+ pKa2 13.9

H3PO4 Í H2PO4- + H+ pKa1 2.17

H2PO4- Í HPO4-2 + H+ pKa2 7.21

HPO4-2 Í PO4-3 + H+ pKa3 12.32

Fe(OH)3(s) + 3H+ Í Fe+3 + 3H2O pKsp -3.96

Fe(OH)2(s) + 2H+ Í Fe+2 + 2H2O pKsp -12.9

Pb(OH)2(s)Í Pb+2 + 2OH- pKsp 19.79

PbCO3(s)Í Pb+2 + CO3-2 pKsp 12.82CaCO3(s)Í Ca+2 + CO3

-2 pKsp 8.35

MnCO3(s)Í Mn+2 + CO3-2 pKsp 9.3

AlPO4(s)Í Al+3 + PO4-3 pKsp 21.0

Al+3 + OH- Í Al(OH)+2 pKf1 -9.0

Al+3 + 2OH- Í Al(OH)2+ pf2 -18.7

Al+3 + 3OH- Í Al(OH)3(s) pf3 -27.0

Al+3 + 4OH- Í Al(OH)4- pf4 -33.0

H3(NTA)Í H+ + H2(NTA)- pKa1 1.66

H2 (NTA)- Í H+ + H(NTA)-2 pK

a22.95

H(NTA)-2 Í H+ + (NTA)-3 pKa3 10.28

Ca+2 + (NTA)-3 Í Ca(NTA)- pKf  -8.2

Mn+2 + (NTA)-3 Í Mn(NTA)- pKf  -8.7

Fe+2 + (NTA)-3 Í Fe(NTA)- pKf  -9.6

Pb+2 + (NTA)-3 Í Pb(NTA)- pKf  -11.4

Al+3 + (NTA)-3 Í Al(NTA) pKf  -13.4

Fe+3 + (NTA)-3 Í Fe(NTA) pKf  -17.9

For the reaction: aA + bBÍ cC + dD,

K = [C]c[D]d at equilibrium, Q = [C]c[D]d at any given moment _______ _______ 

[A]a[B] b  [A]a[B] b

Henry’s Law: [X(aq)] = K H PX

[X(aq)] = concentration of gas X in water 

K H = Henry’s Law constant at specific T,P,S

PX = partial pressure of gas X in solution at equilibrium