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First Homework• If you add 10 g of CaSO4 to 0.5 liters of water, what

would the molarity of the solution be and what would theppm concentration of CaSO4 and of Ca be?

First step is the formula weight of CaSO4Ca=40S=32O=16 but it is O4 so 16 x 4=6440+32+64=136 g per moleMole per liter is conventional measure500 ml =0.5LFor a 1 M solution in 0.5 L = 136/2=68 g M

First Homework10 g is what you added10/68= molar concentration = 0.147 M CaSO4

What is the ppm concentration?Remember ppm = mg kgIt is a measure ofIt is a measure of weight not concentrationweight not concentrationYou added 10 g or 10,000 mgBut again you added this to 0.5 L or 500g of waterThis is the same as 10 000 mg per 500 000 mgSo to get ppm as you used half a kg, you can just

double the concentration10 000/500 000 X 2 = 20 000/1 000 000

First HomeworkThen what portion of what you added was Ca?What is the ppm of Ca in the solution?FW Ca = 40 FW of CaSO4 = 13640/136 is the portion of CaSO4 that is Ca0.294 * 20 000 = ppm concentration of Ca= 5882 mg kg Ca in a 0.147 M CaSO4 solution

If the average amount of water that falls on Africais 10001000 km3km3 and the average need per person is 400400

m3m3, why is it that there are potential watershortages on this continent ?

Realize first what the difference in units ism3 versus km3

1000 m = 1 km1000mx 1000m x1000m = 1 km3

400 m3= 400 x 10-9km3=0.0000004 km3

Then realize

• The continent doesn’t consist of abig collection basin

• Rainfall distribution varies widely byseason and by region

• Water quality is a huge issue

Environmentally importantweak acids and bases

• CO2 in water- acids– H2CO3 - carbonic acid

• CO2 in water- bases– HCO3

- - bicarbonate– CO3

2- -carbonate– CaCO3- calcium carbonate

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How does CO2 get into water?

• Equilibriumbetween what isin air and water

• Governed byHenry’s Law

• Describes howmuch of aparticularsubstance will bein liquid and gasphase

Henry’s Law

[CO2(aq)]= KHP(CO2)CO2 in water will depend ona constant (KH and the partial pressure of

CO2)Pressure will vary by temperature and

concentration

For the weak and the strong-Acid means donates a H+

• CO2 in water- acidsHH22COCO33 - carbonic acid - carbonic acid→→H+ +HCOH+ +HCO33

--

Ka 4.5 x 10Ka 4.5 x 10-7-7

HCO3- - bicarbonate → H+ +CO3

2-

Ka 4.7 x 10-11

•

And we can figure out what the pHof water will be with this weak acid• Ka= 4.45x10-7=[H+][HCO3

-]/[CO2]• 4.45x10-7=[H+][HCO3

-]/1.146 x10-5

• (4.45x10-7 * 1.146x10-5)=[H+]2

• [H+] = (4.45x10-7 * 1.146x10-5)1/2

• [H+] = 2.25 x 10-6

• pH= - log of 2.25 x 10-6=5.65

For the weak and the strong-Base means donates an OH-

• CO2 in water- bases– CaCO3- calcium carbonate⇔Ca2+ +CO3

2-

– CO32- -carbonate +H2O → HCO3

-+OH-

– Kh = 2.1 x 10-4

– If you take into account the dissolution oflimestone

– (CaCO3) with acidification of CO2 in waters,the pH of natural waters would rise to 10

– HCO3- - bicarbonate + H2O→ H2CO3

2- +OH-

– Kb = 2.2 x 10-8

How does carbon in these forms figurein in the ocean?

• CO2 + H2CO3

• HCO3-

• CO32-

• Dead organic• Living organic

• 0.018• 2.6• 0.33• 0.23• 0.007

Units 10 18 mol CStumm and Morgan

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The ocean

•• Big acid base equilibrium reactorBig acid base equilibrium reactor•• Acid from inside the earth isAcid from inside the earth is

neutralized by the dissolution of theneutralized by the dissolution of thebasic rocks on the surfacebasic rocks on the surface

Back to relationships:Other types of home wreckers

• Other things can beadded to water thatwill rip it apart

• Can you name theculprit here (it isn’tjust dirt)?

Some ions in solution arestrong enough to rip apart

parts of the water that formtheir shell of hydration•

Shell of hydration•

Na

K

Remember how ions will beSurrounded by water when They go into solution and thatThe water will be oriented toNeutralize their charge?

For highly charged ions

•

Their charge is strong enoughTo rip the water apart

Fe

Iron and Aluminum will do this to 3 waters

•

And then turn into a solid

Fe

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This is what it looks like

Drainage from SRAP prior to remediation (April 02)

Shallowgroundwatermonitoring

well.

SW6

Iron-staining onconcrete culvert

at SRAP

Corrosion of metal pipes in drainage basin at SRAP.

Large open hole in galvanizedwater control structure allowingdirect bypass of acidic sediments

And the cover of the mostWidely read environmental journal Moving past acid/base

• This discussion has had an emphasison acids, but it all works the same inreverse. Same process for basedominated systems with OH- beingthe driver instead of H+

• These will affect waters in arid areas• As will salts

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Salt or Fresh?• Water will have a range of ions in it• These ions are generally salts• A salt is a precipitate of an anion and

a cation ( a - and a +) NaCl, CaCl, CaSO4,MgNO3 are all examples ofsalts. Some are very solubleand some are only moderatelysoluble

To the Sea, to the Sea• Salt water has about 220 x the

amount of ions as fresh water

1 cubic foot of seawater will contain about 2.2 pounds of salt1 ft3 fresh water will haveless than 0.01 lbs salt

Why?• Hydrological cycle- rains and runoff

from rains go to rivers and lakes• Water then goes to the big

evaporative basins we call oceans

What salts in water do• When salts are in solution, they will

be present as charged ions• What can charged ions do?

Electrical Conductivity

• Ability of water to conductelectricity is the measure that isused to see how salty it is

• The saltier it is, the more ions will bein solution, the better the water willbe able to conduct electricity

The saltier it is-the more you have to think about the

activity of the ion rather than theactual concentration

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Activity affects the ability to do workActivity is directly proportional to ionic

strength(Cars in the carpool versus regular lane)

Until you reach a certain ionicstrength, the behavior of ions insolution will be similar to an ideal

solution

Above that strength, the activity ofthe ion will be less than it would be inan ideal solution, so that the ability ofthe ion to do work, engage in reactions

is reduced

Ionic Strength

• Measure of how salty the solution isor how many ions are floating aroundin it

• Equation• I (ionic strength)=1/2∑MiZi2

• Where M = molarity and Z = charge

Charge of the ions

• I (ionic strength)=1/2∑MiZZii22

• Where M = molarity and Z = charge• Because that charge is squared that

means that the higher valence ionshave a huge effect on ionic strength

Al3+ versus K+

• 1/2∑MiZZii22

• 0.5 ∑ 1 x 32

• 0.5 x 4.5• I = 2.25

• 1/2∑MiZZii22

• 0.5 ∑ 1 x12

• 0.5 x 1• I = 0.5

msnbc

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Dissolved Oxygen (DO) and

Biological Oxygen Demand (BOD)

This is all about eating

We need O2 to eat andBOD is a measure of how much food there is to eat

Dissolved Oxygen (DO)

• DO is a function of a range ofvariables

• We can predict whether DO will behigh or low based on these variables

• We (you) can also measure DO

Biological Oxygen Demand

• Is one of the major variables thatcan impact DO

• It is also the one that is oftenaltered by anthropogenic activities

Dissolved Oxygen• The same way that

there is dissolved CO2in the water, there willalso be dissolved O2 inthe water

Factors• Henry’s law- equilibrium concentrations will

vary with temperature• Ability of O2 to dissolve in water

decreases with increasing temperature• At 20° C the solubility of O2 is about 9 mg

L or 0.3 millimolar or 0.3 x 10-3 M

Fast running versus Still• Moving water, wind, waterfalls, waves will

all incorporate air into water• Still waters are much more likely not be

saturated with O2

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Plants• Photosynthesis creates oxygen• 6CO2+ 6H2O → C6H12O6 +6O2

www.cdatribe-nsn.gov

Ionic strength• Waters with high concentrations of

dissolved ions can hold less gas than waterswith low concentrations of dissolved ionsand dissolved or suspended solids

Where food comes fromWWTP effluent, domestic wastes

Agricultural wastes

Eating- aerobic eaters

CH2O + O2= CO2+H2O+energy

www.co.dane.wi.us

BOD is a measure of how muchfood there is and so how much

oxygen will be used up

• BOD for different types of wastes(mg O2 per liter of wastewater)

• Domestic sewage• All manufacturing• Chemicals and

allied products• Paper• Food• Metals

• 165• 200• 314

• 372• 747• 13

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Example

• Given:– Above a wwtp outfall, DO = 7 ppm– Below the outfall, DO = 4 ppm

• Find: the amount of CH2O degraded

• Assume: no O2 from atm, photosynthesis

Answer

• 7-4 = 3 mg/L O2 consumed– One mole of CH2O per mole O2

– Total demand * FW CH2O/FWO2

(3 mg/L)*(12+2+16)/(2*16) = 2.8mg/L CH2O

Lake in the summer

DO at surface?DO at depth?

Variations in a river

Variations in a riverCity of Boulder-Boulder Creek

• September• 3.4 - 17 mg L

• February• 8.2-12.6 mg L

What accounts for the widerswings in summer?

Would you expect anydifference in a 24 hour period?

DO in Boulder over a 24 hr periodin February

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Yakima RiverUSGS study on water quality