4. Chemical Arithmetic

chapter 9

Greenhouse Gases (Chapt 8)

The Law of Conservation of Mass

Matter can

neither be created nor destroyed in a chemical reaction.

The Mole

• 1 mole of anything contains 6.02 x 1023 units (Avogadro’s number)

• In chemistry 1 mole of anything weighs the same as its molecular weight in grams

• So for methane CH4, atomic weight of C=12 and atomic weight of H=1.

• Thus one mole of methane weighs 12+4(1)=16 grams

Mole Weight What weight of carbon dioxide is generated for every mole of methane that we use as fuel ?

Equation: CH4 + 2O2 CO2 + 2H2O + Heat

Convert to molecular weights (grams)(12+4) + 2(16+16) (12+32) + 2(1+1+16)

Convert to moles (grams)16 g + 64 g 44 g + 36 g

methane watercarbon dioxide oxygen

1 mole (or 16 grams of methane) produces 44 grams of CO2.

Ramifications for Global Warming

• Combustion of carbonaceous fuels produces CO2 (a greenhouse gas) and heat

• Greenhouse effect-glass walls and roof of a greenhouse are transparent to visible light, but opaque to “thermal infrared”

• Surfaces inside the greenhouse absorb solar radiation and reradiate thermal IR

On the molecular level

• Surfaces then reradiate the heat as long wavelength IR radiation.

• Greenhouse gases absorb this IR radiation –causes them to bend & vibrate

• H2O, CH4, CO2 do this ,but major air components N2 (79%) and O2 (20%) are IR opaque

• Collisions between greenhouse gases and air molecules occur and temp of air rises.

In the atmosphere

• Same effect-solar radiation is absorbed by the earth (pavement etc). Eventually heat is re-emitted as IR radiation

• Then greenhouse gases absorb this IR radiation and transfer the heat to N2 and O2 by molecular collisions.

Pictorially…….

Is the greenhouse effect a good thing?

• No

• Yes

Yes! (in general)

• Without it, the Earth’s average temp would drop from a present value of ~15oC, to -15oC

• Problem, excess amounts of CO2, CH4 do contribute to global warming

• But, changes in strength of solar radiation may also be having an effect

Electromagnetic spectrum

Wave and particle character

• Energy of radiation is quantized (particle)

• frequency and wavelength are related (wave)

• 2 equations govern all regions (a continuum)

Concentrations

A solution is a homogeneous mixture of one substance (solute) dissolved in another(solvent).The

solute is present in a smaller proportion than the solvent .

The concentration of a solution expresses the quantity of solute(eg. moles or grams) dissolved in a specific

quantity of solution (usually stated as a volume).

Terms and units used to express concentrations can be: a) imprecise - 'teaspoon per glass' b) approximate (labels for consumer products) - 'percent' c) well-defined - 'molarity'

Molarity

.

The molarity of a solution refers simply to the number of moles of solute per liter of solution.

A 1M (one molar) solution contains one mole of solute in each liter of solution, a 2M solution contains

two moles of solute per liter of solution, etc.

Percentage Concentrations(parts per 100)Commercial labels often express concentrations of solutes as a percentage (%) of weight or volume .

Eg. a typical bottle of table vinegar states the product is "diluted with water to 5% acidity" or contains

5g of acetic acid in every 100g of vinegar.

A percentage concentration expressing the weight of solute for every 100 units of weight of the

solution is a weight / weight percentage, or w/w %.

A 3% solution of H2O2 in water makes up the common antiseptic hydrogen peroxide , but 30% H2O2 bleaches hair (don’t mix them up !)

Concentrations of Fat in milk

• 1% milk contains 1 gram of fat per 100 grams of milk.• Q: How many grams of fat in a 250 mL carton of 1%

milk?

• A: Assume density of milk is 1 gram per mL. (D=M/V).

• Thus the weight of fat in each carton is 250/100 =2.5 grams

• Skim milk contains 0% fat –does it have any calories in

it? Yes, since all milk contains protein and carbohydrate.

It is convenient to express exceedingly small concentrations, such as food contaminants and environmental pollutants, as parts per thousand (ppt), parts per million, parts per billion(ppb)

One part per million (1ppm) represents a convenient unit since it is the concentration of one

milligram (1/1000 gram) of one substance distributed throughout one kilogram (1000grams)

of another, ie. 1mg/kg.

Concentrations: Weight per volume (w/v)

Especially convenient when using aqueous (H2O) solutions because:

Density of H2O = 1 g/mL (cc) or 1kg/L X ppm = X mg/kg or X mg/L

Remember: Molarity = moles/L (>grams/L)

‘deci’ = 1/10 (10-1) ‘centi’ = 1/100 (10 -2)

‘milli’ = 1/1000 (10-3)

Factor Prefix Symbol

1.000.000.000=109 giga g

1.000.000=106 mega M

1000=103 kilo k

100=102 hecto h

10=101 deka da

0.1=10-1 deci d

0.01=10-2 centi c

0.001=10-3 milli m

0.000 00=10-6 micro u

0.000 000 001=10-9 nano n

0.000 000 000 001=10-12 pico p

An Example of Concentrations

Traces of iodide ion in the diet help prevent the enlargement of the thyroid gland, ie. goiter. To provide this dietary iodide KI is added to commercial table salt (NaCl) at about 7.6 x 10-5 g of KI per gram of NaCl .

Convert this concentration into ppm.

The concentration is 7.6 x 10-5 g KI / 1g NaCl

We want to know how many grams of KI there are in 106 g of table salt

7.6 x 10-5 g KI

1 g NaClx

106

106

7.6 x 101 g KI

1x106 g NaCl76 ppm KI==

Homeopathic Dilutions

Homeopathic medicine is based on ‘similarity’ between the disease and symptoms ‘induced’ by the

medicine.

Extreme dilutions are common and the cure is induced (supposedly) by a ‘memory effect’.

The mode of action is unknown but the treatments are effective for some people.

Each dilution is1/100, ie. 10-2.

Six successive dil’ns (10-12) are usual, 12 dilutions (10-24) are common and even 30 dilutions (10-60!) are

used. If a 1M sol’n contains 6 x 1023 molecules (approx.

1 x 1024)

A 12 times diluted sol’n contains 1024 x 10-24 = 1 molecule in1Liter!

If you buy 10mL of this sol’n what are your chances of getting that single molecule ?

Would you buy it?

Odds are 10/1000=.01 or 1 % chance of getting a single

molecule of the medicine in your sample.

Homeopathic Medicines

Fancy Names in Homeopathy

• “Kalium Muriaticum” 5CH means 5 dilutions of a factor of 100 each.

• So 10-10M concentration of KCl (present in normal table salt)

• Muriatic acid is 31% HCl in water

• Kalium is Latin for potassium (K in Periodic Table)