Measurement and Uncertainty

Hello

We are going to cover measurement and different types of uncertainty.

There are some rules that you need to know regarding Measurement and Uncertainty

• no measurement is exact
• every measurement is a best estimate which has some degree of uncertainty
• only when we count a set of objects, we get an exact number

Absolute Uncertainty

This is expressed in units of measurement; not ratio

In order to express uncertainty:

• Make at least 3 measurements
• Calculate average
• Absolute uncertainty is the largest difference between the average and lowest or highest reasonable measurement
• Discard unreasonable data

Eg.1  47cm, 48cm, 45cm, 46cm
          
Calculate the average of these numbers ((47+48+45+46)/4)) = 46.5

Then calculate the difference 46.5- 45 = 1.5 or 48 - 46.5 = 1.5 (does not matter because the result is the same)

Therefore + 1.50cm is the absolute uncertainty.

Absolute uncertainty:

This is determined by uncertainty of each instrument

• Always measure to the best precision
• Estimate to 1/10 or 0.1 of the smallest segment of the instrument scale

Eg.
Take a ruler

The smallest scale is 0.1cm
0.1 of the scale is 0.01 cm
Therefore the absolute uncertainty is 0.01cm

This is how you express it: (the average value) + (absolute uncertainty)

 Eg. 15.6 + 0.03cm

Don't forget the units.

Relative Uncertainty

Relative Uncertainty is absolute uncertainty/estimated measurement
It is expressed in percent or significant figures(the number of significant figures indicates relative uncertainty; the last digit is uncertain)

Written by JK (Oct 31,2010)

Accuracy and Precision

Hello

We are going to explain what accuracy and precision is.

The Textbook says that:

ACCURACY is how close the measurement (or average measurement) comes to real value

PRECISION is how reproducible a measurement is compared to other similar measurements


Written by JK (Oct 31, 2010)

Significant Figures

Hello how are you doing?

Ready to learn more new stuff about chemistry.

We are going to teach you about "Significant Figures" or for a lack of a better expression: How to write you math answers correctly.

I know that right now you people are going to be like "Why are we learning math?", "Isn't this physics or something?" and yapping about how this isn't chemistry.

This part is about units and writing them properly.

Significant Figures are not very big or small numbers, it is MEANINGFUL NUMBERS

There are some rules that you need to know:

• Last digit of a number is uncertain
• That number can be either one digit higher or lower

Eg: Take 2.56g ; numbers 2 and 5 are certain however 6 is uncertain therefore it can either be one digit higher(7) or lower(5)

• Significant Figures  includes all certain digits plus one uncertain digits

There are some exceptions

• Leading zeros DON'T count (eg. 0.01 has 1 sig. digit because 0.0 are leading zeros)
• Trailing Zeros COUNT (eg. 10.050 has 5 sig digits because 0 after 5 is trailing and 0's before 1 and 5 are between certain numbers)
• Trailing zeros without decimal point DON'T count (eg. 12000 has 2 sig. digit because 0's after 12 is not stopped by a decimal)

NOTE:

Some quantities are exact meaning no decimal mubers

Eg. pair of shoes are always 2 shoes

       1 dog not 0.75 dog

       1m = 100cm

These numbers are decided so think carefully.

                                                                                  

Rounding:

Rounding numbers are basically the same as what you would do in math class

However with one set of EXCEPTIONS:

1. Look at the digit after the number you have to round
2. If that digit is greater than 5, round up
3. If that digit is less than 5, keep the same
4. If that digit is equal to 5 and there are more digits after, round up
5. If that digit is equal to 5 and it ends at the 5, make the last number even (eg. 0,2,4,6,8) 

                                                                          

Adding, Subtracting, Multiplying and Dividing:

When adding or subtracting round to the fewest number of decimal places

Eg.  12.544g
       + 1.3g    
         13.844g

In this case you have to round to 1 decimal place because 1.3 has fewer number of decimals than 12.544 so the answer becomes 13. 8g

Eg.  12500mL
       + 6000mL
         18500mL

In this case, you have to round to the thousands because that is fewer digits than 12500 so the answer becomes 18000mL 

When multiplying or dividing, round to the fewest number of significant digits:

Eg.  12.54m
       x  1.3 m
        3762
    +12540
       16.302square meters

In this case, you should round the answer so that there are 2 significant digits so the answer becomes 16meters squared

                            9.646m
Eg.   1.3m |  12.54m
                      117      
                           84
                           78
                             60
                             52
                               8

In this case, the answer is 9.6m because 1.3 has 2 sig. digits and 12.54 has 4 sig digits.

REMEMBER:

Always use the rule: round to the fewest sig. figs for multiplying and dividing

Thank you for tuning in. We will see you next time

Ciao!

Written by JK (Oct 27,2010)

Summary of Textbook (p25-34, 36-39)

Hello

Today we are going to explain CH.2 of the Chemistry textbook.

From the previous posts, we explained to you that matter is everything around us.

However to understand matter fully, we must look at it on a macroscopic level.

This is like look at the sand on a beach.

If we see this sand from far away, all we see is a blob of light brown.

However, if we look closely, we can see the individual grains of sand.

In our world, matter can be in a form of a mixture, which is formed with 2 or more kinds of matter that have separate identities.

Classifying matter into either a pure form or a mixture is a big step in unders
tanding matter.

You can make a mixture by simply adding stuff to your water maybe Kool-Aid or sweetners.
Sometimes, you can find a mixture that does not scatter light and looks uniform throughout. These mixtures are called solutions.

Mixtures can be separated by using different procedures(eg. distillation).

There are few characteristics that you should know about Pure Substances:
actually you need to know this fact: the melting point and the freezing point of a pure substance is the same!!

(Info of chemical and physical changes: refer to "Changes in Matter" article)

There are few rules that you need to know (in general)

Law of Definite Composition: The compounds will have a definite composition.

Law of Multiple Proportions: The fact that two or more compounds with different proportions of the same elements.

(For summary of p36-39) refer to "All about Matter!!" article.

Thanks for tuning in.

Written by JK (Oct4,2010)

PS been edited a few times

More Details in Separation (Hehehe...)

Helloo!! We are back.

We are going to tell you more about different separation techniques.

Hand Separation:

• Used usually for mechanical mixture or heterogeneous mixtures
• separation by a hand, magnet or a sieve
• Evaporation which is solid dissolved in liquid solution or boil away the liquid and solid remains

Filtration:

• deals with solids(not dissolved) and liquids
• passing a mixture that contains solid particles through a porous filter
• the liquid pass through the filter and residue is left on the filter
• separation by using filter paper

Crystallization:

• solids in liquids
• Precipitation: conversion of a solute to solid form by chemical or physical change
• after, solids are separated by filteration or floatation
• saturated solution of a desired solid evaporate or cool - solid come out as crystals

Gravitiy Separation:

• solids based on gravity
• centrifuge whirls the test tube around at high speed forcing the denser materials to the bottom
• Works best for small volumes

Solvent Extraction:

• a component moves into a solvent shaken with the mixture
• works best with solvents that dissolve only one component
• For mechanical mixtures: use liquid to dissolve one solid but not the other so that the desired solid is left behind or dissolved
• Solution: solvent is insoluble with solvent already present

Distillation:

• Heating a mixture creates vapour
• collecting and condensing vollatillized components
• liquid with lowest boiling point boils first
• vapour ascents to distillation flask and enters condenser
• gas cools and condenses > back to liquid and distillates as a purified liquid

Chromatography:

•  Flow mixture over a material and different components goes in different speeds and stops
• mobile phase sweeps the sample over a stationary phase
• can separate complex mixtures
• very small sample sizes; analysis is highly accurate and precise
• separated components can be collected individually

Paper Chromatography:

•  Stationary Phase: liquid soaked into sheet of paper
• Mobile Phase: liquid solvent some components spend more time in the stationary phase than others after drying; spots appear on the paper

Thin layer Chromatography (TLC):

• Stationary Phase: thin layer of absorbent (Al203 or SiO3) coating a sheet of plastic or glass
• some components bond to the absorbent strongly; others more weakly
• appear as spots

Written by JK (Oct 18, 2010)

Separation Techniques

Hello everyone, we are back again.

Today we will talk to you about basics of separating different components and properties.

Separations are all about mixtures and distinguishing the different components and properties in them.

 Before we learn the different techniques to separate mixtures, we need know a few things. :D

• Components in a mixture do not lose their identities

• Components have different properties

• More similar the properties are, more difficult it is to separate.

Also one more thing, when you want to separate a mixture, you need to have a strategy which is to devise a plan with the different techniques to discriminate between components with different properties.

Now onto the different types of separation:

Filtration: This method separates the components by particle size.

Filtration

Floatation: This method separates the components by density.

Crystallization: This method separates the components by solubility(ability to dissolve).

                                 

 Extraction: This method separates the components by solubility.

                                  

Distillation: This method separates by boiling point.

                               

Chromatography: This method separate components by affinity for a stationary phrase.

                                         

Now you have learned the different types of separation.

Tune in soon to learn more about separation in detail.

Written by JK(Oct, 18/2010)

How to Name Complex Acids

Hello we are back with more acids!! (AHHH!! It's burning my face!!)

Here are the rules:

1. any polyatomic atoms(that are negatively charged) that end with --ate replace with --ic
    any polyatomic atoms(that are negatively charged) that end with --ite replace with --ous

2. Put "acid" at the end

Examples:

HCH3C00 - acetic acid (CH3C00 is acetate which you need to replace with --ic)

HClO3      - chloric acid (ClO3 is chlorate which you need to replace with --ic)

HClO2      - chlorous acid (ClO2 is chlorite which you need to replace with --ous)

H2SO3      - sulphurous acid (SO3 is sulphite which you need to replace with --ous)

So this is basically how you name acids!!! :D

Thanks for tuning in!! Bye!


Written by JK (Oct 14,2010)

How to Name Acids

Acids are formed when a compound compsed of hydorogen ions and negatively charged ions become an aqueous or dissolved in water.

*Note: all numbers underlined are subscripts.

In water, these ions separate when dissoved. Also hydrogen ions join with H20(water) to form H30 (hydroniums)



Eg. H+ Cl - => HCl

       HCl (g) +H20 => H30 (aq)  + Cl (aq)

Guidelines in Naming Acids:

For columns 16 and 17, use simple acid formulas.

1. Use "hydro" as the beginning

2. Last syllable of the non metal is dropped and replaced with -ic.

3. Add "acid" at the end

* ________ide  → hydro___ic acid

Eg.

1. HF(aq)  -  hydrofluoric acid

2. HCl(aq) -  hydrochloric acid

3. HBr(aq) - hydrobromic acid

4. HI (aq)  -  hydroiodic acid

5. H20 (aq)  - water  :D

6. H2S (aq) - hydrosulphic acid

7. H2Se (aq) - hydroselenic acid

Written by : JK (Oct 14, 2010)

Answers from the changes in matter questions

Here are the answers:

a) physical change because fog is water vapour that is condensed into droplets of moisture

b)chemical change because new substances are formed: smoke and cancer cells-causing chemicals)

c)chemical change because the growth involves chemicals being produces and used up

d)chemical change because rust is a new substance formed by combination of iron, air and water)

e)physical change because no reaction has occurred to make new substances

f) physical change because we are only separating substances, not producing new ones

If you got perfect, you are ready to go but if you got less than 4 you need to study... :(

Written by JK

Changes in Matter

There are 2 main changes in Matter : Physical and Chemical

In a Physical Change:

no new substance is formed

the chemical composition does not change

it is reversible (you can drink water(liquid) if you melt ice :)

Eg. boiling point, melting point

In a Chemical change:

it is irreversible (you can't undo a salad dressing)

new substances are produced

Eg. burning, cooking

So these are the basic changes in matter

Here are some questions that you can answer.

Classify each of the following as either chemical or a physical change

a) formation of a fog
b)burning a cigarette
c)sprouting of a seed
d)rusting of iron
e) mixing yellow and blue paint to make green paint
f) separating an alcohol- water mixture into water and alcohol

The answers will be up shortly!

Written by JK