ACCURACY & PRECISION and MEASUREMENTS AND UNCERTAINTY!!

   

• Precision: how reproducible a measurement is compared to other similar measurements.
• Accuracy: how close the measurement (for average measurement) comes to the accepted or real value.

-No measurement is exact.  Every measurement is only a best estimate which has some degree of uncertainty.

-Only when e count a set of objects do we get an exact number, for example, a class of 28 students.

Absolute Uncertainty
-The uncertainty expressed in the units of measurement, not as a ratio. 

METHOD 1  Make at least 3 measurements.  Calculate the average,  The absolute uncertainty is the largest difference between the average and the lowest or highest resaonable measurements (ie discard unreasonable data first).

METHOD 2 Determine the uncertainty of each instrument.
                      When making a measurement, always measure to the best precision that you can.  Therefore, you shouild estimate to a fraction 0.1 fo the smallest segment on the instrument scale.

                      On your ruler the smallest division is 1mm.  Your best precision should be to break this into 10 equal pieces or 0.1mm.

Relative Uncertainty and Significant Figures

Relative uncertainty=Absolute uncertaitny/Estimated measurement

Relative Uncertainty Can Be Expressed:

i) in precent (%)

ii) or using significant figures

                 

Significant Figure Oct.26

What is significant figures?
-Measure or meaningful digits.
-More preise mensthere are more significant digits.

A.Significant Digits
-The last digit in a measurement is uncertain as it could be one higher or one digit lower very eaily.
Eg. 2.78g on a balance : the 2&7 are CERTAIN number and 8 is UNCERTAIN
-The significant digits in the measurement includes all of the certain digits plus only the first uncertain digits.
Eg. the 2.78g has "3" significant digits

B.Significant Figures:Details
-Leading zero(s) aren't counted
Eg.0.01 has ONE siginificant digit
-Trailing zero(s) after the decimal point are counted
Eg.1.000 000 has SEVEN significant digits.
-Trailing zero(s) without a decimal point are NOT counted
Eg.1900 has Two significant digit.

C.Exact Numbers
-Some quantities are defined as exactly a certain amount and rounding is required.
Eg. A "pair" of shoes=2 shoes

D.Rounding Rules
-We round answers to the appropriate number of digits using rules similar to math with one set of exception.
-Look at the digits after the position of rounding.
1.Digit >5 ,  round up
Eg.64.36=64.4
2.Digit <5,   round down
64.33=64.3
3.①Digit=5,  more numbers (non-zero) after 5,  round up
Eg. 64.35123=64.4
   ②Digit=5,  ends at the 5, round to make the last digit EVEN(0.2.4.6.8)
Eg. 64.35=64.4
      64.55=64.6

E.Math Rules 1. “+”&“--"
-When adding & subtracting,  round to the fewest number of decial places
Eg      12.333
      +  20.1
-----------------
           32.433
Round to 1 decimal place, the answer=32.4
Eg     12500
   +      6000
---------------
          18500

Round to the thousands place (the first uncertain digit)=18000

F.Math Rules2   "× " & "÷"
-When multiplying or dividing, round to the fewest number significant
Eg.         12.01     4 sig figures
       ×       1.5       2  sig figures
------------------------
                6.005
       +     12.01       
---------------------------
              18.015
Round to 2 sig figures (2<4) or the 1st uncertain digit.
Answer=18.02

Eg. Long Division
12.54÷1.3=9.64
Round to 2 sig figures
Answer=9.6

Lab 3B

Separation of a Mixture by Paper Chromatography

Chromatography is one technique used by chemists to separate mixture of chemical compounds in order to identify or iso;ate their components on chromatography, mixture are separated according to the different solubilities of the components in liquids, or their adsorption on solids.
The method of identifying components is to calculate the Rf value of each.
Rf=d1/d2 where d1=distance traveled by solute
                     d2=distance traveled by solvent
the lab report:

Objective: 

  1. To assemble and operate a paper chromatography apparatus

  2. To study the meaning and significance of Rf values

  3. To test various food colourings and to calculate their Rf values

  4. To compare measured Rf values with standard Rf values

  5. To separate mixtures of food colourings into their components

  6. To identify the components of mixture by means of their Rf values

Material and Equipment:

  Refer to page 33 in Health Lab Text, Lab 3B

Procedure:

  Refer to page 33 in Health Lab Text, Lab 3B

Data and Observation:

  Attached to the back of the lab report.(on the paper....)

Analysis of Results:

1. A, We tested the yellow colouring. It contains Yellow #6

   B. We didn't get any.

2. Green food colouring contains about 1/3 of yellow colouring(Yellow #6) and 2/3 of blue colouring(Blue #2).

3. The unknown mixture is composed of 1/2 of red colouring(Red #4), 1/3 of blue colouring(Blue #2) and 1/6 of Yellow colouring(Yellow #6)

4. The ink mark might be blurred and move up.

5. The green colouring can be decomposed into blue and yellow, but red, yellow and blue colourings cannot be decomposed.

Following-Up Questions

1. The Rf of those colourings are 0.78 and 0.38. They are likely to be Red#3 and Yellow #6.

2. d1= 12*1.0 = 12  d2 = 12*0.41 = 4.92  Δd = 12-4.92 = 7.08cm

3. There is no way that a solute move above the solvent. It is impossible.

Conclusion

  From this lab, we got to know the way we separate a mixture by Paper Chromatography. The Rf value means the ratio of distances raveled by solute and solvent. Different substances usually have different Rf values. We can use this property to separate different substances. And we can also identify the substances by checking the table of Rf values. 

Separating Mixtures Oct 16th

Separation:
-Basis for separation: Different components and properties-components in a mixture retain their identities
-the more similar the properties are, the more difficult it is to separate them.
-Strategy: use any possible way to discriminate between components with different properties.

Hand Separation & Evaporation:
-Hand separation (Solid+Solid) 
-Use a magnet or sieve
-Evaporation(Solid dissolved in liquids)      boil away the liquid and the solids remain

Filtration (Solids NOT dissolve in water)
-use porous filter to separate the solids out. (If the pores are smaller than the solids)
-use filter paper-residue left in filter paper, filtrate, filtrate goes through filter paper

Crystallization(Solid in liquid)
-precipitation is the conversion of a solution to solid form by chemical or physical change
-solids are then separated by filtration or floatation
-saturated solution of a desire solid
-evaporate or cool-solids come out as pure crystals. Then crystals are filtered from remaining solvent.

Gravity Separation
-solids based on density
-A centrifuge whirls the test tube around at high speed forcing the denser materials to the bottom. Work best for small volumes.

Solvent Extraction
-a component moves into a solvent shaken with the mixture
-works best with solvents that dissolve only one components
-mechanical mixture:(Solid & Solid)
2 solids use liquid, one dissolved, get another one.
-solution: solvent is insoluble with solvent already present.
solvent dissolved 1 or more desire solids and leaves unwanted solids behind.
(If shaken in a separator funnel, the liquids will form layers then drain the solvent to leave the wanted material)

Distillation(Liquid in liquid solution)
-heating a mixture can cause low-boiling components to volatilize（vaporize）
-distillation is collecting and condensing volatilized components
-liquids with lowest boiling temperature boil first---vapor ascents to distillation flask and enters condenser; gas cools and condenses back to liquid dropping the distillate as a purified liquid.

Chromatography
-flow the mixture over a material that retains some components more than others, so different components flow over the material at different speeds.
-a mobile phase sweeps the sample over a stationary phase
-can separate very complex mixtures
-very small sample sizes analyses----highly accurate & precise
-separated components can be collected individually

Paper Chromatography(PC):
-stationary phase is liquid soaked into a sheet or strip paper mobile phase is a liquid solvent some components spend more time in stationary phase than other components appear as separate spots spread out on the paper after drying or "developing".

Sheet Chromatography:Thin layer chromatography (TLC）:
-stationary phase is a thin layer of absorbent (Al202 or So2 usually) coating a sheet of plastic or glass some components bond to the absorbent strongly other more weakly as with paper chromatography, components appear as spots on the sheet.

Naming Acids！！！！！！

Acids are formed when a compound composed of Hydrogen ions and a negatively charged ion are dissolved in water(aqueous, aq)！
-ions separate when dissolved in water
-H+ ions join with H2O from H3O+ (Hydronium ion)
ex: H + Cl --> HCl
    HCl(g)+H2O-->H3O(aq)+Cl(aq)

How to name acids?

For Simple Acids:
1. use "hydro" as the beginning
2. last syllable of the nonmetal is dropped and replaced with "-ic"
3. add "acid" at the end.

formula:
*****ide ===> hydro*****ic acid
ex.
HF: hydrofluoric acid
HCl: hydrochloric acid
HBr: hydrobromic acid
HCN: hydrocyanic acid

For Complex Acids:
1. -ate replace with "-ic"
   -ite replace with "-ous"
2. "Acid" at the end of the name

formula:
*****ate ===> *****ic acid
*****ite ===> *****ous acid

ex.
HNO3: Nitric acid
HNO2: Nitrous acid
H3PO4: Phosphoric acid
H2SO4: Sulphuric acid (exception)

SUMMARY OF TEXTBOOK (PAGE25-39)!!

Summary of Chapter 2: Finding out about Matter
Thesis: anything that has mass the takes up space is called matter.

Matter can be basically divided into two majoy groups: pure substance & mixture
Division 1: Pure substance: one set of properties & one kind of particle
elements and compounds are examples of pure substance

Division 2: Mixture: more than one set of properties and substances & physically combined
homogeneous and heterogeneous are examples of mixture 

Physical & Chemical Changes:
Physical change: no new substance is formed
Chemical change: new substance is formed

Characters of Pure Substance:
Pure substance has a constant boiling point
The temperature at which a liquid changes to a solid is called its freezing point while the opposite is melting point

Compounds & Elements:
Pure substance that can be decomposed into new knids of matter are called compounds
The elemental building blocks of all kinds of matter and are called elements

Compounds Have a Definite Composition:
The compounds will have a definite composition which is the different between mixture of elements and compounds of elements------this fact is called the Law of Definite Composition


Matter Is Made of Atom

Atom:
The word atom means this smallest possible piece of something

Element:
Particles made of more than one atom are called molecules

Compound:
particles joining together to form one new particle with different or same property

(LAB) HEATING&COOLING CURVES OF A PURE SUBSTANCE!!

Lab: Heating&Cooling curves of a Pure Substance

Objectives:
1. to investigate the heating process for solid dodecanoic acid
2. to investigate the cooling process for liquid dodecanoic acid
3. to determine and compare the melting and freezing points of dodecanoic acid

Supplies:
Equipment                                                            
ring stand                                                                   beaker (400mL)
buret clamp                                                                thermometer for water bath                   
hot plate                                                                     lab apron
test tube (18mm×150mm)                                          safety goggles      
      assembly (half full with solid
                      dodecanoic acid with 
                      thermometer embedded) 

Chemical Reagents
dodecanoic acid, C11H23COOH
     (in test tube apparatus)

Procedure:
Part I: the Heating Process
1. put on lab apron and goggles
2. deciding the roles with your partner as one is recorder and one is obsever
3. obtain a test tube assembly embedded a solid dodecanoic acid
4. put 300mL water in a 400mL beaker and put it on the hot place, initially turn to the high temperature to boil the water to 55°C to 60°C
5. turn to the low heat as the temperature of water reaches the 55°C, lower the test tube on the clamp ring into the water and start recording the temperature of solid acid every 30s
6. continue recording until the acid reaches a temperature of 50°C. stir to mix the solid and liquid as the acid begins to melt and record the time it melts and it ends
7. turn off the hot plate and quickly move to Part II

Part II: the Cooling Process
1. may exchange the roles with your partner
2. raise the test tube out of the hot water and replace the hot water with the normal one with room temperature
3. immediately start monitoring the cooling process after put the test tube into the normal water, record the temperature
4. record the temperature every 30s until it reaches 25°C, record the solidification begins and ends
5. may stop recording when the temperature of acid is 25°C
6. put every single tool back to the right place properly
7. wash hands thoroughly before leave

Reagent Disposal:
The test tube assembly containing the solidified dodecanoic acid with embedded thermometer is to be stored for future use.

Post Lab Considerations:
The results you have collected can now be analyzed with graphs that will represent the heating and cooling curves of dodecanoic acid.

Experimental Results:
Table 1 Heating and Cooling of Dodecanoic Acid

Analysis of Results:
Part I: the Heating Process
1. using the results you obtained during the heating process, construct a graph of temperature versus time.  use small circles for these data points and sketch a smooth curve through these points.
2. indicate on the graph where melting began and ended.
3. from your heating curve, determine the melting point of dodecanoic acid.

Part II: the Cooling Process
1. on the same graph as the heating curve, plot temperature versus time for the cooling process.  use small squares to distinguish these data points and sketch a smooth curve through these points.
2. indicate on the graph where solidification began and ended.
3. from your heating curve, determine the freezing point of dodecanoic acid.
4. compare your melting and freezing points with each other and with those of two other lab groups.  explain any similarities and differences.
5. what can you conclude about the melting and freezing points of a pure substance?

Follow-up Questions:
1. how would you explain the plateaus in your heating and cooling curves?
2. suppose that more dodecanoic acid had been used in Part I.  what would be the change in appearance of the new cooling curve?  sketch it.

Conclusion:
State the results of Objective 3.

LAW OF DEFINITE/MULTIPLE COMPOSITION!!

Law of definite composition:
                Compounds will have a definite composition
          ex. H2O will be H2O anywhere

               (always have 2 hydrogen and 1 oxygen)

Law of multiple composition:

                When 2 or more compounds with different proportions of the same elements can be made

          ex. CO2 → (×2) C2O4

                carbon           dicarbon               (oxalate)

                dioxide           teraocide                

The Heating/Cooling Curve of a Pure Substance Notes:

First step: solid state at any temperature below its melting popint.  Particles are very closely packed together in an  orderly manner.  Forces between the particles are very strong and can only vibrate at a fixed position.

Second step: As it is heated, heat energy is converted into kinetic energy, kinetic energy increases and the molecules vibrate faster about thier fixed positions and their temperature increases.

Third step: Still solid, melting has begun---solid begins to change into liquid.  Temperature remains the same.

Forth step: It exists in both solid & liquid state.  The temperature remains constant the heat that is supplied to it used to overcome the forces of attraction that hold the particles together.  The constant temperature is called the melting point.  The heat energy that abnsorbed to overcome the intermolecular forces is named as the latent of fusion.

Fifth step: All has completely melted.  Solid has turned into liquid.

Sixth step: It is in liquid state.  As the liquid it is heated, the molecules gain more heat energy and the temperature continues to increase.  The particles move faster and faster because their kinetic energy is increasing.

Seventh step: Exists in liquid state.  Molecules have receieved enough energy to overcome the forces of attraction between the particles in the liquid.  Some of the molecules start to move freely and begin the change into gas.

Eighth step: Exists in both liquid and gaseous states.  Temperature remains unchanged.  Heat energy absorbed is used to overcome the intermolecular forces between the particles of liquid rather than increase the temperature.  The constant temperature is boiling point.

Nineth step: All of the liquid has turned into gas.

Last step: The gas particles continue to absorb more energy and more faster.  The temperature increases as heating continues.

Below is a diagram clealy demonstrating the steps of how solid has been transformed into gas through heat.