Density

What is density?
Density is a physical property of matter. The formula to find density is:

Density = mass / volume

Ways you can tell how dense an object is through weight and sight suggested in these pictures:

-If the density of an object is greater than that of water the object will sink.

-If the density of an object is lower than that of water the object will float.

an example of how to find the density of an iron bar:
1200 g / 1.25 L = 960 g / L

Measurement and Uncertainty

In science, we take measurements. These, however, are never exact. Measurements are only our best estimate, which is subject to some uncertainty.
The only numbers that are exact are when you are counting whole things. Ex. seven apples, 15 pencils, 48 dogs, etc.

There are two types of Uncertainty: Absolute Uncertainty and Relative Uncertainty.

Absolute Uncertainty

- This uncertainty is expressed in units of measurement, not as a ratio
- Two methods can be used

Method One:
- Take at least three measurements
- Calculate the average of these numbers
- The Absolute Uncertainty of these measurements is the difference between the average and the highest or lowest reasonable measurement
- Always remove any measurements that are unreasonable

Ex.
Trial #    Mass of an Object
   1                 27.3 g
   2                 27.5 g
   3                 27.4 g
   4                 27.9 g
   5                 27.4 g

You would remove 27.9 g, since it is unreasonable. You would then add up the other measurements and divide by four, calculating the average:
109.6 g / 4 = 27.4 g
You would then find the difference between 27.4 g and the highest or lowest measurement:
27.4 g - 27.3 g = 0.1 g
27.4 g - 27.5 g - 0.1 g
Therefore, the Absolute Uncertainty of these measurements is 0.1 g. You would record the mass as:
27.4 +- 0.1 g

Method Two:
-Determine the uncertainty of the instrument being used 
-Measure as precisely as possible
-Estimate to 10% of the smallest division made on the instrument
Ex.
On a ruler, the smallest division is 0.1 of a cm, or 1 mm. Therefore, you should measure to the nearest tenth of a mm. If you estimate a measurement to be 25.34 cm on a ruler, you would record the number as 25.34 +- 0.01 cm. If you estimate a measurement to be 6.7oC on a thermostat, you would record the number as 6.7 +- 0.1 oC.

Relative Uncertainty
-This can be expressed as in percent or by using significant figures.
-Percent is most commonly used
-Relative Uncertainty = Absolute Uncertainty / Estimated Measurement
-To get a percent, you would multiply that number by 100.
Ex. Estimated measurement = 27.23 and the Absolute Uncertainty = 0.01
(0.01 / 27.23) X 100 = 0.03672
Therefore, the Relative Uncertainty is 0.04 %

Accuracy Vs. Precision and Sig Figs

Accuracy VS. Precision

Accuracy:

Is how close the measurement is to the actual accepted value!

Precision:

Is how reproducible the measurement is compared to other similar measures!

Significant Figures – Sig. Fig.

Significant figures simply are: how to write a math equation correctly!

The more precise a measurement is… it will result in more significant digits!

What are significant digits??? - They're significant figures

- They are the digits contained within a measurement up until the 1^st uncertain digit which include every certain digit and 1 uncertain digit.

What are Certain and Uncertain Digits???

-          Certain digits are known to be correct without any margin for error; they should include all the digits up until the last digit

-          Uncertain digits may have a margin for error; they should be the last number contained within a given measurement.

For example:

8.27 mL ----> 8 and 2 are certain digits and the 7 is the uncertain digit.

0s:

When counting significant digits one may encounter zeros... so in order to count sig figs...

1.       Trailing zeros are never counted when there is not a # left of the decimal place:                            

2.       E.g. 0.000008 has 1 significant digit

3.       Trailing zeros are counted when there is a number left of the decimal place:                               E.g. 7.08 has 3 significant digits

4.       Trailing zeros without a decimal place aren’t ever counted:                                                             E.g. 5800 has 2 significant digits

Exact #s:

Some values can be written easily as a specific amount and therefore rounding is unnecessary!  These numbers have an infinite number of significant figures.

                                For example: 7 can be expressed as 7.00000…

The Rules of Rounding:

Like Mathematics, in Science we always round to the appropriate number of digits by following a set of rules that slightly differs from Math.

1.       If digit after position of rounding is >5 round up.

2.       If digit after position of rounding is <5 round down.

3.       If digit after position of rounding =5 and there are no more digits (not including 0) round up.

4.       If digit after position of rounding =5 and ends at that number round to the closest even digit.

The Rules of Adding and Subtraction:

-          Always round to the fewest number of decimal places of one of the values within the equation because it is the first uncertain digit.

For example:

19.08 + 2.6 = 21.7

The Rules of Multiplication and Division:

-          Always round to the least amount of significant figures.

For example: 7.89 x 2.1 = 16

For additional information on significant figures and more examples visit:

http://www.ausetute.com.au/sigfig.html

Experiment 3B: Separation of a Mixture by Paper Chromatography

Experiment 3B:

Separation of a Mixture by Paper Chromatography

What is chromatography anyway?

It's a technique to separate mixtures of all kinds of chemical compounds used by chemists.  It's used to isolate and/or identify a mixture's components.



And why is that important?!

Because it has many practical applications such as determining the amount of pesticides in foods or drugs in a sample of urine.


In experiment 3B, my chemistry class tested various food colouring samples to to determine their Rf value (Rf = ratio of fronts).

We determined this value by using the formula: Rf = d1 . 
                                                                                                                   d2
Where d1 represents the distance travelled by the solute
and d2 represents the distance travelled by the solvent.

Note: The Rf value of a subatance will always be between 1 and 0.

For example:


Within Lab 3B, we measured the distance from the origin ( our pencil mark ) to the solute front (where the food colouring rose to); for d1.
Then we measured the distance from the origin to the solvent front ( where the water rose to ); for d2.  Lastly, we divided these two measurements.

Afterwards, we compared our results to Table 4 which contains the dyes approved for food colouring to see whether or not we used any of the dyes listed. 


What did I learn from this epic lab???

1. That food colouring is magical! Just kidding... I learned that despite my class was given the same instructions the results varied. Therefore one can conclude that many samples need to be taken to obtain accurate results.
2. That paper chromatography will not only determine Rf values but will also separate the components tested.  (e.g. the food colouring mixtures separted into their primary colours).

Here's a video of paper chromatography!

The Formation of Acids

The Three Commandments for naming simple acids:


1. Thou shalt use the prefix "hydro" at the beginning of thine acid's name.

2. Thou shalt replace the last syllable of the name of the non-metal with "ic"

3. Thou shalt add the word acid at the end.

Examples:

Formula:                              Ionic Non-acid Name                            Acid Name

HCl(aq)                               hydrogen chloride                               hydrochloric acid

H2S(aq)                              hydrogen sulphide                               hydrosulphuric acid

Note: Ionic non-acid names are similar to naming any other ionic compound.


The Three Commandments for naming complex acids


1. Thou shalt remove the word hydrogen from the ionic non-acid name.

2. -If the negatively charged polyatomic ion name ends with the suffix "ate" thou shalt replace it with "ic".
    -If the negatively charged polatomic ion ends with the suffix "ite" thous shalt replace it with "ous".

3. Thou shalt add the word acid at the end.

Example:

Formula                                    Ionic Non-acid Name                                Acid Name

HClO4(aq)                                hydrogen chlorate                                      chloric acid

Note: You can use the phrase: We ate ic - y sushi and got appendic - ite - ous!


Laying Down the Law of Definite Composition ------- By Proust

Chemical Compounds will always have the same proportion as the elements they contain; in terms of mass.  This applies to anywhere in the universe.

           For example: H2O has two hydrogen atoms and one oxygen atom for a total mass of eighteen grams
                                ( H = 2g and O = 16 g)

Laying Down the Law of Multiple Proportion ------- By Dalton

The same elements can combine in more than one way to from to different compounds.

         For example: FeO and Fe2O are comprised of the same elements but as shown have more than one formation.

Ionic and Covalent Compounds

Ionic and Covalent Compounds


What is an ionic compound?

• It is a chemical compound in which ions are held together by ionic bonds forming a lattice structure.
• It is comprised of a cation (usually metal or it can be a positively charged polyatomic ion) and an anion (usually a negatively charged polyatomic ion).
• It is a very hard and brittle subtsance with a high boiling and melting point.
• It is held together by electrostatic force.

How do you name an ionic compound?

1. Write the name of the cation first.
2. If applicable, write the charge of the cation with roman numerals if it mulivalent.
3. Write the name of the anion with either its polyatomic name or by changing the last syllable of the element to ide.

Examples:

HgClO -------> Mercury(l) hypochlorite

NH4H --------> ammonium hydride

CaSO4 --------> calcium sulphate


How do you write an ionic formula:

1. Write the cation's element/polyatomic ion symbol.
2. Write the anion's element/ polyatomic ion symbol.
3. Write and cancel out the charges as a subscript.

Examples:

Radium chlorite ----------> Ra(ClO2)2

Potassium fluoride -------> KF


What is a covalent compound?

• It is a chemical compound involving the sharing of electrons between atoms.
• It is always formed between two non-metals.
• It is usually has low melting and boiling points.
• It is usually soft and sticky.

How do you name a covalent compound?

1. Write the first element's name corresponding to its symbol
2. Write the second element's name corresponding to its symbol.
3. Write in prefixes to show the number of atoms.

Note: Elements hydrogen, oxygen, fluorine, chlorine and bromine all require the prefix mono if listed second in chemical formula.

Examples:

SF3 ----> sulphur trifluoride

C4H10 ----> tetracarbon decahydride

CO ----> carbon monoxide


How do you write a covalent compound formula?

1. Write the first element's symbol corresponding to its name.
2. Write the second elemen's symbol corresponding to its name.
3. Write in any prefixes if neccessary.

Examples:

carbon disulphide ----> CS2

xenon octafluoride ----> XeF8


Here's a table of commonly used prefixes for covalent compounds:

Separation

Separating Mixtures
What is separation? Separation is essentially a process in which a mixture is converted into two or more distinct products.

There are several basic techniques for separation:
-Filtration: pass mixtures through porous filters

-Distillation: heating a mixture can cause low boiling components to volatize

-Chromatography: components flow over the material at different speeds.

-Crystalization + Extraction: solids are separated by filtration or floatation

-Hand Separation: separated by using magnets + sieve


-Evaporation: boil away the liquid, solid remains

Chromatography-^Components flow over the material at different speeds^

There are different components within the mixtures, along with different properties

-High density/Low density                                    -Reactive/Inert

-Volatile/Non-volatile                                           -Magnetic/Non-Magnetic

-Soluble/Insoluble                                                -Polar/Non-polar