Today we did a really cool lab. It taught us a lot of things....
First, we had to heat our beakers, just to make sure all of the moisture had been removed. Then we got some hydrate in our crucible. We heated it numerous times, removing all of the water and turning it into a anhydrous salt. We then added a drop of water, changing the colour from white back to the original blue colour. This returned the anhydrous salt to the state of hydrate.
Along the way, we were constantly weighing the substance. From our measurements, we were able to determine the percentage of water in the hydrate, the number of moles of anhydrous salt, the number of moles of water removed from the hydrate and the number of moles of water per mole of the anhydrous salt. Using all of this information, we were able to derive the Empirical Formula of the hydrate.
There are two kinds of formulas: Empirical and Molecular
Empirical:
- Gives lowest-term ratio of atoms or moles in formula
- All Ionic compounds are empirical
Molecular:
- Gives all atoms that make up molecule
- Either Ionic or Covalent
Eg. C6H12O6 = Molecular Formula
CH2O = Empirical Formula
Example:
A gas has an Empirical Formula of CH2. What is the Molecular Formula if the mass of one mole is 42.0 g?
Have to find a whole number (N) = Molar Mass/Empiricial Mass = 42.0/14.0 = 3 (N)
(CH2) X 3 = C3H6
Determining Empirical Formula Given Mass
Example:
Determine the Empirical Formula of Fe and O given 10.87 g of Fe and 4.66 g of O.
First, grams must be converted to moles.
Fe - 10.87 g X 1 mole/55.8 g = 0.1948 moles
O - 4.66 g X 1 mole/16.0 g = 0.291 moles
Now, you must divide all of the derived numbers by the smallest amount of moles.
Fe - 0.1948/0.1948 = 1
O - 0.291/0.194 = 1.49, which can be rounded to 1.5
Lastly, you must multiply until you reach a whole number
Fe - 1 X 2 = 2
O - 1.5 X 2 = 3
Therefore, the Empirical Formula is Fe2O3.
Percent Composition:
- The % by mass of elements in a compound
- MUST ADD UP TO ABOUT 100% PERCENT (99.9 or 100.1)
Example:
What is the Percent Composition of CO2?
First, calculate the molar mass.
C - 12.0 g X 1 = 12.0 g
O - 16.0 g X 2 = 32.0 g
Therefore, the Molar Mass is 44.0 g.
Now, calculate each element's % of that total, rounding to one decimal place.
C - 12.0 g/44.0 g X 100 = 27.3 %
O - 32.0 g/44.0 g X 100 = 72.7 %
Percent Composition to Empirical Formula
Example:
A substance is 45.8% Sulphur and 54.2% Fluorine. What is the Empirical Formula?
First, assume 100.0 grams of material.
Then, convert to moles.
S - 45.8 g X 1 mole/32.1 g = 1.43 moles
F - 54.2 g X 1 mole/19.0 g = 2.85 moles
Then, divide by the smallest number.
S - 1.43 g/1.43 g = 1
F - 2.85 g/1.43 g = 2
(Atomic) Mass : The mass of atoms as compared to the very special CARBON-12 ATOM in atomic mass units (u)
(Formula) Mass : The total mass of every atom in a formula of only an ionic compound in u.
(Molecular) Mass : The total mass of every atom that combine to make: a molecule of a covalent compound, a molecule of an organic compound or a polyatomic element in u.
(Molar) Mass : The mass of 1 mole (which has 6.022 x 10^23 particles) of something and is the same value (numerically of course) as atomic, formula or molecular mass but is in GRAMS PER MOLE
What is a mole anyway?
A mole, not to be confused with a small furry rodent, is a unit of measurement used for small particles with a unit conversion factor which allows for say almost weightless atoms be shown in a more manageable format.
To be precise: A mole is 6.022 x 10^23 formula units, particles, molecules or atoms of something.
Which raises the question ???
Why the random number?
Well this number... called Avogadro'a number wasn't created by Amedeo Avogadro!
THIS IS HIM! OMG! ----------------------->
Anyway, apparently he's pretty special, for a french scientist named the number after him in 1909.
By the way...
This number is just like any ordinary way of saying there is so much of something. For instance, a dozen is a simple way of saying there is twelve of something just as a mole is a simple way of saying there is 6.022 x 10^23 of something!
See this website for super in depth information and discussion on this iconic number!
Finding the thickness of aluminum foil combines two different concepts to measure the thickness:
Volume of a solid: V = L x W x H (length x width x height)
Density of a substance: D = m / V (mass / Volume)
Height being the thickness of the aluminum sheet being measured. Therefore in the experiment you determine the thickness in the volume equation.
Aluminum foil 17cm x 15cm (1.04g)
Start with density: (density of aluminum = 2.70g/cm3)
2.70 = 1.04g/V
V = 0.39
Then using the volume in the Volume equation:
0.39 = 17 x 15 H
0.39 = 255 x H
H = 0.39 / 255
H = 1.53x10(-3)cm
Then find the average of 3 pieces of aluminum foil:
1.53x10(-3)cm +
1.48x10(-3)cm +
1.52x10(-3)cm +
/3 = 1.51x10(-3)cm
Then find the % off the acceptable thickness of aluminum foil (1.55x10(-3)cm)
finding the average by (1.51x10(-3)cm) - (1.55x10(-3)cm) / 1.55x10(-3)cm = 2.5%
Today, we did some graphing using Excel. It is a very useful tool. You can alter the graph, making it as large as you want, labelling the axis, changing the colours and styles, as well as many other things. You can also choose to put a line-of-best-fit as well as displaying the equation. Using these tools, you can figure out if the numbers are linear or not.
An example of a linear relationship is:
An example of a non-linear relationship is:
Plotting data makes it easier to read. For substances, for example, you can determine their melting point, boiling point, etc. It is also easy to determine which data is reliable, and which aren't on the line-of-best-fit and may have been a mistake or inaccurately measured.
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 %
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 1st 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:
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???
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.
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).
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 ateic - 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.
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?
Write the name of the cation first.
If applicable, write the charge of the cation with roman numerals if it mulivalent.
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:
Write the cation's element/polyatomic ion symbol.
Write the anion's element/ polyatomic ion symbol.
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?
Write the first element's name corresponding to its symbol
Write the second element's name corresponding to its symbol.
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?
Write the first element's symbol corresponding to its name.
Write the second elemen's symbol corresponding to its name.
Write in any prefixes if neccessary.
Examples:
carbon disulphide ----> CS2
xenon octafluoride ----> XeF8
Here's a table of commonly used prefixes for covalent compounds:
There is a change in atmoic formation through the bonding of atoms.
A new substance is created
The change is irreversable.
The change is automatic ( the reaction can be initiated or act on its own).
Examples:
Burning wood and its creation of smoke, the formation of rust or the act of combining sodium and vinegar.
Physical Changes:
Only occur when:
There is a change in state or phase.
No new substance is created.
Examples:
The melting of glaciers, condensation on a glass, or the cuttting of paper.
STATES OF MATTER - ACCORDING TO THE KINETIC MOLECULAR THEORY
Solid: Liquid: Gas:
The molecules The molecules are The molecules are very
are held close far enough apart far apart and can move
together through to slide past each very fast and freely.
attractions from and move more It will conform to the
charge and can freely. It will shape of its container
only vibrate. It conform to the The molecules will
will not conform shape of its collide but there is no
to the shape of its container. loss of energy at all.
container.
