Percent Composition

- The % by mass of the elements in a compound is called the percent composition

How to find the percent composition: 1. Calculate molar mass

                                                            2. Calculate each element percentage of that total

                                                                Give percentage to 1decimal place

-Percent composition tells what part of a compound each component makes up

               %composition= (Mass of element)/(Mass of compound)×100%

 Ex1. What is the % composition of CO2?

1 mole C= 1×12.0= 12.0g

2mole O= 2 ×16.0= 32.0g

                                  44.0g
                        %C= (mass of carbon)/(mass of CO2)=(12.0×100%)/44.0g=27.3%
                        %O= (mass of oxygen)/(mass of CO2)= (32.0×100%)/44.0g=72.7%

                        Check to see if the result is close to 100%:27.3%+72.7%= 100%

Because of rounding answer may also be shown as 99.9 and 100.1 which are    both OK

Ex2. What is the % composition of the entire element in glucose: C6H12O6

C = 72.0g        H=12.0g          O= 96.0g         Total Mass=180.0g

          %C= (72.0g×100%)/180.0g= 40.0%
          %H= (12.0g×100%)/180.0g= 6.7%
          %O= (96.0g×100%)/180.0g= 53.3%
          40.0+6.7+53.3=100%

To find the percent composition of a group of atoms (Al2(SO4)3), the total of the mass of the group atom is used for the percentage

Ex3. Find the % SO4 in Al2(SO4)4

2Al× 27.0g= 54g

3S × 32.1g= 96.3g                              96.3(Sulphur)+192.0(Oxygen)=288.3g

12O× 16.0g= 192.0g

342.3g

          %SO4= 288.3g/342.3×100%= 84.2%SO4
From the percent composition, ratio of moles can be determined and this will result in an empirical formula

            Step 1: Assume 100.0g of material

            Step 2: Convert all %s to grams            Step 3: Convert grams to moles

Step 4: divide all moles by the smallest; this will give you a ratio to determine the empirical formula

Ex4. Sample of compound: 45.8% Sulphur and 54.2% Flouride. Give the empirical formula

Step 1: Assume the 100.0g of material

Step 2: Therefore: 45.8g S and 54.2g F

Step 3: 45.8/32.1g= 1.4268molS     54.2/19.0g= 2.8526molF 

Step 4: 2.8526/1.4268= 1.99993/(1moleS)= SF2
Therefore the empirical formula for 45.8% Sulphur and 54.2% Flouride is SF2
How to calculate percent composition1: V=xbEeyT8nK84
How to calculate percent composition2: V=_H009sTvYE0

Graph-Lab

Use 'Microsoft Office Excel 2007' to draw the graphs!!!!
There are three variables: Mass, Temperature, Density, Volume
Draw 3 kinds of graphs: Mass VS Volume
                                         Temp VS Density
                                         Volume VS Temp
First, list the statistics:
Example:

TEMP	VOLUME
273	152
320	175
365	203
412	226
455	249
521	273
546	302
580	323
632	351

Draw the graphs by the graphing tools in Excel:

 

Solve the slope:

Slope

0.554318

Lets see a teaching video about how to use Excel Graphing tool:

Significant Figures

Significant Figures?-Measure or meaningful digits.
-More preise mensthere are more significant digits.


1.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

2.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.


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

4.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

5.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

6.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

Aluminum Foil Thickness Lab

Objective: To calculate the thickness of a sheet of aluminum foil and express the answer in terms of proper scientific notation and significant figures

Supplies: 3 rectangular pieces of aluminum foil (minimum 15cm*15cm); metric ruler and centigram balance.

Experimental results:
Table 1
sheet    Length(cm)    Wideth(cm)    Mass(g)
1           16.8                15.4               1.02
2           15.8                15.0               0.95
3           17.2                17.0               1.16

Formulars:
Volume of a rectangular solid:  V=L*W*H

Density: D=M/V

H(average)=(H1+H2+H2)/3

Example:
Sheet1: 2.70g/cm^3=1.02g/(16.8cm*15.4cm*H)   H1=1.46*10^-3cm

Density

What is Density?
- a physical property of matter that describes the ratio of mass to volume
                                            D=M/V
Possible units: g/ml, g/L, g/cm³, kg/L
Important: 1cm³=1ml
                Density of water=1.0g/ml or 1000g/L
    Density object > density liquid objects sinks
    Density object < density liquid object floats
Ex. Iron bar has mass of 1.2kg and a volume of 1.25L. What is it’s density?
D=M/V=1.2kg/1.25L=0.96Kg/L

Lets watch a video about Density demonstration:

Measurements

-no measurement is exact, but there is an estimate of the measurement
-When measuring, at least measure 3 times and calculate the average from the 3 or more
-absolute uncertainty is the largest difference between the average and the lowest and highest reasonable measurement
-biggest difference and smallest difference is usually shown with a positive sign on top and a negative sign on the bottom (±)
Ex. 35.3 ± 0.2, either 35.1 or 35.5
Ex2. 29.95 ± 0.002, either 29.97 or 29.93

Measuring Device Smallest division Measurement nearest Uncertainty  
Ruler                              0.1                     0.01cm                    0.01cm  
Thermometer                      1°C                      0.1°                   0.1°  
100ml graduated cylinder      1ml                         0.1ml                   0.1ml  
400ml beaker                       50ml                            5ml                    5ml

Relative uncertainty=absolute uncertainty/estimated measurement