Little Quiz -Nov.29

1. Solve for the MM and % of each element.
CH3COOH
NH4NO3
C11H22O11

2. what would be the mass of 15.00×10^21 atoms of Cobalt?

3.The empirical formula of a compound is SiH3. If 0.0275 mol of compund has a mass of 1.71g, what is the compound's molecular formula?

4.What's the persentage of water in CuSO4·5H2O?

5. A compound contains 58.5% C, 7.3%H and 34.1% N. What is the formula of the compound?

Percentage Composition --Nov.29

Definition: Percentage by mass of “species” in a chemical formula.

Ex. What is the percentage composition of CO₂?

 *assume you have 1 mole

Total MM=44.0g/mol

Percentage of C=(12g/mol)/(44g/mol)×100% = 27.3%

Percentage of O=(16g/mol×2)/(44g/mol)×100% = 72.7%

*(27.3%+72.3%=100%)

Percentage composition tells you which elements are in the compound and how much of each there is.

Fomula(useless):

                                              Mass of element

Percentage composition=—————————— ×100%

                                             Mass of compound

Mole Conversions

Mole Conversions

1. Coversion from Particles Particles↔mass ( 1 moles= 6.022×10^23 particles)
a)particles →moles

b)moles→particles/molecules/formula units atoms


2.Concersions between moles↔molecules
a)moles→grams

b) grams→ mole


Harder Mole Conversions

Grams↔Moles↔Particle↔# Atom in a Practice

a)Conversions between particles to mass.

Ex.  What is the mass of 2.78×10^22 Fe atoms?

2.78×10^22 ÷1mole／6.022×10^23 molecules × 80.1g／1mole=1.1×10^-3


b)Conversions between mass to particles.

How many atoms of Iron in 20.0g of Iron?

20.0g×1mole／55.8g×6.022×10^23atoms／1 mole=2×10^23

The Mole

-equal volume of different gases has a constant ratio
ex oxygen:hydrogen=16:1
   carbon dioxide:hydrogen=22:1
-expressed by comparing it to the mass of another object


Avogadro's Hypothesis
Equal volume of different gases at the same temperature and pressure have the same number of particles.
If they have the same number of particles, the mass ratio is due to the mass of the particles.


Atomic Mass
Mass of 1 atom of the element in atomic mass units(amu or u or Dalton)
ex fluorine=19.0 amu


Formula Mass
All the atoms of a formula of an ionic compound(in amu)
ex potassium fluoride (KF)   39.1+19.0=581amu


Molecular Mass
All the atoms of a formula in a covalent compound (in amu)
ex carbon dioxide(CO2) = 12.0+16.0×2=44.0 amu


Molar Mass
Atomic/Molecular/Formula mass of any pure substance(in grams per mole)
ex 1mole of oxygen=16.0 g/mol
   1mole of carbon =12.0 g/mol
(same number of particles)


Avogadro's Number
The number of particles in 1 mole of any amount of substance is
6.022×10^23 particles/mol


The mole is extremely important to chemists because it allows them to count atoms and molecules.

Density Nov.11

1cm^3 of watere(H2O)   =   1mL

Density of water   = 1.0g/mL
                      or  =1000g/L


For a solid      →g/cm^3

For a liquid    →g/mL

V=M/D

D object > D liquid      sink
D object < D liquid   float

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