RFM, EF,RAM, percentage composition, isotopes ( includes examples)

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ISOTOPES:
-Takes into consideration he whole mass (all isotopes), this can also be referred to as the relative
abundance
MASS NUMBER X PERCENTAGE + MASS NUMBERS
E.G.
(75X35)+(75=37)
100
=35.3
ISOTOPES ­ atoms of an element with different numbers of neutrons
RAM ­ RELATIVE ATOMIC MASS: (Ar)
RAM- calculates the relative atomic mass of an element from information about relative
abundance of its isotopes.
Multiply each percentage by the massnumber. Add and then divide by 100%. The resulting
number is relative atomic mass of atoms of that element.
i.e
(% of isotope 1 × mass of isotope 1) + (% of isotope 2 × mass of isotope 2) ÷ 100
E.G.
GERMANIUM (Ge)
Ge(24.4%), Ge(32.4%) and Ge(43.2%)
(70x24.4)+(72x32.4)+(74x43.2)
=72.376
RAM- the mean (weighted average) mass of an atom relative to the mass of an atom of carbon ­
12, which is assigned a mass of 12.
PERCENTAGE COMPOSITION:
-used to work out the percentage of once certain element in a compound
E.G.
H2O work out the percentage of oxygen in water?
16 x 100

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RFM ­ RELATIVE FORMULA MASS: (Mr)
RFM- finds out the mass of the whole formula
1. Write the formula of the compound.
2. Write the numbers of each atom in the formula.
3. Insert the relative atomic mass for each type of atom.
4. Calculate the total mass for each element.
5. Add up the total mass for the compound.
Find the relative formula mass of CaCO3?
CaCO3 ­
ca-40
c-12
0-16
So........
40+12+16+16+16 = 100
N.B.…read more

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Fe (iron) O (oxygen)
Mass=44.8g 19.2g
Ar=56 Ar=16
Mass = 44.8 19.2
Ar 56 16
=0.8 =1.2
0.8 = 1 1.2 = 1.5
0.8 0.8
1x2 because it gives
a simpler number
=3
Ratio of Fe to O is 1:3
Using the EF to calculate the MF (and visa versa)
RFM of the compound
E.G.
Ethane has the EF CH3, the MR of ethane is 20
30 = 2
SO.....…read more

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Examples question 2:
A compound containing 62.7% C, 10.33% H and 27.6% O. what is the MF if the Mr is 58.0%?
C H O
Mass 62.07 10.33 27.60
Ar 12 1 16
Mass 62.07 10.33 27.60
Ar 12 1 16
=2.6725 =10.37 =1.725
Divide them all by the smallest number
Formula = C3H6O (3X12)+(1X6)+16=58
Example question 3:
A sample of benzene (7.8g) contained 7.2g C and 0.6 g H, if the Mr is 78.0 what is the
empirical formula?
C H
Mass 7.2 0.…read more

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