calculating the relative molecular mass (rmm)
TRANSCRIPT
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THE RELATIVE MOLECULAR MASS OF
A COMPOUND IS THE SUM OF THE
MASSES OF ALL THE ATOMS PRESENT
IN THE MOLECULE
IT IS OFTEN SHORTENED TO RMM
TO CALCULATE RMM, YOU SHOULD
KNOW THE RELATIVE ATOMIC
MASSES OF THE ELEMENTS, SOMETIMES
CALLED THE RAM OF AN ELEMENT
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Relative Atomic Masses
Element Symbol RAM
Carbon C 12
Hydrogen H 1
Oxygen O 16
Sodium Na 23
Sulphur S 32
Chlorine Cl 35.5
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` What is the Relative Molecular Mass Of
Magnesium Oxide
` Magnesium Oxide has the formula MgO
` MgO has one Magnesium atom and oneOxygen atom.
` RAM Mg,24 ; O, 16
` RMM MgO = RAM Mg +RAM O
` RMM MgO = 24 + 16
` RMM MgO = 40
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` What is the Relative Molecular Mass Of Natrium Hydroxide
` Natrium Hydroxide has the formula NaOH
` NaOH has one Natrium atom, one Oxygen atom and one
Hydrogen atom.
` RAM Na,23 ; O, 16 ; H, 1
` RMM NaOH = RAM Na +RAM O +RAM H
` RMM NaOH = 23 + 16 + 1
` RMM NaOH = 40
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Example Three
What is the Relative Molecular Mass of sodium sulphate?
(Sodium sulphate has the formula Na2SO4 - that's two sodiums, one sulphur, andfour oxygens.)
RAM Na 23; 0 16; S 32
Na 2 x 23 = 46
S 32 = 32
O 4 x 16 = 64
RMM = 46+32+64
Total RMM 142
So we just add up the masses of all these atoms to get the total mass - the relative
molecular mass - 142 in this example.
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Example Four
What is the RMM of ether?
Ether has the formula CH3CH2OCH2CH3 . There are two ways to calculate the
RMM here.
RAM C,12; H,1; O,16
CH3 12 + (3 x 1) = 15
CH2 12 + (2 x 1) = 14
O 16 = 16
CH2 12 + (2 x 1) = 14
CH3 12 + (3 x 1) = 15
RMM=15+14+16+14+15
Total RMM 74
OR
C 4 x 12 = 48
H 10 x 1 = 10O 16 = 16
RMM=48+10+16
Total RMM 74
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Exercise 1
1. Calculate the relative molecular mass of ammonium sulphate.
Ammonium sulphate has the chemical formula (NH4)2SO4.
Exercise 2
2. What is the RMM of (CH2Cl)2CHCH(CHCl2)2?
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` (NH4)2 2 x
[14 + (4 x 1)] = 36
` S = 32
` O4 = 4 x 16= 64
` Total RMM =36+32+64
` 132
` N = 2 x 14= 28
` H= 8 x 1= 8
` S = 32
` O = 4 x 16= 64
` Total RMM =28
+ 8 +32 + 64
` 132
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` (CH2Cl)2 2 x
(12 + (2 x 1) + 35.5)= 99
` CH =12 + 1= 13
` CH =12 + 1= 13
` (CHCl2)2 2 x
[12 + 1 + (2 x 35.5)] = 168
` Total RMM= 99+13+13+168
` 293
C 6 x 12 = 72
H 8 x 1 = 8
Cl 6 x 35.5 = 213
RMM= 72+8+213
Total RMM 293
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Notice that it doesn't matter how complicated the formula is, or whether you
know what the compound is or have never seen it before in your life, you just work
out the answer step by step.
Then you check it again, in case you hit the wrong key on the calculator, or got
the brackets wrong.
Everyone makes the odd arithmatic slip, so always check your answer.
And do use brackets when you write down calculations like this, it prevents little
mistakes ruining your good work.
It only takes a second longer to put them in place, but it helps you get the marks.
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THANK YOU
GOOD LUCK