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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