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Equivalent Weight is an archaic name for 'Moles'.

Moles = mass(g) / Mr ( Relative molecular mass). or Ar (Relative Atomic Mass).

If we take the reaction of sulphuric acid and sodium hydroxide.

First write down the BALANCED reaction eq'n.

H2SO4 + 2NaOH = Na2SO4 + 2H2O

Notice the molar ratios are 1:2::1:2

So if we have 49g (H2SO4) , how much , by mass of sodium hydroxide, do we need to neutralise the acid. ? This mass of sodium hydroxide is the Equivalent Weight.

First calculate the Mr(H2SO4) from atomic masses on the Periodic Table.

2 x H = 2 x 1 = 2

1 x S = 1 x 32 = 32

4 x O = 4 x 16 = 64

2 + 32 + 64 = 98

So moles(H2SO4) = 49g / 98 = 0.5 mole.

By molar ratios above

1 mole reacts with 2 moles

Hence

0.5 moles reacts with 1 mole.

So we need 1 mole(NaOH)

Again calculate the Mr(NaOH)

1 x Na = 1 x 23 = 23

1 x O = 1 x 16 = 16

1 x H = 1 x 1 = 1

23 + 16 + 1 = 40

Using the moles equation again

moles = mass( g)/Mr

Algebraically rearranging

mass(g) = moles X Mr

Mass(NaOH) = 1 moles X 40 =

mass(NaOh) = 40 g ( The 'Equivalent Weight). This is the mass required to neutralise 49 g of sulphuric acid.

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lenpollock

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5mo ago

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