What Is The Effect Of Changing The Concentration Of Alkali On The Volume Of Acid

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An investigation of neutralization reactions

Aim

Question :

What is the effect of changing the concentration of an alkali, on the volume of acid needed to neutralise it, when acid is added to it?

The plan is to use two acids (Hydrochloric & Sulphuric) and one alkali
(Sodium Hydroxide). The reason I need to use two acids is; because the question applies to all acids and alkalis and to just use one of each would not enable me to come to a general conclusion. If there had been more time available, ideally I would have preferred to use two alkalis as well as two acids.

Predictions

First of all, here are the definitions for an acid and an alkali:

An acid is a substance that forms hydrogen (H+) ions when placed in
water. …show more content…

Acids, which form one H+ ion from each acid molecule, are called Monoprotic. Acids, which form two, are called Diprotic. Acids, which form three, are called Triprotic (e.g. Orthophosphoric Acid [H3PO4])

I predict that for Monoprotic acids (e.g. Hydrochloric) :

Concentration of Acid * Volume of Acid = Concentration of Alkali *
Volume of Alkali

The amount of acid needed to neutralise an alkali =

(MAl * VAl)/MA = (Molarity of Alkali * Volume of Alkali)/Molarity of
Acid

I predict that for Diprotic acids (e.g. Sulphuric) :

Concentration of Acid * Volume of Acid = Concentration of Alkali *
Double the Volume of Alkali

Molarity = how many molecules of the acid or alkali per 1000 cm3 (1 litre) of water.

Equipment

Goggles (to protect the eyes)
Mat (to protect the surface of the bench)
Burette (to hold the acid)
Conical Flasks (to hold the alkali)
Indicator (to show the pH of the solution)
Funnel (to pour the acid into the burette)
Measuring Cylinders (to measure out the water, acid & alkali and …show more content…

On my graphs, since 0 concentration of Alkali will mean 0 volume of
Acid, I decided that the best fit line should carry on down to the origin. Because there were no data there, I had to extrapolate the line downwards and because of this it is shown as a red dotted line.
The data must be slightly inaccurate because the best fit line passes very close to the origin rather than actually passing through it.

Improvements

If this experiment was used again, I would try to improve it in the following ways :

* Instead of using Universal Indicator, I would use an indicator which only changed colour when the solution was Neutral rather than gradually changing as the pH became lower (e.g. Methyl Orange or Phenolphthalein).

* To get the pH more accurate next time, a pH meter could be used.

* To get it very accurate, the acids and alkalis supplied should be an exact amount (I am not sure whether the ones I used

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