A2 Chemistry Edexcel pH, buffers, etc

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Acids and Bases

Acids donate H+ ions and base accept H+ ions. The pH of an acid can be wroked out by using -log(concentration of the acid) this is how to work out the pH of  a weak acid.

To work out the concentration of a base you first have to work out the pH and take the pH away from the value of 14 which is the highest pH to give the pH of the base .

Ka is value that is worked out for weak acids ( named weak as they do not fully dissociate, whereas strong acids fully dissociate)

Ka is eqaul to the concentration of H+ times by the concentration of the salt divided by the concentration of the acid.

You assume that none of the H+ come from water , the concentration of the aicd is the same at equlibrium as in the orginal solution and that , you assume that the concentration of salt and H+ ion are the same.

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Buffer solutions and indicators

A buffer is a solution that when small amounts of acid or base are added pH change is resisted.They are made up of a weak acid and the weak acids salt.

The pH of a buffer can be worked out by finding out the conctration of H+ ions by using the formula for Ka , you will be given data which will allow you to work out the moles and therefore the concentration of salt, you will be given the Ka and conentration of the acid so it is just as case of rearranging the formula on the last card.

An indicator is a weak acid or base which has a different colour in the undissociated form to the dissociated form. The best indicator for A tiritation will be one that changes in the vertical portion of  a pH concentration graph. For a strong acid strong base it will centre around a pH of 7. For a strong base weak acid the value would be around pH 9. For a strong base weak acid the point would be around the figure of pH 5. A weak acid and weak base does not have a clear equvilence point because there is a only a very gradual change in pH  around whta could be the pH of the equvilence point.

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Some Definitions

pKa= -log(Ka)

pKw= -log(Kw)

Kw is the ionic product of water which is equal to the concentrations of H+ times concentration of OH-.

 pH is equal to pKa at half of the equvilence point of a tiration.

Dilutions of strong acids a dilution of ten times is equal to an increase in pH of one value this is because the equlibrium of the reaction is shifted to the right hand side. This does not ouccur in weak acids because the decrease in H+ is not as great as expected.

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Conjugate acids and bases

H2SO4 + HNO3  goes to HSO4- and H2NO3+

the conjuate acid is the H2NO3+ as it can donate an proton to get back to the starting reactant

The conjugate base is the HSO4 because it needs to gain a proton to form e starting reactant

You will nedd to be able to work out these pairs in the exam as it quite comman question

label hem, base 1 acid 2 etc etc

some molecules like water are amphoteric meaning that they can act as an acid or a base and you may be asked to work out which way it is acting - look at whether it is gaining or losing protons.

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