chemistry - neutralisation

Question 1

how can you find the volume of sulphuric acid that will exactly neutralise 25cm^3 of sodium hydroxide solution?

Answer 1

• place 25 cm^3 of sodium hydroxide in a conical flask on a white tile, using a pipette and pipette filler
• clamp the burette above the conical flask and fill it with sulphuric acid using a funnel, filling the burette to the 0.00cm^3 line
• add 5-10 drops of phenolphthalein to the conical flask
• slowly add the sulphuric acid to the flask, swirling the flask at the same time
• close the burette tap as soon as the mixture turns from pink to colourless
• record the final volume of sulphuric acid added

Question 2

What apparatus is required for the experiment?

Answer 2

• burette
• conical flask
• pipette and pipette filler
• funnel
• clamp and stand
• white tile

Question 3

why should a white tile be used in a titration experiment?

Answer 3

A white tile is placed under the reacting mixture as it makes the colour change easier to see

Question 4

Why should you repeat a titration?

Answer 4

to allow a mean titre to be calculated, reducing the effect of random error

Question 5

what colour is phenolphthalein in acid and alkali?

Answer 5

acid - colourless
alkali - pink

Question 6

why is the first trial in a titration often called a rough trial?

Answer 6

it gives an approximate idea of where the end point is so that later titrations can be more precise

Question 7

what safety precautions should be taken?

Answer 7

• wear goggles and be careful handling chemicals as they can be corrosive or be an irritant
• burette should be filled below eye level so there is no risk of it spilling or splashing your face
• any broken glass wear needs to be cleared immediately

Question 8

what other indicator can be used, and what is the colour change?

Answer 8

• methyl orange ~ red (acid) to yellow (alkali)
• litmus blue ~ red (acid) to blue (alkali)

Question 9

if you know the volume of acid required, how can you calculate the concentration of the acid, given the alkali concentration and volume?

Answer 9

• calculate the number of moles of the alkali using the known volume and concentration
• use the chemical equation to work out the ratio of acid and alkali that react and hence find how many moles of acid have reacted
• divide the moles of acid by the volume used in neutralisation

Question 10

if you know the concentration of an acid in mol/dm^3, how can you convert it to g/dm^3?

Answer 10

multiply the concentration in mol/dm^3 by the molar mass (Mr) of the acid to give the value in g/dm^3