Core practical 3

Question 1

Outline the 8 steps to the titration practical

Answer 1

1. Wash out 250cm3 volumetric flask using distilled water to ensure no other solutions on it will effect results
2. Use a volumetric pipette to transfer 25cm3 of HCl into the volumetric flask
3. Make the solution up to the mark with deionised water
4. Set up the clamp, stand and burette with NaOH of known concentrations
5. Pour a 25cm3 aliquot of the now DILUTED HCl into conical flask with a few drops of indicator
6. End point of the titration should be when the solution experiences colour change and
   remains for more than 5 seconds
7. Continue to titrate until you get two concordant results
8. Record results on a table

Question 2

Why are we using distilled water to wash the volumetric flask?

Answer 2

Water is un-reactive with reagents therefore won’t effect number moles of acid

Question 3

What two rules must be followed when working out your mean titre?

Answer 3

1. Include concordant results only which are 0.05cm3 readings f each other
2. Do not include the rough titre result in the calc

Question 4

equation for practical 3

Answer 4

NaOH + HCl ———> NaCl + H2O

Question 5

Compare the terms “reading” and “measurement”

Answer 5

Reading are values found from single judgement when using a piece of equipment

Measurements are values taken as difference between judgements of two values

Question 6

How do we calculate measurement uncertainties for titration equipment?

Answer 6

Plus or minus uncertainty / Measurement made on apparatus x 100

Question 7

When do we have to multiply the uncertainty by 2?

Answer 7

When we make two readings using the apparatus

Question 8

What are the two ways in which we could reduce titration uncertainties?

Answer 8

1. Replace measuring cylinder with dropping pipette or burette because they have lower uncertainties
2. Make larger volume titres by increasing volume and concentration of substance in conical flask

Question 9

How can we reduce uncertainties of the apparatus themselves?

Answer 9

Use appratus with greater resolutions to increase sixe of measurement made

Question 10

How many readings ensure least uncertainty?

Answer 10

The greater no of readings as possible

Question 11

State the three ways we can reduce uncertainties of the MEASURING equipment

Answer 11

1. Use a more accurate balance
2. Use larger mass of substance
3. Weighing masses before and after then calculating a difference to accurately of mass added

Question 12

Why does the colour of the end titration change after a while of finishing the experiment?

Answer 12

The sodium hydroxide in excess reacts with carbon dioxide in the air which makes sodium carbonate

Question 13

Suggest 5 precautions and adjustments that would make your burette readings more accurate

Answer 13

1. Use white tile or paper as a background to the conical flask- allows you to identify the colour changes easier
2. Have eye level with the meniscus when making readings
3. Measure at the bottom of the meniscus
4. Ensure the burette is upright or vertical
5. Ensure jet is full without any air bubbles

Question 14

How may we obtain a pure crystal of a hydrated sulphate after a titration?

Answer 14

Repeat titration without indicator so the colour of solution is removed, then heat it up to remove the water

Question 15

Chemical equation for reaction of hydrochloric acid and sodium hydroxide

Answer 15

HCl (aq) + NaOH (aq) —-> NaCl (aq) + H2O (l)

Question 16

Why does pink colour seen at the end fade away?

Answer 16

Because the excess sodium hydroxide reacts with carbon dioxide in air to make sodium carbonate

Question 17

Reaction equation for sodium hydroxide and carbon dioxide

Answer 17

2NaOH (aq) + CO2 (aq) —-> Na2HCO3 (aq) + H2O (l)

Question 18

What happens when sodium hydroxide solution is stored?

Answer 18

It reacts with carbon dioxide in the air, this changes the concentration of sodium hydroxide solution by decreasing it