RP1 - making up a standard solution and carrying out an acid base titration

Question 1

What is the process of making a solution

Answer 1

• weigh sample bottle containing required mass of solid on a balance
• transfer to beaker
• reweigh empty sample bottle
• record difference in mass
• add 100cm3 of distilled water to the beaker . Use a glass rod to stir to help dissolve the solid
  • sometimes substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved
• pour solution into a 250cm3 graduated flask via a funnel
• rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask
• make up to the mark with distilled water using a dropping pipette for last few drops
• invert flask several times to ensure uniform solution

Question 2

how do you measure mass accurately

Answer 2

1) measure mass on 2dp or 3dp balance of a weighing bottle with the required quantity of sold in it
2) empty mass into reaction vessel / flask
3) re weigh the now empty weighing bottle
4) subtract the mass of the empty weighing bottle from the first reading to give exact of mass actually added

Question 3

What’s the method of diluting a solution

Answer 3

• pipette 25cm3 of original solution into a 250cm3 volumetric flask
• make up to the mark with distilled water using a dropping pipette for last few drops
• invert flask several times to ensure uniform solution

Question 4

why is using a volumetric pipette more accurate than a measuring cylinder

Answer 4

because it has a smaller uncertainty

Question 5

why should you use a teat pipette

Answer 5

to make up to the mark in volumetric flask to ensure volume of solution accurately measured and one doesn’t go over the line

Question 6

EQ : “ take a clean 250cm3 volumetric flask . Use the balance provided and a clean dry container to weigh out the amount of solid required . Tip the solid into a clean , dry 250cm3 beaker and add about 100cm3 of distilled water . Use a stirring rod to help solid dissolve , carefully breaking up any lumps of solid with the rod . When solid has dissolved , pour solution into flask using a filter funnel . Add water to flask until level rises to graduation mark ‘
SUGGEST 3 FURTHER INSTRUCUTUON THAT WOULD IMPROVE OVERALL TECHNIQUE (3)

Answer 6

• a method of weighing by difference / wash solid from its weighing container into beaker
• wash the rod into the flask / beaker after use
• wash the beaker into the flask after transfer
• wash filter funnel into the flask
• use a teat pipette to make up to the mark on the volumetric flask
• ensure bottom of liquid meniscus is on the graduation mark
• mix / shake final solution / invert flask

Question 7

EQ : Suggest one reason why it’s important to wash the inside of the flask (1)

Answer 7

To make sure all the solution react with each other / are in the flask

Question 8

EQ : Suggest why washing the titration flask with water does not affect the titre value (1)

Answer 8

Water does not change the number of moles of either reagent / reactants
- Water is not a reagent / does not react with either reactant

Question 9

EQ : Suggest why it’s important to fill the space below the tap in the burette solution with A before beginning an accurate titration (1)

Answer 9

Space will fill during titration / titres or volumes added are too high

Question 10

EQ : suggest one reason why a 250cm3 conical flask is preferred to a 250cm3 beaker for a titration (1)

Answer 10

Less chance of losing liquid on swirling / liquid doesn’t splash on swirling

Question 11

EQ : During a titration , a chemist rinsed the inside of the conical flask with deionised water . The water used for rinsing remained in the conical flask . Give one reason why this rinsing can improve the accuracy of the end point (1)

Answer 11

Returns reagents on the side of flask to the reaction mixture

Question 12

EQ : The student identified use of the burette as the largest source of uncertainty in the experiment . Using the same apparatus , suggest how the procedure could be improved to reduce the percentage uncertainty in using the burette (2)

Answer 12

• Use a larger mass of solid
• So a larger titre

Question 13

EQ : A white solid , MHCO3 dissolves in water and reacts with HCl to give a salt
A student is required to make up 250cm3 of an aqueous solution that contains a known mass of MHCO3 . The student is provided with a sample bottle containing the MHCO3 . Describe the method , including apparatus and practical details that the student should use to prepare the solution (6)

Answer 13

• weigh sample bottle containing the solid on a balance
• transfer solid to a beaker and reweigh sample bottle
• record difference in mass
• add distilled / deionised water
• stir with glass rod or swirl
• until all solid has dissolved
• transfer to a volumetric / graduated flask
• with washings
• make up to 250cm3 / mark with water
• shake / invert flask