1 (1.2) Make volumetric solution + acid-base titration

Question 1

Standard/volumetric solution

Answer 1

A solution of known concentration

Question 2

Making standard solution (brief)

Answer 2

Involves dissolving a known amount of solid in a known amount of water to create a known concentration

Question 3

Why rinse beaker & stirring rod?

Answer 3

Rinse with distilled water and add to flask- ensure no solute clinging/ remaining on beaker or rod

Question 4

Why reweigh boat?

Answer 4

Subtract mass of boat from mass of boat + mass
-to find precise mass of solid used (boat could still contain traces of solid)

Question 5

How would you make a precise volume and concentration of a solution of sodium hydroxide

Answer 5

-First work out how many moles of sodium hydroxide you need using the formula moles = concentration x volume
-work out how many grams of sodium hydroxide you need using the formula mass = moles x Mr
-place a weighing boat on a digital balance and weigh out the mass of solid
-Tip it into a beaker then reweigh the boat
-Subtract the mass of the boat from the mass of the boat and solid together to find precise mass of solid used
-Add distilled water to the beaker and stir until all the sodium hydroxide has dissolved
-tip the solution into a volumetric flask ( 250 cm³) using a funnel to make sure it all goes in
-Rinse the beaker and steering rod with distilled water and add that to the flask too
-Top up flask to correct volume with more distilled water
-ensure bottom of meniscus reaches line
-Stopper the flask and turn it upside down (invert) a few times to make sure it’s mixed

Question 6

What should you do when you get closer to the line?

Answer 6

Add drop by drop- to make sure it’s at meniscus

Question 7

Why rinse out the burette

Answer 7

If not rinsed with substance (alkali/acid) it may be diluted by residue of water in the rat or react with substances left from previous reaction
-would lead to concentration decreasing so larger titre volume added

Question 8

What are concordant results

Answer 8

Titres that are within 0.1 cm³ of each other

Question 9

Safety precautions

Answer 9

-Acid and alkalis corrosive
-Low concentration of acids are irritants
—> wear eye protection and gloves

Question 10

What do you titrations allow you to do

Answer 10

Find out exactly how much acid/alkali is needed to neutralise or measured quantity of alkali/acid
— allows you to work out the concentration of unknown

Question 11

Why is an acid base indicator used

Answer 11

To detect when reaction reaches completion, usually presence of colour change

Question 12

What are the two main indicators for acid/alkali reactions

Answer 12

Methyl orange- red in acid, yellow in alkali
Phenolphthalein- colourless in acid, pink in alkali

Question 13

Why is universal indicator no good

Answer 13

It’s colour change is to gradual
-Require indicators that colour change quickly over very small pH range so know exactly when the reaction has ended

Question 14

How can you make it easier to see endpoint

Answer 14

Stand flask on white tile

Question 15

How to carry out titration

Answer 15

-Use the pipette and pipette filler to transfer one reactant into conical flask (measure set vol of solution you want to know conc of)
-transfer to flask and add a few drops of indicator to flask, place on white tile
-fill burette with standard solution of acid
-Use funnel to carefully pour acid into burette, always do this below eyelevel to avoid any splashing onto face or eyes
-1st do rough titration to get idea where in point is (open tap)
-Do accurate titration and add drop wise when near endpoint
- Record initial and final burette volume
-Repeat until concordant results

Question 16

Common method taken from mark schemes for making volumetric solution

Answer 16

1. Weigh the sample bottle containing the solid on a (2 dp) balance.
2. Transfer solid to beaker and reweigh sample bottle.
3. Record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved.
5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3​ ​ mark with distilled water.
7. Shake flask.