2: Atoms and reactions : titrations

Question 1

What is a standard solution and how is one formed?

Answer 1

A standard solution is a solution of known concentration.

1. dissolve an accurately weighed mass of solid in a known volume of solvent using a volumetric flask
2. fill up to the line with water for optimum accuracy
3. use standard solution to find the concentration of another solution with an unknown concentration

Question 2

What is a titration used for?

Answer 2

A titration is used to identify the concentration of a solution by reacting it with a solution of known concentration and measuring the volume required for a complete reaction.

Question 3

Why does the pipette first need to be rinsed with distilled water and then with the alkali?

Answer 3

• with water to remove any unwanted chemicals
• with alkali to remove any water droplets which could dilute the solution

Question 4

Why does the pipette need to be filled with the alkali just past the 25cm cubed mark?

Answer 4

The level of the liquid will drop slightly when the pipette is lifted out of the alkali

Question 5

Why should only a few drops of indicator be added?

Answer 5

Indicators are weak acids so adding too much could give inaccurate results

Question 6

Why does the conical flask need to be swirled as the acid is being added?

Answer 6

To ensure that the acid and alkali mix thoroughly so they can react.

Question 7

Briefly outline how a student could carry out a titration

Answer 7

1. Using a pipette, add a measured volume (25cm3) of one solution to a conical flask.
2. Add a few drops of a suitable indicator (include example like phenolphthalein)
3. Place the other solution in a burette using a funnel
4. Place conical flask onto a white tile
5. Add the solution in the burette to the solution in the conical flask, whilst swirling it, until the reaction has just been completed (indicator changes colour - pink to colourless) - end point.
6. Measure the volume of the solution added from the burette
7. Repeat the traction until you have 2 concordant titres

Question 8

What does concordant mean?

Answer 8

Within 0.1 cm3 of each other - use the mean of the two concordant values in calculations

Question 9

State the colours of these indicators in acid, in base and at end point

1. Methyl orange
2. Bromothymol blue
3. Phenolphthalein

Answer 9

1. Acid - red, base - yellow, end point - orange
2. Acid - yellow, base - blue, end point - green
3. Acid - colourless, base - pink, end point - pale pink

Question 10

25.0 cm cubes of 0.150moldm-3 NaOH reacted with 23.40cm3 of H2SO4
Calculate the concentration of the H2SO4

Answer 10

0.080 moldm-3

Check notes for workings!

Question 11

A student dissolved 2.794g of an acid HX in water and made the solution up to 250cm3. The student titrated 25.0cm3 of this solution against 0.0614 moldm-3 Na2CO3. 23.45 cm3 of Na2CO3 were needed. Calculate the molar mass of the acid HX

Answer 11

97.0gmol-1

Check notes for workings!!