acid-base titrations

Question 1

What are titrations?

Answer 1

A form of volumetric analysis :

• known volume and concentration of a solution is reacted with a measured volume of a solution
• can be used to find chemical unknowns.

Question 2

What chemical unknowns can be investigated through titrations?

Answer 2

Concentration, molar mass, formula, water of crystallization

Question 3

What are used in titrations?

Answer 3

Solutions with a known concentration (standard solution)

Question 4

What is the process of titrations?

Answer 4

Using a pipe tote add a measured/suitable volume of x into a conical flask (base). Then add 3 drops of a suitable indicator to the flask (phenolphthalein, methyl orange etc). Fill the burette with a suitable volume of y solution (acid usually). Open the valve on the burette to allow a small quantity of y to flow into the flask of x and gently swirl the conical flask containing x solution so that the color change in the indicator would be more accurate. Repeat the process until the end-point is reached. Measure the volume of y that has been added to solution x and then calculate the unknowns.

Question 5

What are standard solutions?

Answer 5

• solutions of a known concentration, allows us to calculate unknowns

Question 6

What is concentration?

Answer 6

How much solute is dissolved in a given volume of solvent

Question 7

What is the method of making a standard solution?

Answer 7

• carefully weigh out the required mass of the solute
• dissolve the solute into the chosen solvent in a beaker.
• transfer the solution into a volumetric flask, rinse the beaker with the initial solvent and add washings to the volumetric flask.
• add some of the solvent to the volumetric flask and make sure not to fill up to the graduation line.
• add solvent, drop by drop until the bottom of the meniscus is sitting on the line, do not allow the solution to fill above the line.
• mix the solution thoroughly and invert the flask multiple times.

Question 8

What are the steps for the initial calculations of a standard solution?

Answer 8

• work out the number of moles required.
• calculate the molar mass of the required substance
• calculate the mass of the substance that is required.

Question 9

Example: the process of the initial calculations with the below info.

• 250 cm^3 solution of NaOH
• 0.1 mol/dm^3

Answer 9

N= cxv
=0.1 x 0.25
->0.025 moles required
Mr of NaOH 
\: 23+1+16
->40
Mass = moles x mr
0.025 x 40
= 1g

Question 10

In general, the calculations can follow certain steps. What are the calculations used to find?

Answer 10

Concentration of the solution, molar mass, etc

Question 11

25 cm3 of 0.1 mol/dm3 of a base NaOH is titrated with 22.5 cm3 of an acid HCl.
What is the concentration of the acid involved in this titration?

Answer 11

HCl + NaOH -> NaCl + H2O

V= 0.025 dm3 
C= 0.1mol/dm3
… 0.025 x 0.1
= 0.0025 moles of base
‘
Moles of acid: 1:1 ratio
So HCl = 0.0025 moles 

0.0025 = 0.0225 x c
C=0.1111….mol/dm3
.. 0.1mol/dm3

Question 12

1.5G of an acid was dissolved in water in order to make a solution with a volume of 250 cm3. 25 cm3 of a solution this acid HX was titrated with 22.5cm3 of a base NaOH which had a concentration of 0.1mol/dm3
What is the Mr of the acid involved in this titration?

Answer 12

Reaction equation:
HX + NaOH -> NaX + H2O
Calculate the amount in mol of base has reacted:
C=0.1
V= 0.0225
Mol= 0.1 x 0.0225
-> 0.00225 moles.
Calculate the amount in mol of acid that was used to make the initial solution:

1:1
… 0.00225 moles of HX in 25 cm3

250/25=10
0.00225 x10
=0.0225mol

Calculate the mr of the acid:
Mr=m/n
1.5/0.0225
=66.667g/mol

Question 13

A student decides to make up a standard solution of sodium hydroxide. She carefully measures out 10 g directly onto a balance. She adds this to a conical flask with 100 ml of water using a glass stirring rod.
- the student made a number of errors in her method. How could she improve her method?

Answer 13

She could use a weighing boat.
She could use a volumetric flask
She could invert the flask rather than stirring

Question 14

A student decides to make up a standard solution of sodium hydroxide. She carefully measures out 10 g directly onto a balance. She adds this to a conical flask with 100 ml of water using a glass stirring rod
— having learnt the correct way of making up a standard solution the student decides to are up a standard solution of Mg(OH)2 the student wants to obtain 1l of 0.1mol/dm3. What mass of Mg(OH)2 should she use?

Answer 14

Mol of Mg(OH)2 
= 0.1 x1
=0.1 mol
N=0.1
Mr= 58.3
Mass = 0.1 x 58.3
= 5.83 g

Question 15

A titration between sulphuric acid and sodium hydroxide is carried out:
H2SO4(aq) +2NaOH (aq) -> Na2SO4(aq) +2H2O (l)
25 cm3 of 0.0440 mol/dm3 NaOH is neutralized by 17.5cm3 of dilute H2SO4 (aq).- -calculate the amount in moles of NaOH used in the titration.

Answer 15

C= 0.0440
V=0.025
N=0.0440x0.025
=0.0011mol

Question 16

A titration between sulphuric acid and sodium hydroxide is carried out:
H2SO4(aq) +2NaOH (aq) -> Na2SO4(aq) +2H2O (l)
25 cm3 of 0.0440 mol/dm3 NaOH is neutralized by 17.5cm3 of dilute H2SO4 (aq)
- calculate the amount in moles of H2SO4 used in the titration
Naoh was 0.0011

Answer 16

2:1
0.0011/2
= 0.00055mol of h2so4

Question 17

A titration between sulphuric acid and sodium hydroxide is carried out:
H2SO4(aq) +2NaOH (aq) -> Na2SO4(aq) +2H2O (l)
25 cm3 of 0.0440 mol/dm3 NaOH is neutralized by 17.5cm3 of dilute H2SO4 (aq)
- calculate the concentration in mol/dm3 of H2SO4 used in the titration

Answer 17

N= 0.00055mol
V=0.0175dm3
0.00055/0.0175
=0.0314 mol/dm3

Question 18

A student carries out a titration using barium hydroxide Ba(OH)2 and nitric acid.
- write an equation for the reaction that takes place.

Answer 18

Ba(OH)2 + 2HNO3 -> Ba(NO3)2 +2H2O

Question 19

A student carries out a titration using barium hydroxide Ba(OH)2 and nitric acid.
- 50ml of barium hydroxide (conc of 0.5 mol/dm3 ) are required to fully titration a 100ml solution of the acid what is the initial concentration of the acid ?
Barium hydroxide + 2 nitric acid -> barium nitrate+2 water

Answer 19

N=cv
0.5x0.5
=0.025 moles 
1:2
0.025:0.05
C=n/v 
0.05/0.1
=0.5mol/dm3

Question 20

A student carries out a titration using barium hydroxide Ba(OH)2 and nitric acid.
Name 2 pieces of apparatus that are used to accurately measure the volume of solutions in a titration. What other chemical substance must be added to the reaction flask for a titration to be successful?

Answer 20

Burette and pipette

Indicator