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chemistry a level > acid-base titrations > Flashcards

acid-base titrations Flashcards

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1
Q

What are titrations?

A

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.
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2
Q

What chemical unknowns can be investigated through titrations?

A

Concentration, molar mass, formula, water of crystallization

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3
Q

What are used in titrations?

A

Solutions with a known concentration (standard solution)

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4
Q

What is the process of titrations?

A

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.

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5
Q

What are standard solutions?

A
  • solutions of a known concentration, allows us to calculate unknowns
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6
Q

What is concentration?

A

How much solute is dissolved in a given volume of solvent

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7
Q

What is the method of making a standard solution?

A
  • 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.
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8
Q

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

A
  • work out the number of moles required.
  • calculate the molar mass of the required substance
  • calculate the mass of the substance that is required.
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9
Q

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

  • 250 cm^3 solution of NaOH
  • 0.1 mol/dm^3
A 
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
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10
Q

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

A

Concentration of the solution, molar mass, etc

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11
Q

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?

A

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

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12
Q

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?

A 
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

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13
Q

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?

A

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

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14
Q

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?

A 
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
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15
Q

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.

A

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

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16
Q

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

A

2:1
0.0011/2
= 0.00055mol of h2so4

17
Q

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

A

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

18
Q

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

A

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

19
Q

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

A 
N=cv
0.5x0.5
=0.025 moles 
1:2
0.025:0.05
C=n/v 
0.05/0.1
=0.5mol/dm3
20
Q

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?

A

Burette and pipette

Indicator

chemistry a level (53 decks)

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