Practical we never did but is in exam - Double Titration

Question 1

What does a double titration use the concept of?

Answer 1

That different indicators change at different values of pH

Question 2

What can a double titration be used to calculate?

Answer 2

The concentrations of 2 different bases in a solution

Question 3

How many stages are there in a double titration and what’s involved in both stages?

Answer 3

2 stages
2 indicators

Question 4

What are the 2 separate indicators used during a double titration and in which order?

Answer 4

Phenolphthalein
Methyl orange

Question 5

What’s the method for this practical?

Answer 5

Titration a 25cm^3 sample of the mixed solution against the HCl solution using pthenylphthalein as the indicator
Do not agitate the flask more than in necessary to mix the acid
Record the volume used at the phenhylpthalein end point
Add methyl orange and continue to titration until its end point
Record the total volume of HCl added to this end point
Calculate the concentration of the sodium hydroids and the sodium carbonate in the mixed solution then the mass in grams

Question 6

What is the colour change of pthenylphthalein and at which pH?

Answer 6

Pink to colourless at pH 9

Question 7

What is the colour change of methyl orange and at what pH?

Answer 7

Yellow to orange-pink at pH 4

Question 8

What are the pthenylphthalein titration reactions?

Answer 8

NaOH + HCl —> NaCl + H2O
Na2CO3+ HCl —> NaHCO3 + NaCl

Question 9

What’s the methyl orange titration reaction?

Answer 9

NaHCO3 + HCl —> NaCl + CO2 + H2O

Question 10

What does the second titre give us the molecules of?

Answer 10

NaHCO3

Question 11

Which moles are equivalent and why?

Answer 11

NaHCO3 = NaCO3
that’s where it came from

Question 12

How do we calculate the moles of NaOH?

Answer 12

Moles of HCl in first titre - moles of Na2CO3

Question 13

What are the stages in working out the concentration of 2 different bases in a solution using a double titration?

Answer 13

1. Moles of HCl used in first stage (this is how many moles of OH- ions and CO32- ions are in the solution)
2. Moles HCl used in second stage (this is how many moles of CO32- ions are in the solution, as the number of moles is equal to the number of moles CO32-)
3. Concentration of CO32-
4. Moles OH- (moles HCl in first titre - moles of CO32-)
5. Concentration of OH-

Question 14

Why is it important not to agitate the flask more than is necessary to mix the acid?

Answer 14

CO2 in the air can dissolve in the flask contents to form carbonate, which could affect results

Question 15

Why can’t we add both indicators at the start of a double titration?

Answer 15

The colour of one will mask the other