Water of crystallisation titration

Question 1

How many Waters of crystallisation does this molecule have?

Answer 1

10

Question 2

What do you Start with in this type of calculation?

Answer 2

X amount of hydrated crystals (given in the question)

Question 3

What is calculated after the titration?

Answer 3

The amount of anhydrous crystals

Question 4

Why is the amount of anhydrous crystals calculated after the titration?

Answer 4

As the water of crystallisation doesn’t react with the other solute.

Question 5

What can you calculate by the subtraction of anhydrous from the hydrated.

Answer 5

The amount of water of crystallisation.

Question 6

What is done after the subtraction of anhydrous from the hydrated?

Answer 6

Then find the % and/or the number of water of crystallisation molecules present.

Question 7

What is the first step in the method for a water of crystallisation calculation?

Answer 7

Sub into V1, M1, n1 and V2, M2, n2. (Be very careful here)

Question 8

What is done after you have subbed into V1, M1, n1 and V2, M2, n2?

Answer 8

Sub into formula

V1M1 ÷ n1 = V2M2 ÷ n2

Question 9

What happens after it has been subbed into the formula?

Answer 9

Solve for unknown molarity (M)

Question 10

What is the answer always in after the molarity has been solved?

Answer 10

MOLES PER LITRE!!

Question 11

What happens if in question a litre of solution wasn’t used?

Answer 11

Adjust accordingly ie if 200cm3 used, divide by 5, if 500cm3 of solution used divide by 2 etc

Question 12

What is done to get grams per volume?

Answer 12

Multiply by Mr of anhydrous

Question 13

What does multiplying by Mr of anhydrous to get grams per volume equal?

Answer 13

This equals grams of anhydrous

Question 14

Can you include the water of crystallisation in the Mr?

Answer 14

No

Question 15

Why can’t you include the water of crystallisation in the Mr?

Answer 15

As you do not know how many there are.

Question 16

What is done to get grams of water of crystallisation?

Answer 16

Subtract grams of anhydrous from grams of hydrated given in the question.

Question 17

To get % water of crystallisation

Answer 17

(Grams of water of crystallisation ÷ Grams of hydrated) x (100 ÷ 1)

Question 18

To solve for ‘x’ in the formula

Answer 18

(Grams of anhydrous you calculated ÷ Grams of water you calculated) = (Mr of anhydrous ÷ 18x)

Question 19

Explain the term standardised (L.C)

Answer 19

Concentration known by another titration

Question 20

Describe in detail how a chemist should have dissolved the weighing sample of washing soda crystals. (L.C)

Answer 20

Wash into beaker of deionised water and stir to dissolve.

Question 21

Explain how adding it drop by drop would have contributed to the accuracy of the titration result. (L.C)

Answer 21

Add dropwise so that end point will be precisely detected

Question 22

Name a suitable indicator for this titration. State the colour change in the titration flak at the end point. (L.C)

Answer 22

• Methyl orange indicator

- Red (pink)

Question 23

State the colour of the indicator before. (L.C)

Answer 23

Orange (yellow)

Question 24

From the mean volume of the hydrochloric acid solution, calculate the concentration of sodium carbonate (Na2CO3) in the original solution in (i) moles per litre, (ii) grams per litre. (L.C)

Answer 24

(i) 0.0432 [(25 x M) ÷ 1 = (21.6 x 0.1) ÷ 2]

ii) 4.6 gl-1 (.0432 x 106

Question 25

Calculate the percentage water of crystallisation in the crystals and the value of x, the average number of water molecules in the formula Na2CO3.xH20. (L.C)

Answer 25

54%

Question 26

Identify a primary standard reagent which could have been used to standardise the hydrochloric acid solution. (L.C)

Answer 26

Anhydrous sodium carbonate

Question 27

Name a suitable indicator for the titration and state the colour change observed in the conical flask at the end point. (L.C)

Answer 27

• Methyl orange indicator

- Red

Question 28

Explain why no more than 1 - 2 drops of indicator should be used. (L.C)

Answer 28

Indicator is a weak acid

Question 29

Describe the correct procedure for rinsing the burette before filling it with the solution it is to deliver. (L.C)

Answer 29

• Rinse with deionised water

- Rinse with reagent

Question 30

Why is it important to fill the part below the tap of the burette? (L.C)

Answer 30

Air will be displaced by the solution

Question 31

Describe in detail how a chemist should have made the solution up to exactly 250cm3. (L.C)

Answer 31

• Pour through funnel into volumetric flask adding rinsings of beaker.
• Add last few drops of deionised water drop by drop to bring bottom of meniscus level with mark reading at eye level.