Water of crystallisation titration Flashcards
How many Waters of crystallisation does this molecule have?
10
What do you Start with in this type of calculation?
X amount of hydrated crystals (given in the question)
What is calculated after the titration?
The amount of anhydrous crystals
Why is the amount of anhydrous crystals calculated after the titration?
As the water of crystallisation doesn’t react with the other solute.
What can you calculate by the subtraction of anhydrous from the hydrated.
The amount of water of crystallisation.
What is done after the subtraction of anhydrous from the hydrated?
Then find the % and/or the number of water of crystallisation molecules present.
What is the first step in the method for a water of crystallisation calculation?
Sub into V1, M1, n1 and V2, M2, n2. (Be very careful here)
What is done after you have subbed into V1, M1, n1 and V2, M2, n2?
Sub into formula
V1M1 ÷ n1 = V2M2 ÷ n2
What happens after it has been subbed into the formula?
Solve for unknown molarity (M)
What is the answer always in after the molarity has been solved?
MOLES PER LITRE!!
What happens if in question a litre of solution wasn’t used?
Adjust accordingly ie if 200cm3 used, divide by 5, if 500cm3 of solution used divide by 2 etc
What is done to get grams per volume?
Multiply by Mr of anhydrous
What does multiplying by Mr of anhydrous to get grams per volume equal?
This equals grams of anhydrous
Can you include the water of crystallisation in the Mr?
No
Why can’t you include the water of crystallisation in the Mr?
As you do not know how many there are.
What is done to get grams of water of crystallisation?
Subtract grams of anhydrous from grams of hydrated given in the question.
To get % water of crystallisation
(Grams of water of crystallisation ÷ Grams of hydrated) x (100 ÷ 1)
To solve for ‘x’ in the formula
(Grams of anhydrous you calculated ÷ Grams of water you calculated) = (Mr of anhydrous ÷ 18x)
Explain the term standardised (L.C)
Concentration known by another titration
Describe in detail how a chemist should have dissolved the weighing sample of washing soda crystals. (L.C)
Wash into beaker of deionised water and stir to dissolve.
Explain how adding it drop by drop would have contributed to the accuracy of the titration result. (L.C)
Add dropwise so that end point will be precisely detected
Name a suitable indicator for this titration. State the colour change in the titration flak at the end point. (L.C)
- Methyl orange indicator
- Red (pink)
State the colour of the indicator before. (L.C)
Orange (yellow)
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)
(i) 0.0432 [(25 x M) ÷ 1 = (21.6 x 0.1) ÷ 2]
ii) 4.6 gl-1 (.0432 x 106
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)
54%
Identify a primary standard reagent which could have been used to standardise the hydrochloric acid solution. (L.C)
Anhydrous sodium carbonate
Name a suitable indicator for the titration and state the colour change observed in the conical flask at the end point. (L.C)
- Methyl orange indicator
- Red
Explain why no more than 1 - 2 drops of indicator should be used. (L.C)
Indicator is a weak acid
Describe the correct procedure for rinsing the burette before filling it with the solution it is to deliver. (L.C)
- Rinse with deionised water
- Rinse with reagent
Why is it important to fill the part below the tap of the burette? (L.C)
Air will be displaced by the solution
Describe in detail how a chemist should have made the solution up to exactly 250cm3. (L.C)
- 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.
