1 In this experiment you are to determine x in the formula of hydrated sodium carbonate, Na2CO3.xH2O, by titration - CIE - A-Level Chemistry - Question 1 - 2014 - Paper 1
Question 1
1 In this experiment you are to determine x in the formula of hydrated sodium carbonate, Na2CO3.xH2O, by titration.
FA 1 is aqueous sodium carbonate, Na2CO3, contai... show full transcript
Worked Solution & Example Answer:1 In this experiment you are to determine x in the formula of hydrated sodium carbonate, Na2CO3.xH2O, by titration - CIE - A-Level Chemistry - Question 1 - 2014 - Paper 1
Step 1
Diluting the solution of FA 1
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Answer
Measure 25.0 cm³ of FA 1 using a pipette and transfer it to a 250 cm³ volumetric flask.
Add distilled water to make the solution reach the mark on the flask.
Thoroughly mix the solution to ensure complete dissolution.
Step 2
Titration
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Answer
Pipette 25.0 cm³ of FA 3 (diluted sodium carbonate) into a conical flask.
Fill the burette with FA 2 (0.110 mol dm⁻³ HNO₃).
Add a few drops of methyl orange indicator to the conical flask.
Conduct a rough titration to estimate the endpoint and record the rough titre value.
Perform multiple accurate titrations, recording all burette readings for each titration.
Step 3
From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.
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Answer
Calculate the mean of all accurate titrations, ensuring that the total spread is within ±0.20 cm³.
If the best titres are 26.5 cm³, 26.6 cm³, and 26.4 cm³, then the mean titre is (26.5 + 26.6 + 26.4) / 3 = 26.5 cm³.
Step 4
Calculate the moles of nitric acid in the volume of FA 2 calculated in (b).
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Answer
Use the following formula:
[
\text{moles of HNO}_3 = \frac{\text{concentration (mol dm}^{-3}) \times , \text{volume (cm}^3)}{1000}
]
2. If the mean titre is 26.5 cm³ then:
[
\text{moles of HNO}_3 = \frac{0.110 \times 26.5}{1000} = 0.002915 \text{ mol}
]
Step 5
Complete the equation below and include the missing state symbols.
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Answer
The balanced equation for the reaction is:
[
\text{Na}_2\text{CO}_3(aq) + 2\text{HNO}_3(aq) \rightarrow 2\text{NaNO}_3(aq) + \text{CO}_2(g) + \text{H}_2\text{O}(l)
]
Step 6
Calculate the number of moles of sodium carbonate in 25.0 cm^3 of solution FA 3.
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Answer
Use the mole ratio from the balanced equation:
2 moles of Na2CO3 react with 2 moles of HNO3. Therefore:
[
\text{moles of Na}_2\text{CO}_3 = \text{moles of HNO}_3 = 0.002915 ext{ mol}
]
Step 7
Calculate the number of moles of sodium carbonate in 250.0 cm^3 of solution FA 3.
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Answer
Since the first calculation was for 25.0 cm³, multiply the moles by 10 for 250.0 cm³:
[
\text{moles of Na}_2\text{CO}_3 = 0.002915 \times 10 = 0.02915 ext{ mol}
]
Step 8
Use the information on page 2 and your answer to (iv) to calculate the relative formula mass, Mr, of hydrated sodium carbonate, Na2CO3.xH2O.
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Answer
Calculate the relative formula mass:
[
M_r = 150.0 + (x \times 18)
]
Using the previously calculated moles, if you have 0.02915 moles, then multiply by the amount per mol to get:
[
M_r = \frac{150.0}{0.02915} = 5130.2232 ext{ g/mol}
]
Step 9
Calculate the value of x in Na2CO3.xH2O. Give your answer to the nearest whole number.
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Answer
Rearranging the formula gives:
[
x = \frac{M_r - 150.0}{18} = \frac{5130.2232 - 150.0}{18} = 275.5 \approx 276
]
Step 10
State the maximum error in any single reading of the burette.
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Answer
Given the precision of the burette, the maximum error is typically ±0.05 cm³.
Step 11
Calculate the maximum percentage error in the volume of FA 2 in your first accurate titration.
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Answer
The maximum percentage error can be calculated using the formula:
[
\text{Percentage Error} = \frac{\text{Maximum Error}}{\text{Reading}} \times 100
]
2. If the first accurate titration reading was 26.5 cm³, then:
[
\text{Percentage Error} = \frac{0.05}{26.5} \times 100 \approx 0.19%
]
